Jvm常见问题诊断---命令行工具

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

示例结果如下：

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

2.jmap

描述：输出给定 java 进程所有的内存使用信息。了解jvm内存分配和状态比较直观的一个方式。
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

以上是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  <constMethodKlass>  
  
  3:         34680        2779112  <methodKlass>  
  
  4:         52947        2340272  <symbolKlass>  
  
  5:          3024        1641744  <constantPoolKlass>  
  
  6:          5182        1618040  [B  
  
  7:          3024        1290544  <instanceKlassKlass>  
  
  8:         12134        1267696  [I  
  
  9:         44622        1070928  java.lang.String  
  
 10:          2565         994128  <constantPoolCacheKlass>  
  
 ………………

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

class_loader    classes     bytes        parent_loader alive?        type  
  
  
<bootstrap>     1102         4283352    null     live   <internal>  
  
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

内存dump 除了直接通过jmap查看内存里各种对象存活情况，也可以直接对整个内存映像dump，再通过工具(jmat)查看。如：

jmap -dump:live,format=b,file=heap.bin pid（注意 加了live参数，会触发fullgc，只dump存活的对象，不加live参数则不会。）

3.jstack

排查疑难问题非常有用的一个命令。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，是一种最直观的了解当前jvm工作状态的命令。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：

"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]

4.jstat

描述：全称“Java Virtual Machine statistics monitoring tool”,用于对jvm进程的资源和性能等各方面情况实时监控的轻量级小工具。
使用：

jstat -<option> [-t] [-h<lines>] <vmid> [<interval> [<count>]]

Options — 监控选项选项。如 -gcutil 查看gc情况，class查看类装载情况。

Option Displays...

class Statistics on the behavior of the class loader.

compiler Statistics of the behavior of the HotSpot Just-in-Time compiler.

gc Statistics of the behavior of the garbage collected heap.

gccapacity Statistics of the capacities of the generations and their corresponding spaces.

gccause Summary of garbage collection statistics (same as -gcutil), with the cause of the last and current (if applicable) garbage collection events.

gcnew Statistics of the behavior of the new generation.

gcnewcapacity Statistics of the sizes of the new generations and its corresponding spaces.

gcold Statistics of the behavior of the old and permanent generations.

gcoldcapacity Statistics of the sizes of the old generation.

gcpermcapacity Statistics of the sizes of the permanent generation.

gcutil Summary of garbage collection statistics.

printcompilation HotSpot compilation method statistics.

vmid — java进程号

interval– 采样间隔时间，单位为毫秒

count — 打印次数，如果缺省则打印无数次

示例 sample1. gc情况监控命令：

jstat -gcutil 1881 3000 10

命令输出：

  S0     S1     E      O      P     YGC     YGCT    FGC    FGCT     GCT   
  0.00  99.86  51.79  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  51.90  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  52.01  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  52.33  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  52.45  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  52.55  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  52.71  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  53.53  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  53.63  48.14  99.92     71    3.203     1    0.040    3.243
  0.00  99.86  53.80  48.14  99.92     71    3.203     1    0.040    3.243

以上每一列的说明：

S0 — Heap上的 Survivor space 0 区已使用空间的百分比

S1 — Heap上的 Survivor space 1 区已使用空间的百分比

E — Heap上的 Eden space 区已使用空间的百分比

O — Heap上的 Old space 区已使用空间的百分比

P — Perm space 区已使用空间的百分比

YGC — 从应用程序启动到采样时发生 Young GC 的次数

YGCT– 从应用程序启动到采样时 Young GC 所用的时间(单位秒)

FGC — 从应用程序启动到采样时发生 Full GC 的次数

FGCT– 从应用程序启动到采样时 Full GC 所用的时间(单位秒)

GCT — 从应用程序启动到采样时用于垃圾回收的总时间(单位秒)

sample2. 类加载情况监控命令：

jstat -class 1881 3000 10

命令输出

Loaded  Bytes  Unloaded  Bytes     Time   
 18302 38220.0        0     0.0      41.74
 18302 38220.0        0     0.0      41.74
 18308 38228.3        0     0.0      41.75
 18308 38228.3        0     0.0      41.75
 18308 38228.3        0     0.0      41.75