JVM问题诊断常用命令：jinfo,jmap,jstack
1.jinfo
描述：输出给定 java 进程所有的配置信息。包括 java 系统属性和 jvm 命令行标记等。
用法：
jinfo [ option ] pid
jinfo [ option ] executable core
jinfo [ option ] [server-id@]remote-hostname-or-IP
例子：
jinfo pid
得到结果如下：
JVM version is 14.0-b16
Java System Properties:
java.runtime.name = Java(TM) SE Runtime Environment
sun.boot.library.path = /work/ProgramFiles/jdk1.6.0_14/jre/lib/i386
java.vm.version = 14.0-b16
java.vm.vendor = Sun Microsystems Inc.
java.vendor.url = http://java.sun.com/
path.separator = :
java.vm.name = Java HotSpot(TM) Server VM
file.encoding.pkg = sun.io
sun.java.launcher = SUN_STANDARD
user.country = CN
sun.os.patch.level = unknown
java.vm.specification.name = Java Virtual Machine Specification
user.dir = /work/RateWorkspace/branches/rate/daemon
java.runtime.version = 1.6.0_14-b08
java.awt.graphicsenv = sun.awt.X11GraphicsEnvironment
java.endorsed.dirs = /work/ProgramFiles/jdk1.6.0_14/jre/lib/endorsed
os.arch = i386
java.io.tmpdir = /tmp
line.separator =
java.vm.specification.vendor = Sun Microsystems Inc.
os.name = Linux
sun.jnu.encoding = UTF-8
java.library.path = /work/ProgramFiles/jdk1.6.0_14/jre/lib/i386/server:/work/ProgramFiles/jdk1.6.0_14/jre/lib/i386:/work/ProgramFiles/jdk1.6.0_14/jre/../lib/i386:/usr/lib/jvm/java-6-openjdk/jre/lib/i386/client:/usr/lib/jvm/java-6-openjdk/jre/lib/i386:/usr/lib/xulrunner-addons:/usr/lib/xulrunner-addons:/usr/java/packages/lib/i386:/lib:/usr/lib
java.specification.name = Java Platform API Specification
java.class.version = 50.0
sun.management.compiler = HotSpot Tiered Compilers
os.version = 2.6.28-19-generic
user.home = /home/yblin
user.timezone = PRC
java.awt.printerjob = sun.print.PSPrinterJob
file.encoding = GBK
java.specification.version = 1.6
java.class.path =
user.name = yblin
java.vm.specification.version = 1.0
java.home = /work/ProgramFiles/jdk1.6.0_14/jre
sun.arch.data.model = 32
user.language = zh
java.specification.vendor = Sun Microsystems Inc.
java.vm.info = mixed mode
java.version = 1.6.0_14
java.ext.dirs = /work/ProgramFiles/jdk1.6.0_14/jre/lib/ext:/usr/java/packages/lib/ext
sun.boot.class.path = /work/ProgramFiles/jdk1.6.0_14/jre/lib/resources.jar:/work/ProgramFiles/jdk1.6.0_14/jre/lib/rt.jar:/work/ProgramFiles/jdk1.6.0_14/jre/lib/sunrsasign.jar:/work/ProgramFiles/jdk1.6.0_14/jre/lib/jsse.jar:/work/ProgramFiles/jdk1.6.0_14/jre/lib/jce.jar:/work/ProgramFiles/jdk1.6.0_14/jre/lib/charsets.jar:/work/ProgramFiles/jdk1.6.0_14/jre/classes
java.vendor = Sun Microsystems Inc.
file.separator = /
java.vendor.url.bug = http://java.sun.com/cgi-bin/bugreport.cgi
sun.io.unicode.encoding = UnicodeLittle
sun.cpu.endian = little
sun.desktop = gnome
sun.cpu.isalist =
VM Flags:
-agentlib:jdwp=transport=dt_socket,suspend=y,address=localhost:55948 -ea -Dfile.encoding=GBK
这个命令包含了 JDK 和 JVM 运行起来时的一些属性。
2.jmap
描述 ：输出给定 java 进程所有的内存使用信息。
使用：
jmap [ option ] pid
jmap [ option ] executable core
jmap [ option ] [server-id@]remote-hostname-or-IP
没有可选项时，会输出所有共享的对象映射。
开始地址，映射大小，全路径。
直接 jmap pid得到如下结果：
0x08048000 46K /work/ProgramFiles/jdk1.6.0_14/bin/java
0xb6763000 74K /work/ProgramFiles/jdk1.6.0_14/jre/lib/i386/libzip.so
0xb6774000 41K /lib/tls/i686/cmov/libnss_files-2.9.so
0xb6780000 37K /lib/tls/i686/cmov/libnss_nis-2.9.so
0xb678b000 29K /lib/tls/i686/cmov/libnss_compat-2.9.so
目前还说不是很理解这个执行结果是什么意思。经询问同事了解，是加载的文件的信息。
-heap 选项
jmap -heap pid : 查看堆的配置和大当前各个区块的使用情况，如伊甸区、老年代，永久代。可以用此来判断内存目前的使用情况以及垃圾回收情况：
jmap -heap pid,得到如下结果：
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 859832320 (820.0MB)
NewSize = 1048576 (1.0MB)
MaxNewSize = 4294901760 (4095.9375MB)
OldSize = 4194304 (4.0MB)
NewRatio = 8
SurvivorRatio = 8
PermSize = 16777216 (16.0MB)
MaxPermSize = 67108864 (64.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 9043968 (8.625MB)
used = 6820384 (6.504425048828125MB)
free = 2223584 (2.120574951171875MB)
75.41362375452898% used
From Space:
capacity = 720896 (0.6875MB)
used = 704680 (0.6720352172851562MB)
free = 16216 (0.01546478271484375MB)
97.7505770596591% used
To Space:
capacity = 720896 (0.6875MB)
used = 0 (0.0MB)
free = 720896 (0.6875MB)
0.0% used
PS Old Generation
capacity = 47710208 (45.5MB)
