为什么80%的码农都做不了架构师？>>> 
Basic Tools(一)
从官网互获取的一部分tools描述
Standard JDK Tools and Utilities
Basic Tools (appletviewer, extcheck, jar, java, javac, javadoc, javah, javap, jdb, jdeps)
Security Tools (keytool, jarsigner, policytool, kinit, klist, ktab)
Internationalization Tools (native2ascii)
Remote Method Invocation (RMI) Tools (rmic, rmiregistry, rmid, serialver)
Java IDL and RMI-IIOP Tools (tnameserv, idlj, orbd, servertool)
Java Deployment Tools (javapackager, pack200, unpack200)
Java Web Start Tools (javaws)
Java Troubleshooting, Profiling, Monitoring and Management Tools (jcmd, jconsole, jmc, jvisualvm)
Java Web Services Tools (schemagen, wsgen, wsimport, xjc)
Experimental JDK Tools and Utilities
NOTE - The tools described in this section are unsupported and experimental in nature and should be used with that in mind. They might not be available in future JDK versions.
Monitoring Tools (jps, jstat, jstatd)
Troubleshooting Tools (jinfo, jhat, jmap, jsadebugd, jstack)
Scripting Tools (jrunscript)主要对一些用的比较多的工具进行描述：
Basic Tools (appletviewer, extcheck, jar, java, javac, javadoc, javah, javap, jdb, jdeps)
appletviewer 现在基本没有使用applet那么这个就工具就跳过。
extcheck命令
主要用于检测指定的jar文件与Java SDK安装的任何扩展之间是否存在版本冲突。在安装一个扩展之前，你可以使用此工具来检测是否已经安装该扩展的相同版本或最新版本。
extcheck工具通过将指定jar文件中清单文件(manifest)的Specification-title和Specification-version头信息与扩展目录中所有的jar文件对应的头信息进行比较，从而检测是否存在版本冲突。
JDK(或JRE)默认的扩展安装目录为jre/lib/ext。extcheck工具通过使用与方法java.lang.Package.isCompatibleWith相同的方式来比较版本号。
如果没有检测出冲突问题，则返回代码为0。
如果扩展目录中任何jar文件的清单文件与指定jar文件有相同的Specification-title以及相同(或更新)的Specification-version版本号，将返回一个非0的错误码。如果指定jar文件的清单文件中没有Specification-title或Specification-version属性，也将返回非0的错误码。
extcheck工具的命令行用法为：
extcheck [-verbose] [-J] target.jar
其中：
可选参数-verbose表示显示详细的检测信息；
可选参数-J可以有多个，用于指定可选的运行时参数，例如：-J-Xms48m。
参数target.jar表示指定的jar文件。
jar命令
一.jar命令参数：
jar命令格式：jar {c t x u f }[ v m e 0 M i ][-C 目录]文件名...
其中{ctxu}这四个参数必须选选其一。[v f m e 0 M i ]是可选参数，文件名也是必须的。
-c 创建一个jar包
-t 显示jar中的内容列表
-x 解压jar包
-u 添加文件到jar包中
-f 指定jar包的文件名
-v 生成详细的报造，并输出至标准设备
-m 指定manifest.mf文件.(manifest.mf文件中可以对jar包及其中的内容作一些一设置)
-0 产生jar包时不对其中的内容进行压缩处理
-M 不产生所有文件的清单文件(Manifest.mf)。这个参数与忽略掉-m参数的设置
-i 为指定的jar文件创建索引文件
-C 表示转到相应的目录下执行jar命令,相当于cd到那个目录，然后不带-C执行jar命令
二.jar使用范例：
(1)创建jar包
jar cf hello.jar hello
利用test目录生成hello.jar包,如hello.jar存在,则覆盖
(2)创建并显示打包过程
jar cvf hello.jar hello
利用hello目录创建hello.jar包,并显示创建过程
例：
E:\>jar cvf hello.jar hello
标明清单(manifest)
增加：hello/(读入= 0) (写出= 0)(存储了 0%)
增加：hello/TestServlet2.class(读入= 1497) (写出= 818)(压缩了 45%)
增加：hello/HelloServlet.class(读入= 1344) (写出= 736)(压缩了 45%)
增加：hello/TestServlet1.class(读入= 2037) (写出= 1118)(压缩了 45%)
(3)显示jar包：
jar tvf hello.jar 查看hello.jar包的内容
指定的jar包必须真实存在，否则会发生FileNoutFoundException。
(4)解压jar包：
jar xvf hello.jar
解压hello.jar至当前目录
(5)jar中添加文件：
jar uf hello.jar HelloWorld.java
将HelloWorld.java添加到hello.jar包中
(6)创建不压缩内容jar包：
jar cvf0 hello.jar *.class
利用当前目录中所有的.class文件生成一个不压缩jar包
(7)创建带manifest.mf文件的jar包：
jar cvfm hello.jar manifest.mf hello
创建的jar包多了一个META-INF目录,META-INF止录下多了一个manifest.mf文件,至于manifest.mf的作用,后面会提到.
(8)忽略manifest.mf文件：
?
1
jar cvfM hello.jar hello
生成的jar包中不包括META-INF目录及manifest.mf文件
(9)加-C应用：
jar cvfm hello.jar mymanifest.mf -C hello/
表示在切换到hello目录下然后再执行jar命令
(10)-i为jar文件生成索引列表：
当一个jar包中的内容很好的时候，你可以给它生成一个索引文件，这样看起来很省事。
jar i hello.jar
执行完这条命令后，它会在hello.jar包的META-INF文件夹下生成一个名为INDEX.LIST的索引文件，它会生成一个列表，最上边为jar包名。
(11)导出解压列表：
jar tvf hello.jar >hello.txt 如果你想查看解压一个jar的详细过程，而这个jar包又很大，屏幕信息会一闪而过，这时你可以把列表输出到一个文件中，慢慢欣赏！
(12)jar -cvf hello.jar hello/*
例如原目录结构如下：
hello
|---com
|---org
你本想只把com目录和org目录打包，而这时jar命令会连同hello目洋也一块打包进。这点大家要注意。jar命令生成的压缩文件会包含它后边出的目录。我们应该进入到hello目录再执行jar命令。
注意：manifest.mf这个文件名，用户可以任指定，但jar命令只认识Manifest.mf，它会对用户指定的文件名进行相应在的转换，这不需用户担心。
三.Manifest.mf文件编写规则：
manifest.mf的编写一定要注意一些细节，它是很苛刻的,我在此也载过不少跟头，谁让它这么小气呢，没办法，所以专门给大家列出来。
(1) 不能有空行和空格的地方
第一行不可以是空行( 第一行的行前不可以有空行)，行与行之间不能有空行，第行的行尾不可以有空格
(2) 一定要有空行的地方
最后一行得是空行(在输完你的内容后加一个回车就OK)
(3) 一定有空格的地方
key: value 在分号后面一定要写写一个空格
四.怎样使用jar包中的类
还是写个小例子吧，这样直观！
public final class Person
{
public static int age()
{
return 30;
}
}
-> javac Person.java
->jar cvf person.jar Person.class
将上面的文件打成一个jar包
再写一个类对其进行调用:
public class MyAge
{
public static void getAge()
{
System.out.println(Person.age());
}
}
->javac MyAge.java
->java -classpath person.jar MyAge
感兴趣的读者可以调试一下这个程序
五.创建可执行jar包
有时自己写个程序，类一大堆，时间一长连自己都不知道那个是主类，而且有可能用到图片或其它文件一大堆，看得也乱，这时你可以考虑把它做成一个可执行jar包...
(1) 编辑manifest.mf文件加入下面一行
Main-Class: MyApplet
注意:Main-Class的大小定，冒号后的空格,MyApplet后一定输入回车，然后保存。
(2) 打包
jar cvfm FirstApplet.jar manifest.mf MyApplet.class
注意:manifest.mf指定为存放Mani-Class: MyApplet文件的class路径(如：hello.Hello)或者文件名(applet)
(3) 可执行jar的使用
java -jar FirstApplet.jar
也可以中使用:
注意：类并没有给出，大家随便写一个就行，类名包名自己随意定，相应的更改就可以...
六.扩展自己的类
在JDK的安装目录\jre\lib\ext目录下，SUN为大家为我们扩展自己类的提供了方便，大家可以将自己的类文件打成.jar包放在此目录下，它由ExtClassLoader类装器负责进行装载，ExtClassLoader类装器是AppClassLoader类装载器的父装载器，AppClassLoader主要负责加载CLASSPATH路径下的文件，而在java中采用的又是委托父装载器的机制，所以此目录下存放的jar中的类文件不做任何的设置，类装载器就可以找到正常的加载，是不是很方便啊，呵...
如果你的.jar是给applet小应用程序看的，可以在打成jar包之前，在其manifest.mf加入下面两行。
Class-Path: FirstApplet.jar
Class-path: SecondApplet.jar
Main-Class: MyApplet
注意：Class-path可以设置多项，直接写jar包名既可。Main-Class主要当jar中有多个.class类文件时,java并不知道那个才是主类，所以要指定，如果jar包中只有一个类当然可以不指定。
Java调用类的顺序：java\lib\ext中的类--->Manifest.mf中指定的类-->当前目录中的类-->set CLASSPATH中指定的类。
七.调用URL网络上的jar包
(1) 生成jar包的URL
URL u=new URL("jar:"+"FirstAppplet.jar"+!/");
(2) 建立jarURLConnection对象
JarURLConnection juc=(JarURLConnection)u.openConnection();
(3) 返回jar包中主类的名字
Attributes attr=juc.getMainAttributes();
String name=attr.getValue("Mani-Class");
一定要确保你的jar包中的manifest.mf中已正确的设置了Mani-Class属性，再强调一下一定要注意规则。
(4) 根据得到的主类名创建Class对象
Class c=Class.forName(name);
(5) 根据Class对象调用其main方法：
Method cm=c.getMethod("main",new Class[]{String.class});
cm.invoke(null,new Object[]{});
提示:上边用到了Reflection反射机制的相关知识，大家如果多反射机制有兴趣，可查看java.lang.reflect包中的相关内容.
八.JAR命令使用技巧：
(1) jar创建压ZIP文件
jar cvfM TestZIP.jar test
加M参数为了不生成META-INF相关内容
然后将TestZIP.jar改为TestZIP.zip就可以，是不是很简单....
