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概要

java [选择] 类名[args]

java [选择] -jar 文件名[args]

javaw [选择] 类名[args]

javaw [选择] -jar 文件名[args]

选项：命令行选项用空格分隔。请参阅选项。班级名称要启动的类的名称。

文件名：要调用的Java归档（JAR）文件的名称。仅与-jar选项一起使用。

args：传递给main（）方法的参数以空格分隔。

描述

java命令启动Java应用程序。它通过启动Java运行时环境（JRE），加载指定的类并调用该类的main（）方法来实现。该方法必须声明为public和static，它不能返回任何值，并且必须接受String数组作为参数。方法声明具有以下形式：

public static void main(String[] args)

通过加载具有main（）方法或扩展javafx.application.Application的类，可以使用java命令来启动JavaFX应用程序。在后一种情况下，启动器构造Application类的实例，调用其init（）方法，然后调用start（javafx.stage.Stage）方法。

默认情况下，不是java命令选项的第一个参数是要调用的类的全限定名。如果指定了-jar选项，则其参数为包含应用程序的类和资源文件的JAR文件的名称。启动类必须由其源代码中的Main-Class清单标头指示。

JRE在三组位置中搜索启动类（以及应用程序使用的其他类）：引导类路径，已安装的扩展名和用户的类路径。

类文件名或JAR文件名之后的参数传递给main（）方法。

javaw命令与java相同，除了javaw没有关联的控制台窗口。当您不希望显示命令提示符窗口时，请使用javaw。但是，如果启动失败，javaw启动器将显示一个对话框，其中包含错误信息。

选项

选件 java命令支持广泛的选项，这些选项可以分为以下几类:

标准选项
非标准选项
高级运行时选项
高级JIT编译器选项
先进的可维修性选项
高级垃圾收集选项

Java虚拟机（JVM）的所有实现都保证支持标准选项。它们用于常见的操作，例如检查JRE的版本、设置类路径、启用详细输出等等。

非标准选项是特定于Java HotSpot虚拟机的通用选项，因此它们不能保证得到所有JVM实现的支持，并且可能会发生更改。这些选项以-X开头。

不建议临时使用高级选项。这些是开发人员选项，用于调整Java热点虚拟机操作的特定区域，这些区域通常具有特定的系统要求，并且可能需要对系统配置参数进行特权访问。它们也不能保证得到所有JVM实现的支持，并且可能会发生更改。高级选项从-XX开始。

为了跟踪在最新版本中已弃用或删除的选项，文档末尾有一个名为deprecated and removed options的部分。

布尔选项用于启用默认禁用的功能或禁用默认启用的功能。这样的选项不需要参数。布尔-XX选项使用加号（-XX:+OptionName）启用，使用减号（-XX:-OptionName）禁用。

对于需要参数的选项，参数可以用空格、冒号（：）或等号（=）与选项名分隔，或者参数可以直接跟在选项后面（每个选项的确切语法不同）。如果希望指定字节大小，可以不使用后缀，也可以使用后缀k或k表示千字节（KB），m或m表示兆字节（MB），g或g表示千兆字节（GB）。例如，要将大小设置为8GB，可以指定8g、8192m、8388608k或8589934592作为参数。如果希望指定百分比，请使用0到1之间的数字（例如，指定0.25作为25%）。

标准选项

这些是所有JVM实现都支持的最常用选项。

-agentlib:libname[=options]

加载指定的本机代理库。在库名称之后，可以使用特定于库的选项的逗号分隔列表。

如果指定了-agentlib:foo选项，那么JVM将尝试在PATH系统变量指定的位置加载名为foo.dll的库。

下面的示例演示如何加载堆分析工具（HPROF）库，并在堆栈深度为3的情况下每隔20毫秒获取一次CPU信息示例：

-agentlib:hprof=cpu=samples,interval=20,depth=3

面的示例演示如何加载Java调试连线协议（JDWP）库并侦听端口8000上的套接字连接，在主类加载之前挂起JVM：

-agentlib:jdwp=transport=dt_socket,server=y,address=8000

有关本机代理库的详细信息，请参阅以下内容：

网站上的java.lang.instrument包说明
JVM工具界面指南中的代理命令行选项，网址

-agentpath:pathname[=options]

