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jmap命令

发表于 专题于 Disqus：

使用的JDK版本为jdk1.7.0_79。

一、常用用法

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jmap [-F] [-dump:format=b,file=FILENAME|-heap|-histo|-permstat] 进程ID

二、含义

“进程ID”是指JVM进程ID，以上常用用法的含义是：打印或者保存相应JVM进程的Java堆信息。

三、选项

假设现在运行有一个进程ID为3937的JVM进程，以该JVM进程为实验对象，对“选项”进行说明。

3.1、“-F”

“-F”选项可与接下来的几个选项结合使用，不能单独使用。使用“-F”选项后，会开启“HotSpot Serviceability Agent”机制，这使得JVM进程收到一个“SIGSTOP”信号而被“Hung”，只有当请求的信息获取完成后，该JVM进程才会收到一个“SIGCONT”信号而重新运行。[3][4][5]

3.2、“-dump:format=b,file=FILENAME”

保存Java堆堆转存快照到文件“FILENAME”，供后续进行分析，堆转存快照的格式为“hprof二进制格式”。

3.3、“-heap”

打印Java堆的如下信息：使用的垃圾收集器，Java堆相关的几个虚拟机参数值，Java堆不同内存区域的使用情况，“字符串常量池”使用情况等。注意跟“jstat”命令的统计信息有所重叠。
执行jmap -heap 3937命令，得到如下所示结果：

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Attaching to process ID 3937, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 24.79-b02

using thread-local object allocation.
Mark Sweep Compact GC

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 536870912 (512.0MB)
   NewSize          = 1048576 (1.0MB)
   MaxNewSize       = 4294901760 (4095.9375MB)
   OldSize          = 4194304 (4.0MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 12582912 (12.0MB)
   MaxPermSize      = 67108864 (64.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 7471104 (7.125MB)
   used     = 790304 (0.753692626953125MB)
   free     = 6680800 (6.371307373046875MB)
   10.578142132675438% used
Eden Space:
   capacity = 6684672 (6.375MB)
   used     = 790304 (0.753692626953125MB)
   free     = 5894368 (5.621307373046875MB)
   11.822629442401961% used
From Space:
   capacity = 786432 (0.75MB)
   used     = 0 (0.0MB)
   free     = 786432 (0.75MB)
   0.0% used
To Space:
   capacity = 786432 (0.75MB)
   used     = 0 (0.0MB)
   free     = 786432 (0.75MB)
   0.0% used
tenured generation:
   capacity = 16429056 (15.66796875MB)
   used     = 7500296 (7.152839660644531MB)
   free     = 8928760 (8.515129089355469MB)
   45.65262909810521% used
Perm Generation:
   capacity = 12582912 (12.0MB)
   used     = 9098864 (8.677352905273438MB)
   free     = 3484048 (3.3226470947265625MB)
   72.31127421061198% used

4104 interned Strings occupying 306480 bytes.

3.4、“-histo”

打印每个类的统计信息：类的实例数量，类的实例字节数，类的全限定名称等等。
执行jmap -histo 3937命令，得到如下所示结果（限于篇幅，只截取了第一页结果）：