(2) 使用WinRAR解压.jar文件
上边我们已经说过了，说JAR文件是一种特殊的压缩文件，所以它当然可以用我们常用的一些解压 缩工具来解了，至于怎么解，这就不用我说了吧。
(3) 用WinRAR生成.jar文件
我们已经说过JAR包与ZIP包主要区别就是JAR包中多一个META-INF的目录，META-INF目录下有一个manifest.mf文件，我们只要建立好相关的目录一压缩就可以了。
目录的结构如下：
TestJar
|--META-INF
|--manifest.mf
|--相关的类文件
JAVA命令
Java是通过java虚拟机来装载和执行编译文件（class文件）的，java虚拟机通过命令java option 来启动，-option为虚拟机参数，通过这些参数可对虚拟机的运行状态进行调整.
一、如何查看参数列表:
Standard Options
Non-Standard Options
Advanced Runtime Options
Advanced JIT Compiler Options
Advanced Serviceability Options
Advanced Garbage Collection Options
二、标准参数说明：
1. -classpath,-cp
虚拟机在运行一个类时，需要将其装入内存，虚拟机搜索类的方式和顺序如下：
Bootstrap classes，Extension classes，User classes。
Bootstrap 中的路径是虚拟机自带的jar或zip文件，虚拟机首先搜索这些包文件，用System.getProperty("sun.boot.class.path")可得到虚拟机搜索的包名。
Extension是位于jre"lib"ext目录下的jar文件，虚拟机在搜索完Bootstrap后就搜索该目录下的jar文件。用System. getProperty("java.ext.dirs”)可得到虚拟机使用Extension搜索路径。
User classes搜索顺序为当前目录、环境变量 CLASSPATH、-classpath。
-cp 和 -classpath 是同义词，参数意义是一样的。classpath参数太长了，所以提供cp作为缩写形式,它们用于告知虚拟机搜索目录名、jar文档名、zip文档名，之间用分号;分隔。
例如当你自己开发了公共类并包装成一个common.jar包，在使用 common.jar中的类时，就需要用-classpath common.jar 告诉虚拟机从common.jar中查找该类，
否则虚拟机就会抛出java.lang.NoClassDefFoundError异常，表明未找到类定义。
在运行时可用System.getProperty(“java.class.path”)得到虚拟机查找类的路径。
使用-classpath后虚拟机将不再使用CLASSPATH中的类搜索路径，如果-classpath和CLASSPATH都没有设置，则虚拟机使用当前路径(.)作为类搜索路径。
推荐使用-classpath来定义虚拟机要搜索的类路径，而不要使用环境变量 CLASSPATH的搜索路径，以减少多个项目同时使用CLASSPATH时存在的潜在冲突。
例如应用1要使用a1.0.jar中的类G，应用2要使用 a2.0.jar中的类G,a2.0.jar是a1.0.jar的升级包，当a1.0.jar，a2.0.jar都在CLASSPATH中，虚拟机搜索到第一个包中的类G时就停止搜索，
如果应用1应用2的虚拟机都从CLASSPATH中搜索，就会有一个应用得不到正确版本的类G。
实例1：java -classpath lib\Launcher.jar com.teleca.robin.Launcher getProcessState.bat 5000
实例2：java -classpath classes com.teleca.robin.Launcher getProcessState.bat 5000
如果需要指定各个JAR文件具体的存放路径，相同路径有多个可使用通配符。
示例1：
java -cp .;c:\classes\myClass.jar;d:\classes\*.jar packname.mainclassname
实例3：
java -cp lib\*.jar com.teleca.robin.Launcher getProcessState.bat 5000
实例3-1：
java -cp Util.jar;lib\sqljdbc.jar com.harry.j2se.AppEntrance
2.-D=value
在虚拟机的系统属性中设置属性名/值对，运行在此虚拟机之上的应用程序可用System.getProperty(“propertyName”)得到value的值。
如果value中有空格，则需要用双引号将该值括起来，如-Dname=”space string”。
该参数通常用于设置系统级全局变量值，如配置文件路径，应为该属性在程序中任何地方都可访问。
示例1：
设置虚拟机使用的Extension搜索路径
java -Djava.ext.dirs=lib MyClass
3. -client，-server
这两个参数用于设置虚拟机使用何种运行模式，client模式启动比较快，但运行时性能和内存管理效率不如server模式，通常用于客户端应用程序。相反，server模式启动比client慢，但可获得更高的运行性能。
在windows上，缺省的虚拟机类型为client模式，如果要使用 server模式，就需要在启动虚拟机时加-server参数，以获得更高性能，对服务器端应用，推荐采用server模式，尤其是多个CPU的系统。在 Linux，Solaris上缺省采用server模式。
4. -hotspot
含义与client相同，jdk1.4以前使用的参数，jdk1.4开始不再使用，代之以client。
5. -verbose[:class|gc|jni]
在输出设备上显示虚拟机运行信息。
verbose和verbose:class含义相同，输出虚拟机装入的类的信息，显示的信息格式如下：
[Loaded java.io.FilePermission$1 from shared objects file]
当虚拟机报告类找不到或类冲突时可用此参数来诊断来查看虚拟机从装入类的情况。
6. -verbose:gc
在虚拟机发生内存回收时在输出设备显示信息，格式如下：
[Full GC 268K->168K(1984K), 0.0187390 secs]
该参数用来监视虚拟机内存回收的情况。
7.-verbose:jni
在虚拟机调用native方法时输出设备显示信息，格式如下：
[Dynamic-linking native method HelloNative.sum ... JNI]
该参数用来监视虚拟机调用本地方法的情况，在发生jni错误时可为诊断提供便利。
9.-version
显示可运行的虚拟机版本信息然后退出。一台机器上装有不同版本的JDK时
10.-showversion
显示版本信息以及帮助信息。
11.-ea[:...|:]和-enableassertions[:...|:]
从JDK1.4开始，java可支持断言机制，用于诊断运行时问题。通常在测试阶段使断言有效，在正式运行时不需要运行断言。断言后的表达式的值是一个逻辑值，为true时断言不运行，为false时断言运行，抛出java.lang.AssertionError错误。
上述参数就用来设置虚拟机是否启动断言机制，缺省时虚拟机关闭断言机制，用-ea 可打开断言机制，不加和classname时运行所有包和类中的断言，如果希望只运行某些包或类中的断言，可将包名或类名加到-ea之后。例如要启动包com.foo.util中的断言，可用命令 ea:com.foo.util 。
注：ea是enableassertions的缩写
12.-da[:...|:]和-disableassertions[:...|:]
用来设置虚拟机关闭断言处理，packagename和classname的使用方法和-ea相同。
注：da应该是disableassertions的缩写
13.-esa 和-enablesystemassertions
设置虚拟机显示系统类的断言。
注：esa是enablesystemassertions的缩写
15.-dsa 和-disablesystemassertions
设置虚拟机关闭系统类的断言。
注：dsa是disablesystemassertions的缩写
16.-agentlib:[=]
该参数是JDK5新引入的，用于虚拟机装载本地代理库。
Libname为本地代理库文件名，虚拟机的搜索路径为环境变量PATH中的路径，options为传给本地库启动时的参数，多个参数之间用逗号分隔。在Windows平台上虚拟机搜索本地库名为libname.dll的文件，在Unix上虚拟机搜索本地库名为libname.so的文件，搜索路径环境变量在不同系统上有所不同，Linux、SunOS、IRIX上为LD_LIBRARY_PATH，AIX上为LIBPATH，HP-UX上为SHLIB_PATH。
例如可使用-agentlib:hprof来获取虚拟机的运行情况，包括CPU、内存、线程等的运行数据，并可输出到指定文件中，可用-agentlib:hprof=help来得到使用帮助列表。在jre"bin目录下可发现hprof.dll文件。
17. -agentpath:[=]
设置虚拟机按全路径装载本地库，不再搜索PATH中的路径。其他功能和agentlib相同。
18.-javaagent:[=]
虚拟机启动时装入java语言设备代理。Jarpath文件中的mainfest 文件必须有Agent-Class属性。代理类要实现public static void premain(String agentArgs, Instrumentation inst)方法。当虚拟机初始化时，将按代理类的说明顺序调用premain方法。
三、非标准参数说明（看虚拟机是否接受）
1.-Xmixed
设置-client模式虚拟机对使用频率高的方式进行Just-In-Time编译和执行，对其他方法使用解释方式执行。该方式是虚拟机缺省模式。
2.-Xint
设置-client模式下运行的虚拟机以解释方式执行类的字节码，不将字节码编译为本机码。
3. -Xbootclasspath:path和-Xbootclasspath/a:path及-Xbootclasspath/p:path
改变虚拟机装载缺省系统运行包rt.jar而从-Xbootclasspath中设定的搜索路径中装载系统运行类。除非你自己能写一个运行时，否则不会用到该参数。
/a:将在缺省搜索路径后加上path 中的搜索路径。
/p:在缺省搜索路径前先搜索path中的搜索路径。
4.-Xnoclassgc
关闭虚拟机对class的垃圾回收功能。
5.-Xincgc
启动增量垃圾收集器，缺省是关闭的。增量垃圾收集器能减少偶然发生的长时间的垃圾回收造成的暂停时间。但增量垃圾收集器和应用程序并发执行，因此会占用部分CPU在应用程序上的功能。
6.-Xloggc:
将虚拟机每次垃圾回收的信息写到日志文件中，文件名由file指定，文件格式是平文件，内容和-verbose:gc输出内容相同。
7.-Xbatch
虚拟机的缺省运行方式是在后台编译类代码，然后在前台执行代码，使用-Xbatch参数将关闭虚拟机后台编译，在前台编译完成后再执行。
8-Xms
设置虚拟机可用内存堆的初始大小，缺省单位为字节，该大小为1024的整数倍并且要大于1MB，可用k(K)或m(M)为单位来设置较大的内存数。初始堆大小为2MB。
例如：-Xms6400K，-Xms256M
9.-Xmx
设置虚拟机内存堆的最大可用大小，缺省单位为字节。该值必须为1024整数倍，并且要大于2MB。可用k(K)或m(M)为单位来设置较大的内存数。缺省堆最大值为64MB。
例如：-Xmx81920K，-Xmx80M
当应用程序申请了大内存运行时虚拟机抛出java.lang.OutOfMemoryError: Java heap space错误，就需要使用-Xmx设置堆的最大值
10.-Xss
设置线程栈的大小，缺省单位为字节。与-Xmx类似，也可用K或M来设置较大的值。通常操作系统分配给线程栈的缺省大小为1MB。
另外也可在java中创建线程对象时设置栈的大小，构造函数原型为Thread(ThreadGroup group, Runnable target, String name, long stackSize)。
11.-Xprof
输出CPU运行时的诊断信息。
12.-Xfuture
对类文件进行严格格式检查，以保证类代码符合类代码规范。为保持向后兼容，虚拟机缺省不进行严格的格式检查。
13.-Xrs
减少虚拟机中操作系统的信号（singals）的使用。该参数通常用在虚拟机以后台服务方式运行时使用（如Servlet）。
14.-Xcheck:jni
调用JNI函数时进行附加的检查，特别地虚拟机将校验传递给JNI函数参数的合法性，在本地代码中遇到非法数据时，虚拟机将报一个致命错误而终止。使用该参数后将造成性能下降。
四、Advanced Runtime 参数
These options control the runtime behavior of the Java HotSpot VM.