加载由绝对路径名指定的本机代理库。此选项相当于-agentlib，但使用库的完整路径和文件名。

-client

选择Java HotSpot 客户端虚拟机。64位版本的Java SE开发工具包（JDK）目前忽略了这个选项，而是使用服务器JVM。

有关默认的JVM选择，请参阅

-Dproperty=value

设置系统属性值。属性变量是一个字符串，没有表示属性名称的空格。值变量是表示属性值的字符串。如果值是带空格的字符串，则将其括在引号中（例如-Dfoo=“foo bar”）。

-disableassertions[:[packagename]...|:classname]

-da[:[packagename]...|:classname]

禁用断言。默认情况下，在所有包和类中都禁用断言。

不带参数，-disableassertions（-da）禁用所有包和类中的断言。packagename参数以…结尾时，开关将禁用指定包和任何子包中的断言。如果参数只是…，则开关将禁用当前工作目录中未命名包中的断言。使用classname参数，开关将禁用指定类中的断言。

-disableassertions（-da）选项适用于所有类装入器和系统类（它们没有类装入器）。这个规则有一个例外：如果选项没有参数，那么它不适用于系统类。这使得在除了系统类之外的所有类中禁用断言变得很容易。使用-disable system assertions选项可以禁用所有系统类中的断言。

要显式启用特定包或类中的断言，请使用-enablessertions（-ea）选项。两个选项都可以同时使用。例如，要在包com.wombat.ruitbat（和任何子包）中启用断言但在类com.wombat.ruitbat.Brickbat中禁用断言的情况下运行MyClass应用程序，请使用以下命令：

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

-disablesystemassertions

-dsa

在所有系统类中禁用断言。

-enableassertions[:[packagename]...|:classname]

-ea[:[packagename]...|:classname]

启用断言。默认情况下，在所有包和类中都禁用断言。

不带参数，-enablessertions（-ea）在所有包和类中启用断言。packagename参数以…结尾时，开关启用指定包和任何子包中的断言。如果参数是简单的…，那么开关将启用当前工作目录中未命名包中的断言。使用classname参数，开关启用指定类中的断言。

-enablessertions（-ea）选项适用于所有类装入器和系统类（它们没有类装入器）。这个规则有一个例外：如果选项没有参数，那么它不适用于系统类。这使得在除系统类之外的所有类中启用断言变得容易。-enablesystemassertions选项提供了一个单独的开关，用于在所有系统类中启用断言。

要显式禁用特定包或类中的断言，请使用-disable assertions（-da）选项。如果一个命令包含这些开关的多个实例，则在加载任何类之前，将按顺序对它们进行处理。例如，要在包com.wombat.ruitbat（和任何子包）中启用断言但在类com.wombat.ruitbat.Brickbat中禁用断言的情况下运行MyClass应用程序，请使用以下命令：

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

-enablesystemassertions

-esa

在所有系统类中启用断言。

-help

-?

在不实际运行JVM的情况下显示java命令的使用信息。

-jar filename

执行封装在JAR文件中的程序。file name参数是具有清单的JAR文件的名称，清单中包含Main Class:classname形式的行，该行使用作为应用程序起点的public static void Main（String[]args）方法定义类。

使用-jar选项时，指定的jar文件是所有用户类的源，其他类路径设置将被忽略。

有关JAR文件的更多信息，请参阅以下参考资料：

jar(1)
Java Archive（JAR）文件指南
Lesson: 在JAR文件中打包程序

-javaagent:jarpath[=options]