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num     #instances         #bytes  class name
----------------------------------------------
   1:         26723        3048040  <constMethodKlass>
   2:          4967        2283080  [B
   3:         11530        2245392  [C
   4:         26723        1927712  <methodKlass>
   5:           322        1648520  [I
   6:          2768        1435312  <constantPoolKlass>
   7:          2768        1051720  <instanceKlassKlass>
   8:          2118         783680  <constantPoolCacheKlass>
   9:          9216         368640  org.apache.lucene.index.FreqProxTermsWriter$PostingList
  10:         10801         259224  java.lang.String
  11:          2959         254872  java.lang.Class
  12:          4057         198384  [S
  13:          4340         193960  [[I
  14:          1802         160184  [Ljava.util.HashMap$Entry;
  15:          3964         126848  java.util.LinkedHashMap$Entry
  16:           288         122976  [Lorg.apache.lucene.index.RawPostingList;
  17:          1798         108408  [Ljava.lang.Object;
  18:          4136          99264  java.util.concurrent.ConcurrentHashMap$HashEntry
  19:          3254          78096  java.util.HashMap$Entry
  20:          1014          56784  java.util.LinkedHashMap
  21:          1133          54384  java.util.HashMap
  22:           174          54288  <objArrayKlassKlass>
  23:           190          39152  [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;
  24:           456          36480  java.lang.reflect.Method
  25:           550          35200  java.lang.reflect.Field
  26:          4059          32472  java.lang.Object
  27:          1201          28824  java.util.ArrayList
  28:          1079          25896  com.google.inject.TypeLiteral
  29:           366          23424  org.apache.lucene.index.SegmentInfo
  30:           290          18560  java.lang.reflect.Constructor
  31:           622          14928  com.google.inject.Key
  32:           586          14064  java.util.Hashtable$Entry
  33:           437          13984  java.lang.ref.SoftReference
  34:           545          13080  com.google.common.collect.CustomConcurrentHashMap$StrongEntry
  35:           144          12672  org.apache.lucene.index.TermsHashPerField
  36:           513          12312  sun.reflect.annotation.AnnotationInvocationHandler
  37:           204           9792  java.lang.Package
  38:            96           9216  java.io.ObjectStreamClass
  39:           383           9192  com.google.inject.spi.Dependency
  40:           555           8880  com.google.common.collect.CustomConcurrentHashMap$StrongValueReference
  41:           152           8512  java.net.URL
  42:           210           8400  com.google.inject.internal.ProviderInstanceBindingImpl
  43:           255           8160  java.lang.ref.Finalizer
  44:           507           8112  java.lang.Integer

3.5、“-permstat”

打印类加载器信息：类加载器名称，类加载器加载的类数量，类加载器加载的类字节数，父“类加载器”名称，是否存活标志，类加载器类型等等。
执行jmap -permstat 3937命令，得到如下所示结果（限于篇幅，只截取了第一页结果）：

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Attaching to process ID 3937, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 24.79-b02
finding class loader instances ..done.
computing per loader stat ..done.
please wait.. computing liveness............................................................................done.
class_loader	classes	bytes	parent_loader		alive?	type

<bootstrap>	1143	4158120	  null  	live	<internal>
0x9b343750	0	0	0x9b2c85c0	live	java/util/ResourceBundle$RBClassLoader@0xb0bdf870
0x9b69c608	1	1752	0x9b2c85c0	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b3a17d0	0	0	0x9b2c85c0	live	org/sonatype/guice/plexus/scanners/CloningClassLoader@0xb0d62e30
0x9b744a98	1	1752	0x9b2c85c0	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b2c9490	1	1768	  null  	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b2c93e8	1	1768	  null  	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b2c85c0	1827	5868816	0x9b2c9440	live	sun/misc/Launcher$AppClassLoader@0xb0957068
0x9b5d9d70	1	1752	0x9b2c85c0	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b995630	1	1768	  null  	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b744a60	1	1864	0x9b2c85c0	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b2c94e8	1	1768	  null  	dead	sun/reflect/DelegatingClassLoader@0xb0835250
0x9b2c9440	8	40640	  null  	live	sun/misc/Launcher$ExtClassLoader@0xb091e048
0x9b339138	1	16760	0x9b2c85c0	live	org/codehaus/plexus/classworlds/realm/ClassRealm@0xb0b389f0

total = 14	2987	10098528	    N/A    	alive=6, dead=8	    N/A

参考文献： [1]http://docs.oracle.com/javase/7/docs/technotes/tools/share/jmap.html [2]man jmap [3]http://stackoverflow.com/questions/26140182/running-jmap-getting-unable-to-open-socket-file [4]https://major.io/2009/06/15/two-great-signals-sigstop-and-sigcont/ [5]http://velocity.oreilly.com.cn/2013/ppts/java_web_serviceability_agent.pdf
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