-XX:+CheckEndorsedAndExtDirs
Enables the option to prevent the java command from running a Java application if it uses the endorsed-standards override mechanism or the extension mechanism. This option checks if an application is using one of these mechanisms by checking the following:
The java.ext.dirs or java.endorsed.dirs system property is set.
The lib/endorsed directory exists and is not empty.
The lib/ext directory contains any JAR files other than those of the JDK.
The system-wide platform-specific extension directory contains any JAR files.
-XX:+DisableAttachMechanism
Enables the option that disables the mechanism that lets tools attach to the JVM. By default, this option is disabled, meaning that the attach mechanism is enabled and you can use tools such as jcmd, jstack, jmap, and jinfo.
-XX:ErrorFile=filename
Specifies the path and file name to which error data is written when an irrecoverable error occurs. By default, this file is created in the current working directory and named hs_err_pidpid.log where pid is the identifier of the process that caused the error. The following example shows how to set the default log file (note that the identifier of the process is specified as %p):
-XX:ErrorFile=./hs_err_pid%p.log
The following example shows how to set the error log to /var/log/java/java_error.log:
-XX:ErrorFile=/var/log/java/java_error.log
If the file cannot be created in the specified directory (due to insufficient space, permission problem, or another issue), then the file is created in the temporary directory for the operating system. The temporary directory is /tmp.
-XX:+FailOverToOldVerifier
Enables automatic failover to the old verifier when the new type checker fails. By default, this option is disabled and it is ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode.
-XX:+FlightRecorder
Enables the use of the Java Flight Recorder (JFR) during the runtime of the application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option as follows:
java -XX:+UnlockCommercialFeatures -XX:+FlightRecorder
If this option is not provided, Java Flight Recorder can still be enabled in a running JVM by providing the appropriate jcmd diagnostic commands.
-XX:-FlightRecorder
Disables the use of the Java Flight Recorder (JFR) during the runtime of the application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option as follows:
java -XX:+UnlockCommercialFeatures -XX:-FlightRecorder
If this option is provided, Java Flight Recorder cannot be enabled in a running JVM.
-XX:FlightRecorderOptions=parameter=value
Sets the parameters that control the behavior of JFR. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. This option can be used only when JFR is enabled (that is, the -XX:+FlightRecorder option is specified).
The following list contains all available JFR parameters:
defaultrecording={true|false}
Specifies whether the recording is a continuous background recording or if it runs for a limited time. By default, this parameter is set to false (recording runs for a limited time). To make the recording run continuously, set the parameter to true.
disk={true|false}
Specifies whether JFR should write a continuous recording to disk. By default, this parameter is set to false (continuous recording to disk is disabled). To enable it, set the parameter to true, and also set defaultrecording=true.
dumponexit={true|false}
Specifies whether a dump file of JFR data should be generated when the JVM terminates in a controlled manner. By default, this parameter is set to false (dump file on exit is not generated). To enable it, set the parameter to true, and also set defaultrecording=true.
The dump file is written to the location defined by the dumponexitpath parameter.
dumponexitpath=path
Specifies the path and name of the dump file with JFR data that is created when the JVM exits in a controlled manner if you set the dumponexit=true parameter. Setting the path makes sense only if you also set defaultrecording=true.
If the specified path is a directory, the JVM assigns a file name that shows the creation date and time. If the specified path includes a file name and if that file already exists, the JVM creates a new file by appending the date and time stamp to the specified file name.
globalbuffersize=size
Specifies the total amount of primary memory (in bytes) used for data retention. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the size is set to 462848 bytes.
loglevel={quiet|error|warning|info|debug|trace}
Specify the amount of data written to the log file by JFR. By default, it is set to info.
maxage=time
Specifies the maximum age of disk data to keep for the default recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age is set to 15 minutes (15m).
This parameter is valid only if you set the disk=true parameter.
maxchunksize=size
Specifies the maximum size (in bytes) of the data chunks in a recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of data chunks is set to 12 MB.
maxsize=size
Specifies the maximum size (in bytes) of disk data to keep for the default recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of disk data is not limited, and this parameter is set to 0.
This parameter is valid only if you set the disk=true parameter.
repository=path
Specifies the repository (a directory) for temporary disk storage. By default, the system's temporary directory is used.
samplethreads={true|false}
Specifies whether thread sampling is enabled. Thread sampling occurs only if the sampling event is enabled along with this parameter. By default, this parameter is enabled.
settings=path
Specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/jre/lib/jfr.
stackdepth=depth
Stack depth for stack traces by JFR. By default, the depth is set to 64 method calls. The maximum is 2048, minimum is 1.
threadbuffersize=size
Specifies the per-thread local buffer size (in bytes). Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. Higher values for this parameter allow more data gathering without contention to flush it to the global storage. It can increase application footprint in a thread-rich environment. By default, the local buffer size is set to 5 KB.
You can specify values for multiple parameters by separating them with a comma. For example, to instruct JFR to write a continuous recording to disk, and set the maximum size of data chunks to 10 MB, specify the following:
-XX:FlightRecorderOptions=defaultrecording=true,disk=true,maxchunksize=10M
-XX:LargePageSizeInBytes=size
On Solaris, sets the maximum size (in bytes) for large pages used for Java heap. The size argument must be a power of 2 (2, 4, 8, 16, ...). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for large pages automatically.
The following example illustrates how to set the large page size to 4 megabytes (MB):
-XX:LargePageSizeInBytes=4m
-XX:MaxDirectMemorySize=size
Sets the maximum total size (in bytes) of the New I/O (the java.nio package) direct-buffer allocations. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations automatically.
The following examples illustrate how to set the NIO size to 1024 KB in different units:
-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576
-XX:NativeMemoryTracking=mode
Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:
off
Do not track JVM native memory usage. This is the default behavior if you do not specify the -XX:NativeMemoryTracking option.
summary
Only track memory usage by JVM subsystems, such as Java heap, class, code, and thread.
detail
In addition to tracking memory usage by JVM subsystems, track memory usage by individual CallSite, individual virtual memory region and its committed regions.
-XX:ObjectAlignmentInBytes=alignment
Sets the memory alignment of Java objects (in bytes). By default, the value is set to 8 bytes. The specified value should be a power of two, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.
The heap size limit in bytes is calculated as:
4GB * ObjectAlignmentInBytes
Note: As the alignment value increases, the unused space between objects will also increase. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes.
-XX:OnError=string
Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.
The following example shows how the -XX:OnError option can be used to run the gcore command to create the core image, and the debugger is started to attach to the process in case of an irrecoverable error (the %p designates the current process):
-XX:OnError="gcore %p;dbx - %p"
-XX:OnOutOfMemoryError=string
Sets a custom command or a series of semicolon-separated commands to run when an OutOfMemoryError exception is first thrown. If the string contains spaces, then it must be enclosed in quotation marks. For an example of a command string, see the description of the -XX:OnError option.
-XX:+PerfDataSaveToFile
If enabled, saves jstat(1) binary data when the Java application exits. This binary data is saved in a file named hsperfdata_, where is the process identifier of the Java application you ran. Use jstat to display the performance data contained in this file as follows:
jstat -class file:////hsperfdata_
jstat -gc file:////hsperfdata_
-XX:+PrintCommandLineFlags
Enables printing of ergonomically selected JVM flags that appeared on the command line. It can be useful to know the ergonomic values set by the JVM, such as the heap space size and the selected garbage collector. By default, this option is disabled and flags are not printed.
-XX:+PrintNMTStatistics
Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default, this option is disabled and native memory tracking data is not printed.
-XX:+RelaxAccessControlCheck
Decreases the amount of access control checks in the verifier. By default, this option is disabled, and it is ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode.
-XX:+ResourceManagement
Enables the use of Resource Management during the runtime of the application.
This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option as follows:
java -XX:+UnlockCommercialFeatures -XX:+ResourceManagement
-XX:ResourceManagementSampleInterval=value (milliseconds)
Sets the parameter that controls the sampling interval for Resource Management measurements, in milliseconds.
This option can be used only when Resource Management is enabled (that is, the -XX:+ResourceManagement option is specified).
-XX:SharedArchiveFile=path
Specifies the path and name of the class data sharing (CDS) archive file
-XX:SharedClassListFile=file_name
Specifies the text file that contains the names of the class files to store in the class data sharing (CDS) archive. This file contains the full name of one class file per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:
java/lang/Object
hello/Main
The class files that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths.
-XX:+ShowMessageBoxOnError
Enables displaying of a dialog box when the JVM experiences an irrecoverable error. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error. By default, this option is disabled.
-XX:StartFlightRecording=parameter=value
Starts a JFR recording for the Java application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. This option is equivalent to the JFR.start diagnostic command that starts a recording during runtime. You can set the following parameters when starting a JFR recording:
compress={true|false}
Specifies whether to compress the JFR recording log file (of type JFR) on the disk using the gzip file compression utility. This parameter is valid only if the filename parameter is specified. By default it is set to false (recording is not compressed). To enable compression, set the parameter to true.
defaultrecording={true|false}
Specifies whether the recording is a continuous background recording or if it runs for a limited time. By default, this parameter is set to false (recording runs for a limited time). To make the recording run continuously, set the parameter to true.
delay=time
Specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default, there is no delay, and this parameter is set to 0.
dumponexit={true|false}
Specifies whether a dump file of JFR data should be generated when the JVM terminates in a controlled manner. By default, this parameter is set to false (dump file on exit is not generated). To enable it, set the parameter to true.