加载指定的Java编程语言代理。有关检测Java应用程序的更多信息，请参阅Java API文档中的Java.lang.instrument包说明，网址

-jre-restrict-search

在版本搜索中包含用户专用JRE。

-no-jre-restrict-search

从版本搜索中排除用户专用JRE。

-server

选择Java HotSpot服务器虚拟机。64位版本的JDK只支持服务器VM，因此在这种情况下，该选项是隐式的。

有关默认的JVM选择，请参阅

-showversion

显示版本信息并继续执行应用程序。此选项与-version选项等效，只是后者指示JVM在显示版本信息后退出。

-splash:imgname

显示由imgname指定的图像的启动屏幕。例如，要在启动应用程序时显示images目录中的splash.gif文件，请使用以下选项：

-splash:images/splash.gif

-verbose:class

显示有关每个加载类的信息。

-verbose:gc

显示有关每个垃圾收集（GC）事件的信息。

-verbose:jni

显示有关使用本机方法和其他Java本机接口（JNI）活动的信息。

-version

显示版本信息，然后退出。此选项与-showversion选项等效，只是后者不指示JVM在显示版本信息后退出。

-version:release

指定用于运行应用程序的版本。如果调用的java命令的版本不符合此规范，并且在系统上找到了适当的实现，则将使用适当的实现。

release参数指定精确的版本字符串，或由空格分隔的版本字符串和范围列表。版本字符串是开发人员对版本号的指定，格式如下：1.x.0_（其中x是主要版本号，u是更新版本号）。版本范围由版本字符串后跟加号（＋）来指定此版本或更高版本，或由版本字符串的一部分后跟星号（*）来指定具有匹配前缀的任何版本字符串组成。版本字符串和范围可以使用空格来表示逻辑或组合，或者使用与号（&）来表示两个版本字符串/范围的逻辑和组合。例如，如果运行类或JAR文件需要JRE 6u13（1.6.0_13）或任何从6u10（1.6.0_10）开始的JRE 6，请指定以下内容：

-version:"1.6.0_13 1.6* & 1.6.0_10+"

只有当release参数中有空格时，引号才是必需的。

对于JAR文件，首选项是在JAR文件清单中而不是在命令行中指定版本要求。

非标准选项

这些选项是特定于Java热点虚拟机的通用选项。

-X

显示所有可用-X选项的帮助。

-Xbatch

禁用后台编译。默认情况下，JVM将该方法编译为后台任务，在解释器模式下运行该方法，直到后台编译完成。-Xbatch标志禁用后台编译，以便所有方法的编译作为前台任务进行，直到完成为止。

此选项相当于-XX:-BackgroundCompilation。

-Xbootclasspath:path

指定用分号（；）分隔的目录、JAR文件和ZIP存档的列表，以搜索引导类文件。这些文件用于代替JDK中包含的引导类文件。

不要部署使用此选项重写rt.jar中的类的应用程序，因为这违反了JRE二进制代码许可证。

-Xbootclasspath/a:path

指定要附加到默认引导类路径末尾的目录、JAR文件和以分号（；）分隔的ZIP存档的列表。

不要部署使用此选项重写rt.jar中的类的应用程序，因为这违反了JRE二进制代码许可证。

-Xbootclasspath/p:path

指定以分号（；）分隔的目录、JAR文件和ZIP存档的列表，以放在默认引导类路径前面。

不要部署使用此选项重写rt.jar中的类的应用程序，因为这违反了JRE二进制代码许可证。

-Xcheck:jni

对Java本机接口（JNI）函数执行附加检查。具体来说，它在处理JNI请求之前验证传递给JNI函数的参数和运行时环境数据。遇到的任何无效数据都表示本机代码有问题，在这种情况下，JVM将以不可恢复的错误终止。使用此选项时，预期性能会降低。

-Xcomp

在第一次调用时强制编译方法。默认情况下，客户端VM（-Client）执行1000个解释方法调用，服务器VM（-Server）执行10000个解释方法调用，以收集有效编译所需的信息。指定-Xcomp选项将禁用解释方法调用以提高编译性能，但会降低效率。