used = 589184 (0.5618896484375MB)
free = 47121024 (44.9381103515625MB)
1.2349223042582418% used
PS Perm Generation
capacity = 16777216 (16.0MB)
used = 5179648 (4.939697265625MB)
free = 11597568 (11.060302734375MB)
30.87310791015625% used
-histo 选项
内存使用的柱状图，包含每个类，对象使用的内存大小。 VM 内部的类名以 * 前缀开头。
num #instances #bytes class name
----------------------------------------------
1: 57195 5639792 [C
2: 34680 3991104
3: 34680 2779112
4: 52947 2340272
5: 3024 1641744
6: 5182 1618040 [B
7: 3024 1290544
8: 12134 1267696 [I
9: 44622 1070928 java.lang.String
10: 2565 994128
………………
-permstat 选项
输出所有类装载器在堆里产生的对象
包括每个装载器的名字，活跃，地址，父装载器，和其总共加载的类大小。
jmap -permstat pid
得到如下结果：
class_loader classes bytes parent_loader alive? type
1102 4283352 null live
0x80753d50 1 1992 null dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0x8034d9e0 8 32272 null live sun/misc/Launcher$ExtClassLoader@0x7c432400
0x8095a4e0 1 1784 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0xadc25868 1 1784 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0xaebf74c0 1 1784 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0xadc2b5c8 1 1784 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0x808cd448 1 1768 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0x80758288 1 1768 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0x80959e80 1 1784 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0x80756248 1 1768 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
0x808e6ae8 1 1784 0x8034d998 dead sun/reflect/DelegatingClassLoader@0x7c2e6c28
3.jstack - Stack Trace
为指定的线程输出 java 的线程堆栈信息，包括了进程里的所有线程。每一个线程 frame ，包括类全名，方法名，代码行。
java.lang.Thread.State ： RUNNABLE BLOCKED TIMED_WATTING(sleep 后会进入这种状态 ) （如果是 BLOCKED 状态就要注意了，看看 blocked 的状态在等待什么？因为什么而阻塞？）最常见的情况是线程在等待网络的读写，比如当网络数据没有准备好读时，线程处于这种等待状态，而一旦有数据准备好读之后，线程会重新激活，读取并处理数据。
在线程中，有一些 JVM 内部的后台线程，来执行譬如垃圾回收，或者低内存的检测等等任务，这些线程往往在 JVM 初始化的时候就存在，如下所示：
"Low Memory Detector" daemon prio=10 tid=0x081465f8 nid=0x7 runnable [0x00000000..0x00000000]
"CompilerThread0" daemon prio=10 tid=0x08143c58 nid=0x6 waiting on condition [0x00000000..0xfb5fd798]
"Signal Dispatcher" daemon prio=10 tid=0x08142f08 nid=0x5 waiting on condition [0x00000000..0x00000000]
"Finalizer" daemon prio=10 tid=0x08137ca0 nid=0x4 in Object.wait() [0xfbeed000..0xfbeeddb8]
写了个代码， 100 个线程同时往控制台输出，代码如下：
public class MyThread extends Thread {
public MyThread(String name){
super(name);
}
@Override
public void run() {
super.run();
while (true) {
try {
System.out.println("1");
} catch (Exception e) {
}
}
}
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
MyThread my = new MyThread("thrad" + i);
my.start();
}
}
}
通过 jstack pid, 得到如下部分结果：
"thrad76" prio=10 tid=0x096ce400 nid=0x4305 waiting for monitor entry [0x7a280000]
java.lang.Thread.State: BLOCKED (on object monitor)
at java.io.PrintStream.println(PrintStream.java:755)
- waiting to lock <0x805782c0> (a java.io.PrintStream)
at org.jtester.service.MyThread.run(MyThread.java:15)
"thrad75" prio=10 tid=0x096ccc00 nid=0x4304 runnable [0x7a2d1000]
java.lang.Thread.State: RUNNABLE
at java.io.PrintStream.write(PrintStream.java:428)
- locked <0x805782c0> (a java.io.PrintStream)
at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:202)
at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:272)
at sun.nio.cs.StreamEncoder.flushBuffer(StreamEncoder.java:85)
- locked <0x805783c8> (a java.io.OutputStreamWriter)
at java.io.OutputStreamWriter.flushBuffer(OutputStreamWriter.java:168)
at java.io.PrintStream.write(PrintStream.java:477)
- locked <0x805782c0> (a java.io.PrintStream)
at java.io.PrintStream.print(PrintStream.java:619)
at java.io.PrintStream.println(PrintStream.java:756)
- locked <0x805782c0> (a java.io.PrintStream)
at org.jtester.service.MyThread.run(MyThread.java:15)
"thrad74" prio=10 tid=0x096cb000 nid=0x4303 waiting for monitor entry [0x7a322000]
java.lang.Thread.State: BLOCKED (on object monitor)
at java.io.PrintStream.println(PrintStream.java:755)
- waiting to lock <0x805782c0> (a java.io.PrintStream)
at org.jtester.service.MyThread.run(MyThread.java:15)
可以看到只有线程 thrad75 是 RUNNABLE 状态的，其他的都是 BLOCKED 状态。因为其他的线程在竞争控制台输出的时候都被阻塞了。