The dump file is written to the location defined by the filename parameter.
Example:
-XX:StartFlightRecording=name=test,filename=D:\test.jfr,dumponexit=true
duration=time
Specifies the duration of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 5h means 5 hours). By default, the duration is not limited, and this parameter is set to 0.
filename=path
Specifies the path and name of the JFR recording log file.
name=identifier
Specifies the identifier for the JFR recording. By default, it is set to Recording x.
maxage=time
Specifies the maximum age of disk data to keep for the default recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age is set to 15 minutes (15m).
maxsize=size
Specifies the maximum size (in bytes) of disk data to keep for the default recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of disk data is not limited, and this parameter is set to 0.
settings=path
Specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/jre/lib/jfr.
You can specify values for multiple parameters by separating them with a comma. For example, to save the recording to test.jfr in the current working directory, and instruct JFR to compress the log file, specify the following:
-XX:StartFlightRecording=filename=test.jfr,compress=true
-XX:ThreadStackSize=size
Sets the thread stack size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value depends on the platform:
Linux/ARM (32-bit): 320 KB
Linux/i386 (32-bit): 320 KB
Linux/x64 (64-bit): 1024 KB
OS X (64-bit): 1024 KB
Oracle Solaris/i386 (32-bit): 320 KB
Oracle Solaris/x64 (64-bit): 1024 KB
The following examples show how to set the thread stack size to 1024 KB in different units:
-XX:ThreadStackSize=1m
-XX:ThreadStackSize=1024k
-XX:ThreadStackSize=1048576
This option is equivalent to -Xss.
-XX:+TraceClassLoading
Enables tracing of classes as they are loaded. By default, this option is disabled and classes are not traced.
-XX:+TraceClassLoadingPreorder
Enables tracing of all loaded classes in the order in which they are referenced. By default, this option is disabled and classes are not traced.
-XX:+TraceClassResolution
Enables tracing of constant pool resolutions. By default, this option is disabled and constant pool resolutions are not traced.
-XX:+TraceClassUnloading
Enables tracing of classes as they are unloaded. By default, this option is disabled and classes are not traced.
-XX:+TraceLoaderConstraints
Enables tracing of the loader constraints recording. By default, this option is disabled and loader constraints recording is not traced.
-XX:+UnlockCommercialFeatures
Enables the use of commercial features. Commercial features are included with Oracle Java SE Advanced or Oracle Java SE Suite packages, as defined on the Java SE Products page at http://www.oracle.com/technetwork/java/javase/terms/products/index.html
By default, this option is disabled and the JVM runs without the commercial features. Once they were enabled for a JVM process, it is not possible to disable their use for that process.
If this option is not provided, commercial features can still be unlocked in a running JVM by using the appropriate jcmd diagnostic commands.
-XX:+UseAltSigs
Enables the use of alternative signals instead of SIGUSR1 and SIGUSR2 for JVM internal signals. By default, this option is disabled and alternative signals are not used. This option is equivalent to -Xusealtsigs.
-XX:+UseAppCDS
Enables application class data sharing (AppCDS). To use AppCDS, you must also specify values for the options -XX:SharedClassListFile and -XX:SharedArchiveFile during both CDS dump time (see the option -Xshare:dump) and application run time.
This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option. This is also an experimental feature; it may change in future releases.
See "Application Class Data Sharing".
-XX:-UseBiasedLocking
Disables the use of biased locking. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, whereas applications with certain patterns of locking may see slowdowns. For more information about the biased locking technique, see the example in Java Tuning White Paper at http://www.oracle.com/technetwork/java/tuning-139912.html#section4.2.5
By default, this option is enabled.
-XX:-UseCompressedOops
Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB. When this option is enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes less than 32 GB. This option works only for 64-bit JVMs.
It is also possible to use compressed pointers when Java heap sizes are greater than 32GB. See the -XX:ObjectAlignmentInBytes option.
-XX:+UseHugeTLBFS
This option for Linux is the equivalent of specifying -XX:+UseLargePages. This option is disabled by default. This option pre-allocates all large pages up-front, when memory is reserved; consequently the JVM cannot dynamically grow or shrink large pages memory areas; see -XX:UseTransparentHugePages if you want this behavior.
For more information, see "Large Pages".
-XX:+UseLargePages
Enables the use of large page memory. By default, this option is disabled and large page memory is not used.
For more information, see "Large Pages".
-XX:+UseMembar
Enables issuing of membars on thread state transitions. This option is disabled by default on all platforms except ARM servers, where it is enabled. (It is recommended that you do not disable this option on ARM servers.)
-XX:+UsePerfData
Enables the perfdata feature. This option is enabled by default to allow JVM monitoring and performance testing. Disabling it suppresses the creation of the hsperfdata_userid directories. To disable the perfdata feature, specify -XX:-UsePerfData.
-XX:+UseTransparentHugePages
On Linux, enables the use of large pages that can dynamically grow or shrink. This option is disabled by default. You may encounter performance problems with transparent huge pages as the OS moves other pages around to create huge pages; this option is made available for experimentation.
For more information, see "Large Pages".
-XX:+AllowUserSignalHandlers
Enables installation of signal handlers by the application. By default, this option is disabled and the application is not allowed to install signal handlers.
五、Advanced Runtime 参数
These options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.
-XX:+AggressiveOpts
Enables the use of aggressive performance optimization features, which are expected to become default in upcoming releases. By default, this option is disabled and experimental performance features are not used.
-XX:AllocateInstancePrefetchLines=lines
Sets the number of lines to prefetch ahead of the instance allocation pointer. By default, the number of lines to prefetch is set to 1:
-XX:AllocateInstancePrefetchLines=1
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchDistance=size
Sets the size (in bytes) of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point.
Negative values denote that prefetch distance is chosen based on the platform. Positive values are bytes to prefetch. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is set to -1.
The following example shows how to set the prefetch distance to 1024 bytes:
-XX:AllocatePrefetchDistance=1024
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchInstr=instruction
Sets the prefetch instruction to prefetch ahead of the allocation pointer. Only the Java HotSpot Server VM supports this option. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0:
-XX:AllocatePrefetchInstr=0
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchLines=lines
Sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code. The default value is 1 if the last allocated object was an instance, and 3 if it was an array.
The following example shows how to set the number of loaded cache lines to 5:
-XX:AllocatePrefetchLines=5
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchStepSize=size
Sets the step size (in bytes) for sequential prefetch instructions. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the step size is set to 16 bytes:
-XX:AllocatePrefetchStepSize=16
Only the Java HotSpot Server VM supports this option.
-XX:AllocatePrefetchStyle=style
Sets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3:
0
Do not generate prefetch instructions.
1
Execute prefetch instructions after each allocation. This is the default parameter.
2
Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.
3
Use BIS instruction on SPARC for allocation prefetch.
Only the Java HotSpot Server VM supports this option.
-XX:+BackgroundCompilation
Enables background compilation. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (this is equivalent to specifying -Xbatch).
-XX:CICompilerCount=threads
Sets the number of compiler threads to use for compilation. By default, the number of threads is set to 2 for the server JVM, to 1 for the client JVM, and it scales to the number of cores if tiered compilation is used. The following example shows how to set the number of threads to 2:
-XX:CICompilerCount=2
-XX:CodeCacheMinimumFreeSpace=size
Sets the minimum free space (in bytes) required for compilation. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. When less than the minimum free space remains, compiling stops. By default, this option is set to 500 KB. The following example shows how to set the minimum free space to 1024 MB:
-XX:CodeCacheMinimumFreeSpace=1024m
-XX:CompileCommand=command,method[,option]
Specifies a command to perform on a method. For example, to exclude the indexOf() method of the String class from being compiled, use the following:
-XX:CompileCommand=exclude,java/lang/String.indexOf
Note that the full class name is specified, including all packages and subpackages separated by a slash (/). For easier cut and paste operations, it is also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:
-XX:CompileCommand=exclude,java.lang.String::indexOf
If the method is specified without the signature, the command will be applied to all methods with the specified name. However, you can also specify the signature of the method in the class file format. In this case, you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as command end. For example, if you want to exclude only the indexOf(String) method of the String class from being compiled, use the following:
-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I"
You can also use the asterisk (*) as a wildcard for class and method names. For example, to exclude all indexOf() methods in all classes from being compiled, use the following:
-XX:CompileCommand=exclude,*.indexOf
The commas and periods are aliases for spaces, making it easier to pass compiler commands through a shell. You can pass arguments to -XX:CompileCommand using spaces as separators by enclosing the argument in quotation marks:
-XX:CompileCommand="exclude java/lang/String indexOf"
Note that after parsing the commands passed on the command line using the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to this file or specify a different file using the -XX:CompileCommandFile option.
To add several commands, either specify the -XX:CompileCommand option multiple times, or separate each argument with the newline separator (\n). The following commands are available:
break
Set a breakpoint when debugging the JVM to stop at the beginning of compilation of the specified method.
compileonly
Exclude all methods from compilation except for the specified method. As an alternative, you can use the -XX:CompileOnly option, which allows to specify several methods.
dontinline
Prevent inlining of the specified method.
exclude
Exclude the specified method from compilation.
help
Print a help message for the -XX:CompileCommand option.
inline
Attempt to inline the specified method.
log
Exclude compilation logging (with the -XX:+LogCompilation option) for all methods except for the specified method. By default, logging is performed for all compiled methods.
option
This command can be used to pass a JIT compilation option to the specified method in place of the last argument (option). The compilation option is set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class, use the following:
-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency
You can specify multiple compilation options, separated by commas or spaces.
print
Print generated assembler code after compilation of the specified method.
quiet
Do not print the compile commands. By default, the commands that you specify with the -XX:CompileCommand option are printed; for example, if you exclude from compilation the indexOf() method of the String class, then the following will be printed to standard output:
CompilerOracle: exclude java/lang/String.indexOf
You can suppress this by specifying the -XX:CompileCommand=quiet option before other -XX:CompileCommand options.