还可以在编译之前使用-XX:CompileThreshold选项更改解释方法调用的次数。

-Xdebug

什么都不做。提供向后兼容性。

-Xdiag

显示其他诊断信息。

-Xfuture

启用严格的类文件格式检查，以强制严格遵守类文件格式规范。鼓励开发人员在开发新代码时使用此标志，因为在将来的版本中，更严格的检查将成为默认检查。

-Xint

以仅解释模式运行应用程序。对本机代码的编译被禁用，所有字节码都由解释器执行。准时制（JIT）编译器提供的性能优势在此模式中不存在。

-Xinternalversion

显示比-version选项更详细的JVM版本信息，然后退出。

-Xloggc:filename

设置要将详细GC事件信息重定向到其中进行日志记录的文件。写入此文件的信息类似于-verbose:gc的输出，其中包含自每个记录的事件之前的第一个gc事件以来经过的时间。-Xloggc选项重写-verbose:gc，如果这两个选项都是用同一个java命令给出的。

示例:

-Xloggc:garbage-collection.log

-Xmaxjitcodesize=size

指定JIT编译代码的最大代码缓存大小（字节）。附加字母k或k表示千字节，m或m表示兆字节，g或g表示千兆字节。默认的最大代码缓存大小为240 MB；如果使用-XX:-tiered compilation选项禁用分层编译，则默认大小为48 MB：

-Xmaxjitcodesize=240m

此选项相当于-XX:ReservedCodeCacheSize。

-Xmixed

由解释器执行所有字节码，但编译为本机代码的热方法除外。

-Xmnsize

设置新一代（年轻代）堆的初始大小和最大大小（字节）。附加字母k或k表示千字节，m或m表示兆字节，g或g表示千兆字节。

堆的年轻一代区域用于新对象。GC在这个区域的执行频率高于其他区域。如果年轻一代的大小太小，那么将执行许多小的垃圾收集。如果大小太大，则只执行完整的垃圾回收，这可能需要很长时间才能完成。Oracle建议您将年轻一代的大小保持在整个堆大小的一半到四分之一之间。

以下示例说明如何使用各种单位将年轻一代的初始大小和最大大小设置为256 MB：

-Xmn256m
-Xmn262144k
-Xmn268435456

您可以使用-XX:NewSize设置初始大小，使用-XX:MaxNewSize设置最大大小，而不是使用-Xmn选项设置新一代堆的初始大小和最大大小。

-Xmssize

设置堆的初始大小（字节）。此值必须是1024的倍数且大于1 MB。附加字母k或k表示千字节，m或m表示兆字节，g或g表示千兆字节。
下面的示例演示如何使用不同的单元将分配的内存大小设置为6MB：

-Xms6291456
-Xms6144k
-Xms6m

如果不设置此选项，则初始大小将设置为为为旧代和年轻代分配的大小之和。年轻一代堆的初始大小可以使用-Xmn选项或-XX:NewSize选项设置。

-Xmxsize

指定内存分配池的最大大小（字节）。此值必须是1024的倍数且大于2 MB。附加字母k或k表示千字节，m或m表示兆字节，g或g表示千兆字节。默认值在运行时根据系统配置选择。对于服务器部署，-Xms和-Xmx通常设置为相同的值。请参阅Java SE HotSpot虚拟机垃圾收集优化指南中的“人体工程学”一节

以下示例说明如何使用不同的单元将分配内存的最大允许大小设置为80 MB：

-Xmx83886080
-Xmx81920k
-Xmx80m

-Xmx选项相当于-XX:MaxHeapSize。

-Xnoclassgc

禁用类的垃圾收集（GC）。这可以节省一些GC时间，从而缩短应用程序运行期间的中断。

当您在启动时指定-Xnoclassgc时，应用程序中的类对象将在GC期间保持不变，并且将始终被视为活动的。这会导致更多的内存被永久占用，如果不小心使用，将抛出内存不足异常。