-XX:CompileCommandFile=filename
Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.
Each line in the command file represents a command, a class name, and a method name for which the command is used. For example, this line prints assembly code for the toString() method of the String class:
print java/lang/String toString
For more information about specifying the commands for the JIT compiler to perform on methods, see the -XX:CompileCommand option.
-XX:CompileOnly=methods
Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods will be compiled. Specify each method with the full class name (including the packages and subpackages). For example, to compile only the length() method of the String class and the size() method of the List class, use the following:
-XX:CompileOnly=java/lang/String.length,java/util/List.size
Note that the full class name is specified, including all packages and subpackages separated by a slash (/). For easier cut and paste operations, it is also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:
-XX:CompileOnly=java.lang.String::length,java.util.List::size
Although wildcards are not supported, you can specify only the class or package name to compile all methods in that class or package, as well as specify just the method to compile methods with this name in any class:
-XX:CompileOnly=java/lang/String
-XX:CompileOnly=java/lang
-XX:CompileOnly=.length
-XX:CompileThreshold=invocations
Sets the number of interpreted method invocations before compilation. By default, in the server JVM, the JIT compiler performs 10,000 interpreted method invocations to gather information for efficient compilation. For the client JVM, the default setting is 1,500 invocations. This option is ignored when tiered compilation is enabled; see the option -XX:+TieredCompilation. The following example shows how to set the number of interpreted method invocations to 5,000:
-XX:CompileThreshold=5000
You can completely disable interpretation of Java methods before compilation by specifying the -Xcomp option.
-XX:+DoEscapeAnalysis
Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis. Only the Java HotSpot Server VM supports this option.
-XX:InitialCodeCacheSize=size
Sets the initial code cache size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is set to 500 KB. The initial code cache size should be not less than the system's minimal memory page size. The following example shows how to set the initial code cache size to 32 KB:
-XX:InitialCodeCacheSize=32k
-XX:+Inline
Enables method inlining. This option is enabled by default to increase performance. To disable method inlining, specify -XX:-Inline.
-XX:InlineSmallCode=size
Sets the maximum code size (in bytes) for compiled methods that should be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. Only compiled methods with the size smaller than the specified size will be inlined. By default, the maximum code size is set to 1000 bytes:
-XX:InlineSmallCode=1000
-XX:+LogCompilation
Enables logging of compilation activity to a file named hotspot.log in the current working directory. You can specify a different log file path and name using the -XX:LogFile option.
By default, this option is disabled and compilation activity is not logged. The -XX:+LogCompilation option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.
You can enable verbose diagnostic output with a message printed to the console every time a method is compiled by using the -XX:+PrintCompilation option.
-XX:MaxInlineSize=size
Sets the maximum bytecode size (in bytes) of a method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:
-XX:MaxInlineSize=35
-XX:MaxNodeLimit=nodes
Sets the maximum number of nodes to be used during single method compilation. By default, the maximum number of nodes is set to 65,000:
-XX:MaxNodeLimit=65000
-XX:MaxTrivialSize=size
Sets the maximum bytecode size (in bytes) of a trivial method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size of a trivial method is set to 6 bytes:
-XX:MaxTrivialSize=6
-XX:+OptimizeStringConcat
Enables the optimization of String concatenation operations. This option is enabled by default. To disable the optimization of String concatenation operations, specify -XX:-OptimizeStringConcat. Only the Java HotSpot Server VM supports this option.
-XX:+PrintAssembly
Enables printing of assembly code for bytecoded and native methods by using the external disassembler.so library. This enables you to see the generated code, which may help you to diagnose performance issues.
By default, this option is disabled and assembly code is not printed. The -XX:+PrintAssembly option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.
-XX:+PrintCompilation
Enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled. This enables you to see which methods actually get compiled. By default, this option is disabled and diagnostic output is not printed.
You can also log compilation activity to a file by using the -XX:+LogCompilation option.
-XX:+PrintInlining
Enables printing of inlining decisions. This enables you to see which methods are getting inlined.
By default, this option is disabled and inlining information is not printed. The -XX:+PrintInlining option has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.
-XX:ReservedCodeCacheSize=size
Sets the maximum code cache size (in bytes) for JIT-compiled code. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default maximum code cache size is 240 MB; if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB. This option has a limit of 2 GB; otherwise, an error is generated. The maximum code cache size should not be less than the initial code cache size; see the option -XX:InitialCodeCacheSize. This option is equivalent to -Xmaxjitcodesize.
-XX:RTMAbortRatio=abort_ratio
The RTM abort ratio is specified as a percentage (%) of all executed RTM transactions. If a number of aborted transactions becomes greater than this ratio, then the compiled code will be deoptimized. This ratio is used when the -XX:+UseRTMDeopt option is enabled. The default value of this option is 50. This means that the compiled code will be deoptimized if 50% of all transactions are aborted.
-XX:RTMRetryCount=number_of_retries
RTM locking code will be retried, when it is aborted or busy, the number of times specified by this option before falling back to the normal locking mechanism. The default value for this option is 5. The -XX:UseRTMLocking option must be enabled.
-XX:-TieredCompilation
Disables the use of tiered compilation. By default, this option is enabled. Only the Java HotSpot Server VM supports this option.
-XX:+UseAES
Enables hardware-based AES intrinsics for Intel, AMD, and SPARC hardware. Intel Westmere (2010 and newer), AMD Bulldozer (2011 and newer), and SPARC (T4 and newer) are the supported hardware. UseAES is used in conjunction with UseAESIntrinsics.
-XX:+UseAESIntrinsics
UseAES and UseAESIntrinsics flags are enabled by default and are supported only for Java HotSpot Server VM 32-bit and 64-bit. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:
-XX:+UseAES -XX:+UseAESIntrinsics
To support UseAES and UseAESIntrinsics flags for 32-bit and 64-bit use -server option to choose Java HotSpot Server VM. These flags are not supported on Client VM.
-XX:+UseCodeCacheFlushing
Enables flushing of the code cache before shutting down the compiler. This option is enabled by default. To disable flushing of the code cache before shutting down the compiler, specify -XX:-UseCodeCacheFlushing.
-XX:+UseCondCardMark
Enables checking of whether the card is already marked before updating the card table. This option is disabled by default and should only be used on machines with multiple sockets, where it will increase performance of Java applications that rely heavily on concurrent operations. Only the Java HotSpot Server VM supports this option.
-XX:+UseRTMDeopt
Auto-tunes RTM locking depending on the abort ratio. This ratio is specified by -XX:RTMAbortRatio option. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock will be deoptimized and recompiled with all locks as normal locks. This option is disabled by default. The -XX:+UseRTMLocking option must be enabled.
-XX:+UseRTMLocking
Generate Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. This option is disabled by default. Options related to RTM are only available for the Java HotSpot Server VM on x86 CPUs that support Transactional Synchronization Extensions (TSX).
RTM is part of Intel's TSX, which is an x86 instruction set extension and facilitates the creation of multithreaded applications. RTM introduces the new instructions XBEGIN, XABORT, XEND, and XTEST. The XBEGIN and XEND instructions enclose a set of instructions to run as a transaction. If no conflict is found when running the transaction, the memory and register modifications are committed together at the XEND instruction. The XABORT instruction can be used to explicitly abort a transaction and the XEND instruction to check if a set of instructions are being run in a transaction.
A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking the thread that did not originally request access to the transaction. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails. An RTM lock is a lock that has been delegated to the TSX's system.
RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently). RTM also improves the performance of coarse-grain locking, which typically does not perform well in multithreaded applications. (Coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible.) Also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong. This occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line. As a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache.
-XX:+UseSHA
Enables hardware-based intrinsics for SHA crypto hash functions for SPARC hardware. UseSHA is used in conjunction with the UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics options.
The UseSHA and UseSHA*Intrinsics flags are enabled by default, and are supported only for Java HotSpot Server VM 64-bit on SPARC T4 and newer.
This feature is only applicable when using the sun.security.provider.Sun provider for SHA operations.
To disable all hardware-based SHA intrinsics, specify -XX:-UseSHA. To disable only a particular SHA intrinsic, use the appropriate corresponding option. For example: -XX:-UseSHA256Intrinsics.
-XX:+UseSHA1Intrinsics
Enables intrinsics for SHA-1 crypto hash function.
-XX:+UseSHA256Intrinsics
Enables intrinsics for SHA-224 and SHA-256 crypto hash functions.
-XX:+UseSHA512Intrinsics
Enables intrinsics for SHA-384 and SHA-512 crypto hash functions.
-XX:+UseSuperWord
Enables the transformation of scalar operations into superword operations. This option is enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord. Only the Java HotSpot Server VM supports this option.
六、Advanced Serviceability 参数
These options provide the ability to gather system information and perform extensive debugging.
-XX:+ExtendedDTraceProbes
Enables additional dtrace tool probes that impact the performance. By default, this option is disabled and dtrace performs only standard probes.
-XX:+HeapDumpOnOutOfMemory
Enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default, this option is disabled and the heap is not dumped when an OutOfMemoryError exception is thrown.
-XX:HeapDumpPath=path
Sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError option is set. By default, the file is created in the current working directory, and it is named java_pidpid.hprof where pid is the identifier of the process that caused the error. The following example shows how to set the default file explicitly (%p represents the current process identificator):
-XX:HeapDumpPath=./java_pid%p.hprof
The following example shows how to set the heap dump file to /var/log/java/java_heapdump.hprof:
-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
-XX:LogFile=path
Sets the path and file name where log data is written. By default, the file is created in the current working directory, and it is named hotspot.log.
The following example shows how to set the log file to /var/log/java/hotspot.log:
-XX:LogFile=/var/log/java/hotspot.log
-XX:+PrintClassHistogram
Enables printing of a class instance histogram after a Control+C event (SIGTERM). By default, this option is disabled.
Setting this option is equivalent to running the jmap -histo command, or the jcmd pid GC.class_histogram command, where pid is the current Java process identifier.
-XX:+PrintConcurrentLocks
Enables printing of java.util.concurrent locks after a Control+C event (SIGTERM). By default, this option is disabled.
Setting this option is equivalent to running the jstack -l command or the jcmd pid Thread.print -l command, where pid is the current Java process identifier.
-XX:+UnlockDiagnosticVMOptions
Unlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options are not available.
七、Advanced Garbage Collection Options 参数
These options control how garbage collection (GC) is performed by the Java HotSpot VM.
-XX:+AggressiveHeap
Enables Java heap optimization. This sets various parameters to be optimal for long-running jobs with intensive memory allocation, based on the configuration of the computer (RAM and CPU). By default, the option is disabled and the heap is not optimized.
-XX:+AlwaysPreTouch
Enables touching of every page on the Java heap during JVM initialization. This gets all pages into the memory before entering the main() method. The option can be used in testing to simulate a long-running system with all virtual memory mapped to physical memory. By default, this option is disabled and all pages are committed as JVM heap space fills.
-XX:+CMSClassUnloadingEnabled
Enables class unloading when using the concurrent mark-sweep (CMS) garbage collector. This option is enabled by default. To disable class unloading for the CMS garbage collector, specify -XX:-CMSClassUnloadingEnabled.
-XX:CMSExpAvgFactor=percent
Sets the percentage of time (0 to 100) used to weight the current sample when computing exponential averages for the concurrent collection statistics. By default, the exponential averages factor is set to 25%. The following example shows how to set the factor to 15%:
-XX:CMSExpAvgFactor=15
-XX:CMSInitiatingOccupancyFraction=percent
Sets the percentage of the old generation occupancy (0 to 100) at which to start a CMS collection cycle. The default value is set to -1. Any negative value (including the default) implies that -XX:CMSTriggerRatio is used to define the value of the initiating occupancy fraction.
The following example shows how to set the occupancy fraction to 20%:
-XX:CMSInitiatingOccupancyFraction=20
-XX:+CMSScavengeBeforeRemark
Enables scavenging attempts before the CMS remark step. By default, this option is disabled.
-XX:CMSTriggerRatio=percent
Sets the percentage (0 to 100) of the value specified by -XX:MinHeapFreeRatio that is allocated before a CMS collection cycle commences. The default value is set to 80%.
The following example shows how to set the occupancy fraction to 75%:
-XX:CMSTriggerRatio=75
-XX:ConcGCThreads=threads
Sets the number of threads used for concurrent GC. The default value depends on the number of CPUs available to the JVM.
For example, to set the number of threads for concurrent GC to 2, specify the following option:
-XX:ConcGCThreads=2
-XX:+DisableExplicitGC
Enables the option that disables processing of calls to System.gc(). This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, the JVM still performs GC when necessary.
-XX:+ExplicitGCInvokesConcurrent
Enables invoking of concurrent GC by using the System.gc() request. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option.
-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses
Enables invoking of concurrent GC by using the System.gc() request and unloading of classes during the concurrent GC cycle. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option.
-XX:G1HeapRegionSize=size
Sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value can be between 1 MB and 32 MB. The default region size is determined ergonomically based on the heap size.
The following example shows how to set the size of the subdivisions to 16 MB:
-XX:G1HeapRegionSize=16m
-XX:+G1PrintHeapRegions
Enables the printing of information about which regions are allocated and which are reclaimed by the G1 collector. By default, this option is disabled.
-XX:G1ReservePercent=percent
Sets the percentage of the heap (0 to 50) that is reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. By default, this option is set to 10%.
The following example shows how to set the reserved heap to 20%:
-XX:G1ReservePercent=20
-XX:InitialHeapSize=size
Sets the initial size (in bytes) of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.
The following examples show how to set the size of allocated memory to 6 MB using various units:
-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6m
If you set this option to 0, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The size of the heap for the young generation can be set using the -XX:NewSize option.
-XX:InitialSurvivorRatio=ratio
Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC and/or -XX:+UseParallelOldGC options). Adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC and -XX:+UseParallelOldGC options, and survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.
The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R):
S=Y/(R+2)
The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size.
By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), the initial size of the survivor space will be 0.2 MB.
The following example shows how to set the initial survivor space ratio to 4:
-XX:InitialSurvivorRatio=4
-XX:InitiatingHeapOccupancyPercent=percent
Sets the percentage of the heap occupancy (0 to 100) at which to start a concurrent GC cycle. It is used by garbage collectors that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (for example, the G1 garbage collector).
By default, the initiating value is set to 45%. A value of 0 implies nonstop GC cycles. The following example shows how to set the initiating heap occupancy to 75%:
-XX:InitiatingHeapOccupancyPercent=75
-XX:MaxGCPauseMillis=time
Sets a target for the maximum GC pause time (in milliseconds). This is a soft goal, and the JVM will make its best effort to achieve it. By default, there is no maximum pause time value.
The following example shows how to set the maximum target pause time to 500 ms:
-XX:MaxGCPauseMillis=500
-XX:MaxHeapSize=size
Sets the maximum size (in byes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.
The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:
-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80m
On Oracle Solaris 7 and Oracle Solaris 8 SPARC platforms, the upper limit for this value is approximately 4,000 MB minus overhead amounts. On Oracle Solaris 2.6 and x86 platforms, the upper limit is approximately 2,000 MB minus overhead amounts. On Linux platforms, the upper limit is approximately 2,000 MB minus overhead amounts.
The -XX:MaxHeapSize option is equivalent to -Xmx.
-XX:MaxHeapFreeRatio=percent
Sets the maximum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space expands above this value, then the heap will be shrunk. By default, this value is set to 70%.
The following example shows how to set the maximum free heap ratio to 75%:
-XX:MaxHeapFreeRatio=75
-XX:MaxMetaspaceSize=size
Sets the maximum amount of native memory that can be allocated for class metadata. By default, the size is not limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system.
The following example shows how to set the maximum class metadata size to 256 MB:
-XX:MaxMetaspaceSize=256m
-XX:MaxNewSize=size
Sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically.
-XX:MaxTenuringThreshold=threshold
Sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector, and 6 for the CMS collector.
The following example shows how to set the maximum tenuring threshold to 10:
-XX:MaxTenuringThreshold=10
-XX:MetaspaceSize=size
Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform.
-XX:MinHeapFreeRatio=percent
Sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap will be expanded. By default, this value is set to 40%.
The following example shows how to set the minimum free heap ratio to 25%:
-XX:MinHeapFreeRatio=25
-XX:NewRatio=ratio
Sets the ratio between young and old generation sizes. By default, this option is set to 2. The following example shows how to set the young/old ratio to 1:
-XX:NewRatio=1
-XX:NewSize=size
Sets the initial size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.
The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs will be performed. If the size is too high, then only full GCs will be performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size.
The following examples show how to set the initial size of young generation to 256 MB using various units:
-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456
The -XX:NewSize option is equivalent to -Xmn.
-XX:ParallelGCThreads=threads
Sets the number of threads used for parallel garbage collection in the young and old generations. The default value depends on the number of CPUs available to the JVM.
For example, to set the number of threads for parallel GC to 2, specify the following option:
-XX:ParallelGCThreads=2
-XX:+ParallelRefProcEnabled
Enables parallel reference processing. By default, this option is disabled.
-XX:+PrintAdaptiveSizePolicy
Enables printing of information about adaptive generation sizing. By default, this option is disabled.
-XX:+PrintGC
Enables printing of messages at every GC. By default, this option is disabled.
-XX:+PrintGCApplicationConcurrentTime
Enables printing of how much time elapsed since the last pause (for example, a GC pause). By default, this option is disabled.
-XX:+PrintGCApplicationStoppedTime
Enables printing of how much time the pause (for example, a GC pause) lasted. By default, this option is disabled.
-XX:+PrintGCDateStamps
Enables printing of a date stamp at every GC. By default, this option is disabled.
-XX:+PrintGCDetails
Enables printing of detailed messages at every GC. By default, this option is disabled.
-XX:+PrintGCTaskTimeStamps
Enables printing of time stamps for every individual GC worker thread task. By default, this option is disabled.
-XX:+PrintGCTimeStamps
Enables printing of time stamps at every GC. By default, this option is disabled.
-XX:+PrintStringDeduplicationStatistics
Prints detailed deduplication statistics. By default, this option is disabled. See the -XX:+UseStringDeduplication option.
-XX:+PrintTenuringDistribution
Enables printing of tenuring age information. The following is an example of the output:
Desired survivor size 48286924 bytes, new threshold 10 (max 10)
- age 1: 28992024 bytes, 28992024 total
- age 2: 1366864 bytes, 30358888 total
- age 3: 1425912 bytes, 31784800 total
...
Age 1 objects are the youngest survivors (they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space). Age 2 objects have survived two scavenges (during the second scavenge they were copied from one survivor space to the next). And so on.
In the preceding example, 28 992 024 bytes survived one scavenge and were copied from eden to survivor space, 1 366 864 bytes are occupied by age 2 objects, etc. The third value in each row is the cumulative size of objects of age n or less.
By default, this option is disabled.
-XX:+ScavengeBeforeFullGC
Enables GC of the young generation before each full GC. This option is enabled by default. Oracle recommends that you do not disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. To disable GC of the young generation before each full GC, specify -XX:-ScavengeBeforeFullGC.
-XX:SoftRefLRUPolicyMSPerMB=time
Sets the amount of time (in milliseconds) a softly reachable object is kept active on the heap after the last time it was referenced. The default value is one second of lifetime per free megabyte in the heap. The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM). This difference means that the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references. In the latter case, the value of the -Xmx option has a significant effect on how quickly soft references are garbage collected.
The following example shows how to set the value to 2.5 seconds:
-XX:SoftRefLRUPolicyMSPerMB=2500
-XX:StringDeduplicationAgeThreshold=threshold
String objects reaching the specified age are considered candidates for deduplication. An object's age is a measure of how many times it has survived garbage collection. This is sometimes referred to as tenuring; see the -XX:+PrintTenuringDistribution option. Note that String objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. The default value for this option is 3. See the -XX:+UseStringDeduplication option.
-XX:SurvivorRatio=ratio
Sets the ratio between eden space size and survivor space size. By default, this option is set to 8. The following example shows how to set the eden/survivor space ratio to 4:
-XX:SurvivorRatio=4
-XX:TargetSurvivorRatio=percent
Sets the desired percentage of survivor space (0 to 100) used after young garbage collection. By default, this option is set to 50%.
The following example shows how to set the target survivor space ratio to 30%:
-XX:TargetSurvivorRatio=30
-XX:TLABSize=size
Sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. If this option is set to 0, then the JVM chooses the initial size automatically.