-Xprof

分析正在运行的程序并将分析数据发送到标准输出。此选项是作为一个实用程序提供的，在程序开发中很有用，不打算在生产系统中使用。

-Xrs

减少了JVM对操作系统信号的使用。

关闭挂钩通过在关闭时运行用户清理代码（例如关闭数据库连接）来实现Java应用程序的有序关闭，即使JVM突然终止。

JVM监视控制台控制事件以实现意外终止的关闭挂钩。具体来说，JVM注册一个控制台控制处理程序，该处理程序开始关闭钩子处理，并为CTRL_C_事件、CTRL_CLOSE_事件、CTRL_LOGOFF_事件和CTRL_shutdown_事件返回TRUE。

JVM使用类似的机制来实现转储线程堆栈以进行调试的功能。JVM使用CTRL_BREAK_事件来执行线程转储。

如果JVM作为服务运行（例如，作为web服务器的servlet引擎），那么它可以接收CTRL_LOGOFF_事件，但不应该启动关机，因为操作系统实际上不会终止进程。为了避免这种可能的干扰，可以使用-Xrs选项。使用-Xrs选项时，JVM不会安装控制台控制处理程序，这意味着它不会监视或处理CTRL_C_事件、CTRL_CLOSE_事件、CTRL_LOGOFF_事件或CTRL_SHUTDOWN_事件。

指定-Xrs有两个结果：

Ctrl + Break 线程转储不可用。
用户代码负责导致关闭挂钩运行，例如，在终止JVM时调用System.exit（）。

-Xshare:mode

设置类数据共享（CDS）模式。此选项可能的模式参数包括：

auto：如果可能，请使用CD。这是Java HotSpot 32位客户端虚拟机的默认值。

on：需要使用CD。如果无法使用类数据共享，则打印错误消息并退出。

off：不要使用CD。这是Java HotSpot 32位服务器虚拟机、Java HotSpot 64位客户端虚拟机和Java HotSpot 64位服务器虚拟机的默认值。

dump：手动生成CDS存档。按照“设置类路径”中的说明指定应用程序类路径。

您应该在每个新的JDK版本中重新生成CDS存档。

-XshowSettings:category

显示设置并继续。此选项的可能类别参数包括：

all：显示所有设置类别。这是默认值。

locale：显示与区域设置相关的设置。

properties：显示与系统属性相关的设置。

vm：显示JVM的设置。

-Xsssize

设置线程堆栈大小（字节）。附加字母k或k表示KB，m或m表示MB，g或g表示GB。默认值取决于虚拟内存。

以下示例以不同的单位将线程堆栈大小设置为1024 KB：

-Xss1m
-Xss1024k
-Xss1048576

这个选项相当于-XX:ThreadStackSize.

-Xverify:mode

设置字节码验证程序的模式。字节码验证确保类文件的格式正确，并满足Java虚拟机规范中第4.10节“类文件验证”中列出的约束。

不要关闭验证，因为这会降低Java提供的保护，并可能由于类文件格式不正确而导致问题。

此选项可能的模式参数包括：

remote：验证引导类加载器未加载的所有字节码。如果不指定-Xverify选项，则这是默认行为。

all：启用对所有字节码的验证。

none：禁用所有字节码的验证。不支持使用-Xverify:none。

高级运行时选项

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 file to C:/log/java/java_error.log:

-XX:ErrorFile=C:/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 specified by the value of the TMP environment variable; if that environment variable is not defined, then the value of the TEMP environment variable is used.

-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 userdump.exe utility to obtain a crash dump in case of an irrecoverable error (the %p designates the current process):

-XX:OnError="userdump.exe %p"

The preceding example assumes that the path to the userdump.exe utility is specified in the PATH environment variable.

-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}

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}

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

maxsize=size

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 virtual memory.

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:+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 .

-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:+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:+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.

高级JIT编译器选项

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:

Do not generate prefetch instructions.

Execute prefetch instructions after each allocation. This is the default parameter.

Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.

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 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

-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.