The following example shows how to set the initial TLAB size to 512 KB:
-XX:TLABSize=512k
-XX:+UseAdaptiveSizePolicy
Enables the use of adaptive generation sizing. This option is enabled by default. To disable adaptive generation sizing, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly (see the -XX:SurvivorRatio option).
-XX:+UseCMSInitiatingOccupancyOnly
Enables the use of the occupancy value as the only criterion for initiating the CMS collector. By default, this option is disabled and other criteria may be used.
-XX:+UseConcMarkSweepGC
Enables the use of the CMS garbage collector for the old generation. Oracle recommends that you use the CMS garbage collector when application latency requirements cannot be met by the throughput (-XX:+UseParallelGC) garbage collector. The G1 garbage collector (-XX:+UseG1GC) is another alternative.
By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. When this option is enabled, the -XX:+UseParNewGC option is automatically set and you should not disable it, because the following combination of options has been deprecated in JDK 8: -XX:+UseConcMarkSweepGC -XX:-UseParNewGC.
-XX:+UseG1GC
Enables the use of the garbage-first (G1) garbage collector. It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. It meets GC pause time goals with high probability, while maintaining good throughput. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (stable and predictable pause time below 0.5 seconds).
By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM.
-XX:+UseGCOverheadLimit
Enables the use of a policy that limits the proportion of time spent by the JVM on GC before an OutOfMemoryError exception is thrown. This option is enabled, by default and the parallel GC will throw an OutOfMemoryError if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress. To disable this option, specify -XX:-UseGCOverheadLimit.
-XX:+UseNUMA
Enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application's use of lower latency memory. By default, this option is disabled and no optimization for NUMA is made. The option is only available when the parallel garbage collector is used (-XX:+UseParallelGC).
-XX:+UseParallelGC
Enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors.
By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. If it is enabled, then the -XX:+UseParallelOldGC option is automatically enabled, unless you explicitly disable it.
-XX:+UseParallelOldGC
Enables the use of the parallel garbage collector for full GCs. By default, this option is disabled. Enabling it automatically enables the -XX:+UseParallelGC option.
-XX:+UseParNewGC
Enables the use of parallel threads for collection in the young generation. By default, this option is disabled. It is automatically enabled when you set the -XX:+UseConcMarkSweepGC option. Using the -XX:+UseParNewGC option without the -XX:+UseConcMarkSweepGC option was deprecated in JDK 8.
-XX:+UseSerialGC
Enables the use of the serial garbage collector. This is generally the best choice for small and simple applications that do not require any special functionality from garbage collection. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM.
-XX:+UseSHM
On Linux, enables the JVM to use shared memory to setup large pages.
For more information, see "Large Pages".
-XX:+UseStringDeduplication
Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector. See the -XX:+UseG1GC option.
String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array.
-XX:+UseTLAB
Enables the use of thread-local allocation blocks (TLABs) in the young generation space. This option is enabled by default. To disable the use of TLABs, specify -XX:-UseTLAB.
关于JAVA命令的一些过期的参数：
Deprecated and Removed Options
These options were included in the previous release, but have since been considered unnecessary.
-Xincgc
Enables incremental garbage collection. This option was deprecated in JDK 8 with no replacement.
-Xrunlibname
Loads the specified debugging/profiling library. This option was superseded by the -agentlib option.
-XX:CMSIncrementalDutyCycle=percent
Sets the percentage of time (0 to 100) between minor collections that the concurrent collector is allowed to run. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.
-XX:CMSIncrementalDutyCycleMin=percent
Sets the percentage of time (0 to 100) between minor collections that is the lower bound for the duty cycle when -XX:+CMSIncrementalPacing is enabled. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.
-XX:+CMSIncrementalMode
Enables the incremental mode for the CMS collector. This option was deprecated in JDK 8 with no replacement, along with other options that start with CMSIncremental.
-XX:CMSIncrementalOffset=percent
Sets the percentage of time (0 to 100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.
-XX:+CMSIncrementalPacing
Enables automatic adjustment of the incremental mode duty cycle based on statistics collected while the JVM is running. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.
-XX:CMSIncrementalSafetyFactor=percent
Sets the percentage of time (0 to 100) used to add conservatism when computing the duty cycle. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.
-XX:CMSInitiatingPermOccupancyFraction=percent
Sets the percentage of the permanent generation occupancy (0 to 100) at which to start a GC. This option was deprecated in JDK 8 with no replacement.
-XX:MaxPermSize=size
Sets the maximum permanent generation space size (in bytes). This option was deprecated in JDK 8, and superseded by the -XX:MaxMetaspaceSize option.
-XX:PermSize=size
Sets the space (in bytes) allocated to the permanent generation that triggers a garbage collection if it is exceeded. This option was deprecated un JDK 8, and superseded by the -XX:MetaspaceSize option.
-XX:+UseSplitVerifier
Enables splitting of the verification process. By default, this option was enabled in the previous releases, and verification was split into two phases: type referencing (performed by the compiler) and type checking (performed by the JVM runtime). This option was deprecated in JDK 8, and verification is now split by default without a way to disable it.
-XX:+UseStringCache
Enables caching of commonly allocated strings. This option was removed from JDK 8 with no replacement.关于JAVA命令的一些简单的例子：
Performance Tuning Examples
The following examples show how to use experimental tuning flags to either optimize throughput or to provide lower response time.
Example 1 - Tuning for Higher Throughput
java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g
Example 2 - Tuning for Lower Response Time
java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStampJAVA大页面内存
Large Page Memory
The default memory page size in most operating systems is 4 kilobytes (kb). For a 32-bit operating system the maximum amount of memory is 4 GB, which equates to 1,048,576 ((1024*1024*1024*4)/4096) memory pages. A 64-bit operating system can address 18 Exabytes of memory in theory which equates to a huge number of memory pages. The overhead of managing such a large number of memory pages is significant, regardless of the operating system. The largest heap size used for tests covered in this book was 20 GB, which equates to 5,242,880 memory pages, a five-fold increase over 4 GB.
Large memory pages are pages of memory which are significantly larger than 4 kb, usually 2 Mb. In some instances it's configurable, from 2MB to 256MB. For the systems used in the tests for this book, the page size is 2MB. With 2MB pages, the number of pages for a 20GB heap memory drops from 5,242,880 to 10,240! A 99.8% reduction in the number of memory pages means significantly less management overhead for the underlying operating system.
Large memory pages are locked in memory, and cannot be swapped to disk like regular memory pages which has both advantages and disadvantages. The advantage is that if the heap is using large page memory it can not be paged or swapped to disk so it's always readily available. For Linux the disadvantage is that for applications to use it they have to attach to it using the correct flag for the shmget() system call, also they need to have the proper security permissions for the memlock() system call. For any application that does not have the ability to use large page memory, the server will look and behave as if the large page memory does not exist, which could be a major problem. Care must be taken when configuring large page memory, depending on what else is running on your server besides the JVM.
To enable large page memory, add the following option to the command-line used to start the platform:
-XX:+UseLargePages
This option applies to OpenJDK, and the Oracle proprietary HotSpot-based JVM but there are similar options for IBM's and Oracle's JRockit JVMs. Refer to their documentation for further details. It's also necessary to change the following parameters:
kernel.shmmax = n
Where n is equal to the number of bytes of the maximum shared memory segment allowed on the system. You should set it at least to the size of the largest heap size you want to use for the JVM, or alternatively you can set it to the total amount of memory in the system.
vm.nr_hugepages = n
Where n is equal to the number of large pages. You will need to look up the large page size in /proc/meminfo.
vm.huge_tlb_shm_group = gid
Where gid is a shared group id for the users you want to have access to the large pages.
The next step is adding the following in /etc/security/limits.conf:
soft memlock n
hard memlock n
Where is the runtime user of the JVM and n is the number of pages from vm.nr_hugepages * the page size in KB from /proc/meminfo.
Instead of setting n to a specific value, this can be set to unlimited, which reduces maintenance.
Enter the command sysctl -p and these settings will be persistent. To confirm that this had taken effect, check that in the statistics available via /proc/meminfo, HugePages_Total is greater than 0. If HugePages_Total is either zero (0) or less than the value you configured, there are one of two things that could be wrong:
the specified number of memory pages was greater than was available;
there were not enough contiguous memory pages available.
When large page memory is allocated by the operating system, it must be in contiguous space. While the operating system is running, memory pages get fragmented. If the request failed because of this it may be necessary to reboot, so that the allocation of memory pages occurs before applications are started.
With the release of Red Hat Enterprise Linux 6, a new operating system capability called transparent huge pages (huge pages are the same as large pages) is available. This feature gives the operating system the ability to combine standard memory pages and make them large pages dynamically and without configuration. It enables the option of using large page memory for any application, even if it's not directly supported. Consider using this option since it reduces the configuration effort at a relatively small performance cost. Consult the Red Hat Enterprise Linux 6 documentation for specific configuration instructions.
The graph below shows the performance difference of the standard workload used in the 32-bit vs. 64-bit JVM comparison section.
JVM Throughput - 32-bit versus 64-bit
Figure 9.3. JVM Throughput - 32-bit versus 64-bit
The peak line that was created with the 16GB heap is included to illustrate the difference. All heap sizes, even down to 4GB were substantially faster than the best without large page memory. The peak was actually the 18GB heap size run, which had 6.58% higher throughput than the 4GB result. This result was also 17.48% more throughput than the 16GB test run without large page memory. In these results it's evident that using large page memory is worthwhile, even for rather small heap sizes.
JVM Throughput - comparison of with and without large pages enabled
Figure 9.4. JVM Throughput - comparison of with and without large pages enabled
This graph compares two runs, with and without large page memory, using the EJB 3 OLTP application that has been referenced throughout this book. The results are similar to what we saw in the Java only workload. When executing this test, using the same 12GB heap size but without large page memory, the result was just under 3,900 transactions per second (3,899.02 to be exact). With large page memory enabled the result was over 4,100 transactions per second (4.143.25 to be exact). This represents a 6.26% throughput improvement which is not as large as the Java-only workload but this is to be expected, as this is a more complex workload, with a fairly large percentage of the time not even in the JVM itself. It's still significant however, because it equates to 244+ transactions per second more or 14,000+ extra transactions per minute. In real terms this means more work (processing) can be done in the same amount of time.