先进的可维修性选项

These options provide the ability to gather system information and perform extensive debugging.

-XX:+HeapDumpOnOutOfMemoryError

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 C:/log/java/java_heapdump.log:

-XX:HeapDumpPath=C:/log/java/java_heapdump.log

-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 C:/log/java/hotspot.log:

-XX:LogFile=C:/log/java/hotspot.log

-XX:+PrintClassHistogram

Enables printing of a class instance histogram after a Control+Break event. 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+Break event. 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.

高级垃圾收集选项

These options control how garbage collection (GC) is performed by the Java HotSpot VM.

-XX:ActiveProcessorCount=x

Overrides the number of CPUs that the VM uses to calculate the size of thread pools it uses for various operations such as Garbage Collection and ForkJoinPool.

The VM normally determines the number of available processors from the operating system. This flag can be useful for partitioning CPU resources when running multiple Java processes in docker containers. This flag is honored even if UseContainerSupport is not enabled. See -XX:-UseContainerSupport for a description of enabling and disabling container support.

-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:InitialRAMPercentage=percent

Sets the initial amount of memory that the JVM will use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 1.5625 percent.

The following example shows how to set the percentage of the initial amount of memory used for the Java heap:

-XX:InitialRAMPercentage=5

-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

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:MaxRAMPercentage=percent

Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 25 percent.

Specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops. See -XX:UseCompressedOops for further information about compressed oops.

The following example shows how to set the percentage of the maximum amount of memory used for the Java heap:

-XX:MaxRAMPercentage=75

-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:MinRAMPercentage=percent

The following example shows how to set the percentage of the maximum amount of memory used for the Java heap for small heaps:

-XX:MinRAMPercentage=75

-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:+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.

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.

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:+PrintGCTimeStamp

Large Pages

Also known as huge pages, large pages are memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system). Large pages optimize processor Translation-Lookaside Buffers.

A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations. TLB is a scarce system resource. A TLB miss can be costly as the processor must then read from the hierarchical page table, which may require multiple memory accesses. By using a larger memory page size, a single TLB entry can represent a larger memory range. There will be less pressure on TLB, and memory-intensive applications may have better performance.

However, large pages page memory can negatively affect system performance. For example, when a large mount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When this happens, either the OS or JVM reverts to using regular pages.

Large Pages Support

Only Windows Server 2003 supports large pages. To use this feature, the administrator must first assign additional privilege to the user who will be running the application:

Select Control Panel -> Administrative Tools -> Local Security Policy.
Select Local Policies -> User Rights Assignment.
Double click Lock pages in memory; add users and/or groups.
Reboot your system.

Note that these steps are required even if it is the administrator who will be running the application, as administrators by default do not have the privilege to lock pages in memory.

Application Class Data Sharing

Application Class Data Sharing (AppCDS) extends CDS (see https://docs.oracle.com/javase/8/docs/technotes/guides/vm/class-data-sharing.html) to enable classes from the standard extensions directories (specified by the system property java.ext.dirs; see https://docs.oracle.com/javase/8/docs/technotes/guides/extensions/spec.html) and the application class path (see "Setting the Class Path") to be placed in the shared archive. AppCDS reduces the footprint and decreases start-up time of your applications provided that a substantial number of classes are loaded from the application class path.

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.

Creating a Shared Archive File, and Running an Application with It

The following steps create a shared archive file that contains all the classes used by the test.Hello application. The last step runs the application with the shared archive file.