HugePage，就是指的大页内存管理方式。与传统的4kb的普通页管理方式相比，HugePage为管理大内存(8GB以上)更为高效。本文描述了什么是HugePage，以及HugePage的一些特性。
1、Hugepage的引入
操作系统对于数据的存取直接从物理内存要比从磁盘读写数据要快的多，但是物理内存是有限的，这样就引出了物理内存与虚拟内存的概念。虚拟内存就是为了满足物理内存的不足而提出的策略，它是利用磁盘空间虚拟出的一块逻辑内存，这部分磁盘空间Windows下称之为虚拟内存，Linux下被称为交换空间(Swap Space)。
对于这个大内存的管理(物理内存+虚拟内存)，大多数操作系统采用了分段或分页的方式进行管理。分段是粗粒度的管理方式，而分页则是细粒度管理方式，分页方式可以避免内存空间的浪费。相应地，也就存在内存的物理地址与虚拟地址的概念。通过前面这两种方式，CPU必须把虚拟地址转换程物理内存地址才能真正访问内存。为了提高这个转换效率，CPU会缓存最近的虚拟内存地址和物理内存地址的映射关系，并保存在一个由CPU维护的映射表中。为了尽量提高内存的访问速度，需要在映射表中保存尽量多的映射关系。
linux的内存管理采取的是分页存取机制，为了保证物理内存能得到充分的利用，内核会按照LRU算法在适当的时候将物理内存中不经常使用的内存页自动交换到虚拟内存中，而将经常使用的信息保留到物理内存。通常情况下，Linux默认情况下每页是4K，这就意味着如果物理内存很大，则映射表的条目将会非常多，会影响CPU的检索效率。因为内存大小是固定的，为了减少映射表的条目，可采取的办法只有增加页的尺寸。因此Hugepage便因此而来。也就是打破传统的小页面的内存管理方式，使用大页面2m,4m,16m等等。如此一来映射条目则明显减少。如果系统有大量的物理内存（大于8G），则物理32位的操作系统还是64位的，都应该使用Hugepage。
2、Hugepage的相关术语
Page Table:
A page table is the data structure of a virtual memory system in an operating system to store the mapping between virtual addresses and physical addresses. This means that on a virtual memory system, the memory is accessed by first accessing a page table and then accessing the actual memory location implicitly.
如前所述，page table也就是一种用于内存管理的实现方式，用于物理地址到虚拟之间的映射。因此对于内存的访问，先是访问Page Table，然后根据Page Table 中的映射关系，隐式的转移到物理地址来存取数据。
TLB:
A Translation Lookaside Buffer (TLB) is a buffer (or cache) in a CPU that contains parts of the page table. This is a fixed size buffer being used to do virtual address translation faster.
CPU中的一块固定大小的cache，包含了部分page table的映射关系，用于快速实现虚拟地址到物理地址的转换。
hugetlb:
This is an entry in the TLB that points to a HugePage (a large/big page larger than regular 4K and predefined in size). HugePages are implemented via hugetlb entries, i.e. we can say that a HugePage is handled by a "hugetlb page entry". The 'hugetlb" term is also (and mostly) used synonymously with a HugePage (See Note 261889.1). In this document the term "HugePage" is going to be used but keep in mind that mostly "hugetlb" refers to the same concept.
hugetlb 是TLB中指向HugePage的一个entry(通常大于4k或预定义页面大小)。 HugePage 通过hugetlb entries来实现，也可以理解为HugePage 是hugetlb page entry的一个句柄。
hugetlbfs:
This is a new in-memory filesystem like tmpfs and is presented by 2.6 kernel. Pages allocated on hugetlbfs type filesystem are allocated in HugePages.
一个类似于tmpfs的新的in-memory filesystem，在2.6内核被提出。
3、常见的错误概念
WRONG: HugePages is a method to be able to use large SGA on 32-bit VLM systems
RIGHT: HugePages is a method to have larger pages where it is useful for working with very large memory. It is both useful in 32- and 64-bit configurations
WRONG: HugePages cannot be used without USE_INDIRECT_DATA_BUFFERS
RIGHT: HugePages can be used without indirect buffers. 64-bit systems does not need to use indirect buffers to have a large buffer cache for the RDBMS instance and HugePages can be used there too.
WRONG: hugetlbfs means hugetlb
RIGHT: hugetlbfs is a filesystem type **BUT** hugetlb is the mechanism employed in the back where hugetlb can be employed WITHOUT hugetlbfs
WRONG: hugetlbfs means hugepages
RIGHT: hugetlbfs is a filesystem type **BUT** HugePages is the mechanism employed in the back (synonymously with hugetlb) where HugePages can be employed WITHOUT hugetlbfs.
4、Regular Pages 与 HugePages
a、Regular Pages
在下图中有两个不同的进程，两个进程对于内存的访问是首先访问本地的page table，而本地的page table又参照了system-wide table的page(也就是前面描述的TLB)，最终system-wide table中的entry指向了实际的物理地址。图中物理地址page size大小4kb。也可以看到进程1和进程2在system-wide table中都指向了page2，也就是同一个物理地址。Oracle sga中共享内存的使用会出现上述情形。
b、Huge Pages
在下图中，本地的page table 与system page table中都包含了huge page属性。因此page table中的任意一个page可能使用了常规的page，
也有可能使用了huge page。同样进程1和进程2都共享了其中的Hpage2。图中的物理内存常规的page size是4kb，huge page size 是4mb。
--Author : Robinson
--Blog : http://blog.csdn.net/robinson_0612
5、huge page 的大小
huge page 的大小取决于所使用的操作系统的内核版本以及不同的硬件平台
可以使用$grep Hugepagesize /proc/meminfo来查看huge page 的大小
下面是不同平台常用的huge page 的大小。
HW Platform Source Code Tree Kernel 2.4 Kernel 2.6
----------------- --------------------- ------------ -------------
Linux x86 (IA32) i386 4 MB 4 MB *
Linux x86-64 (AMD64, EM64T) x86_64 2 MB 2 MB
Linux Itanium (IA64) ia64 256 MB 256 MB
IBM Power Based Linux (PPC64) ppc64/powerpc N/A ** 16 MB
IBM zSeries Based Linux s390 N/A 1 MB
IBM S/390 Based Linux s390 N/A N/A
6、使用huge page的优点
对于较大的系统内存以及sga，使用hugepage可以极大程度的提高Oracle数据库性能。
a、Not swappable
HugePages are not swappable. Therefore there is no page-in/page-out mechanism overhead.HugePages are universally regarded as pinned.
无需交换。也就是不存在页面由于内存空间不足而存在换入换出的问题
b、Relief of TLB pressure
Hugepge uses fewer pages to cover the physical address space, so the size of “book keeping” (mapping from the virtual to the physical address) decreases, so it requiring fewer entries in the TLB
TLB entries will cover a larger part of the address space when use HugePages, there will be fewer TLB misses before the entire or most of the SGA is mapped in the SGA
Fewer TLB entries for the SGA also means more for other parts of the address space
减轻TLB的压力，也就是降低了cpu cache可缓存的地址映射压力。由于使用了huge page，相同的内存大小情况下，管理的虚拟地址数量变少。
TLB entry可以包含更多的地址空间，cpu的寻址能力相应的得到了增强。
c、Decreased page table overhead
Each page table entry can be as large as 64 bytes and if we are trying to handle 50GB of RAM, the pagetable will be approximately 800MB in size which is practically will not fit in 880MB size lowmem (in 2.4 kernels - the page table is not necessarily in lowmem in 2.6 kernels) considering the other uses of lowmem. When 95% of memory is accessed via 256MB hugepages, this can work with a page table of approximately 40MB in total. See also Document 361468.1.
降低page table负载，对于普通的page，每个entry需要64bytes进行管理，对于50gb的内存，管理这些entry，需要800mb的大小
(50*1024*1024)kb/4kb*64bytes/1024/1024=800mb。
d、Eliminated page table lookup overhead
Since the pages are not subject to replacement, page table lookups are not required.( 消除page table查找负载)
e、Faster overall memory performance
On virtual memory systems each memory operation is actually two abstract memory operations. Since there are fewer pages to work on, the possible bottleneck on page table access is clearly avoided.(提高内存的整体性能)
7、未正确配值huge page的风险
基于大内存(>8GB)的管理，如果配值或正确配值huge page，可能存在下列不确定的隐性问题
HugePages not used (HugePages_Total = HugePages_Free) at all wasting the amount configured for
Poor database performance
System running out of memory or excessive swapping
Some or any database instance cannot be started
Crucial system services failing (e.g.: CRS)
8、基于2.6内核的配值步骤
The kernel parameter used for HugePages is vm.nr_hugepages which is based on the number of the pages. SLES9, RHEL4 and Asianux 2.0 are examples of distributions with the 2.6 kernel. For the configuration, follow steps below:
a. Start instance(s)
b. Calculate nr_hugepages using script from Document 401749.1
c. Set kernel parameter:
# sysctl -w vm.nr_hugepages=
and make sure that the parameter is persistent to reboots. e.g. On SLES9:
# chkconfig boot.sysctl on
d. Check available hugepages:
$ grep Huge /proc/meminfo
e. Restart instances
f. Check available hugepages:
$ grep Huge /proc/meminfo
9、注意事项
a、HugePage使用的是共享内存，在操作系统启动期间被动态分配并被保留，因为他们不会被置换。
b、由于不会被置换的特点，在使用hugepage的内存不能被其他的进程使用。所以要合理设置该值，避免造成内存浪费。
c、对于只使用Oracle的服务器来说，把Hugepage设置成SGA(所有instance SGA之和)大小即可。
d、如果增加HugePage或添加物理内存或者是当前服务器增加了新的instance以及SGA发生变化，应该重新设置所需的HugePage。
e、reference: HugePages on Linux: What It Is... and What It Is Not... [ID 361323.1] To Bottom
参考链接：http://www.admin10000.com/document/5125.html
http://www.cnblogs.com/princessd8251/articles/4025140.html
https://access.redhat.com/documentation/en-US/JBoss_Enterprise_Application_Platform/5/html/Performance_Tuning_Guide/sect-Performance_Tuning_Guide-Java_Virtual_Machine_Tuning-Large_Page_Memory.html
http://www.2cto.com/os/201304/201735.html