Create a list of all classes used by the test.Hello application. The following command creates a file named hello.classlist that contains a list of all classes used by this application:

java -Xshare:off -XX:+UnlockCommercialFeatures -XX:DumpLoadedClassList=hello.classlist -XX:+UseAppCDS -cp hello.jar test.Hello

Note that the -cp parameter must contain only JAR files; the -XX:+UseAppCDS option does not support class paths that contain directory names.
Create a shared archive, named hello.jsa, that contains all the classes in hello.classlist:

java -XX:+UnlockCommercialFeatures -Xshare:dump -XX:+UseAppCDS -XX:SharedArchiveFile=hello.jsa -XX:SharedClassListFile=hello.classlist -cp hello.jar

Note that the -cp parameter used at archive creation time must be the same as (or a prefix of) the -cp used at run time.
Run the application test.Hello with the shared archive hello.jsa:

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:+UseAppCDS -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello

Ensure that you have specified the option -Xshare:on or -Xshare:auto.
Verify that the test.Hello application is using the class contained in the hello.jsa shared archive:

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:+UseAppCDS -XX:SharedArchiveFile=hello.jsa -cp hello.jar -verbose:class test.Hello

The output of this command should contain the following text:

Loaded test.Hello from shared objects file by sun/misc/Launcher$AppClassLoader

Sharing a Shared Archive across Multiple Application Processes

You can share the same archive file across multiple applications processes that have the exact same class path or share a common class path prefix. This reduces memory usage as the archive is memory-mapped into the address space of the processes. The operating system automatically shares the read-only pages across these processes.

The following steps create a shared archive that both applications Hello and Hi can use.

Create a list of all classes used by the Hello application and another list for the Hi application:

java -XX:+UnlockCommercialFeatures -XX:DumpLoadedClassList=hello.classlist -XX:+UseAppCDS -cp common.jar:hello.jar Hello

java -XX:+UnlockCommercialFeatures -XX:DumpLoadedClassList=hi.classlist -XX:+UseAppCDS -cp common.jar:hi.jar Hi

Note that because the Hello and Hi applications share a common class path prefix (both of their class paths start with common.jar), these two applications can share a shared archive file.
Create a single list of classes used by all the applications that will share the shared archive file.

The following commands combine the files hello.classlist and hi.classlist to one file, common.classlist:

type hello.classlist hi.classlist > common.classlist
Create a shared archive, named common.jsa, that contains all the classes in common.classlist:

java -XX:+UnlockCommercialFeatures -Xshare:dump -XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -XX:SharedClassListFile=common.classlist -cp common.jar

The value of the -cp parameter is the common class path prefix shared by the Hello and Hi applications.
Run the Hello and Hi applications with the same shared archive:

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -cp common.jar:hello.jar Hello

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -cp common.jar:hi.jar Hi

Exit Status

The following exit values are typically returned by the launcher when the launcher is called with the wrong arguments, serious errors, or exceptions thrown by the JVM. However, a Java application may choose to return any value by using the API call System.exit(exitValue). The values are:

0: Successful completion
>0: An error occurred

JDK8 VM配置 官方

概要

描述

选项

标准选项

-agentlib:libname[=options]

-agentpath:pathname[=options]

-client

-Dproperty=value

-disablesystemassertions

-dsa

-enableassertions[:[packagename]...|:classname]

-ea[:[packagename]...|:classname]

-enablesystemassertions

-esa

-help

-?

-jar filename

-javaagent:jarpath[=options]

-jre-restrict-search

-no-jre-restrict-search

-server

-showversion

-splash:imgname

-verbose:class

-verbose:gc

-verbose:jni

-version

-version:release

非标准选项

-X

-Xbatch

-Xbootclasspath:path

-Xcheck:jni

-Xcomp

-Xdebug

-Xdiag

-Xfuture

-Xint

-Xinternalversion

-Xloggc:filename

-Xmaxjitcodesize=size

-Xmixed

-Xmnsize

-Xmssize

-Xnoclassgc

-Xprof

-Xrs

-Xshare:mode

-XshowSettings:category

-Xsssize

-Xverify:mode

高级运行时选项

高级JIT编译器选项

先进的可维修性选项

高级垃圾收集选项

Deprecated and Removed Options

Performance Tuning Examples

Large Pages

Large Pages Support

Application Class Data Sharing

Creating a Shared Archive File, and Running an Application with It

Sharing a Shared Archive across Multiple Application Processes

Exit Status

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JDK8 VM配置官方