jmap详解

最新推荐文章于 2024-07-20 09:59:36 发布

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最新推荐文章于 2024-07-20 09:59:36 发布

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文章标签： java

原文链接：https://my.oschina.net/xionghui/blog/491774

本文详细介绍了Java内存管理工具jmap的使用方法及输出信息解析，包括heap、histo、permstat等参数的使用场景及含义，帮助开发者更好地理解和优化Java应用的内存使用。

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jmap（Java Memory Map）主要用于查看jvm内存，是jvm自带的一种内存映像工具。

jmap常用参数：

-heap：打印jvm heap的情况
　　-histo：打印jvm heap的直方图。其输出信息包括类名，对象数量，对象占用大小。
　　-histo：live ：同上，但是只打印存活对象的情况。
　　-permstat：打印permanent generation heap情况。

-heap

执行 jmap -heap pid 可以查看进程堆内存使用情况，包括使用的GC算法、堆配置参数和各代中堆内存使用情况。导出日志文件如下：

PROCESS PATH：/export/App/...
DATETIME: 2015-08-13 10:39:10
EXECUTE COMMAND：/export/servers/jdk1.6.0_25/bin/jmap -heap 30119

using thread-local object allocation.
Parallel GC with 23 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 2147483648 (2048.0MB)
   NewSize          = 536870912 (512.0MB)
   MaxNewSize       = 536870912 (512.0MB)
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 67108864 (64.0MB)
   MaxPermSize      = 134217728 (128.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 534904832 (510.125MB)
   used     = 281308384 (268.2765808105469MB)
   free     = 253596448 (241.84841918945312MB)
   52.590361344875646% used
From Space:
   capacity = 983040 (0.9375MB)
   used     = 720896 (0.6875MB)
   free     = 262144 (0.25MB)
   73.33333333333333% used
To Space:
   capacity = 983040 (0.9375MB)
   used     = 0 (0.0MB)
   free     = 983040 (0.9375MB)
   0.0% used
PS Old Generation
   capacity = 1610612736 (1536.0MB)
   used     = 1192251304 (1137.0194473266602MB)
   free     = 418361432 (398.98055267333984MB)
   74.02470360199611% used
PS Perm Generation
   capacity = 67108864 (64.0MB)
   used     = 33232488 (31.692970275878906MB)
   free     = 33876376 (32.307029724121094MB)
   49.52026605606079% used

-histo[:live]

使用jmap -histo[:live] pid查看堆内存中的对象数目、大小统计直方图，如果带上live则只统计活对象，如下：

PROCESS PATH：/export/App/...
DATETIME: 2015-08-13 10:13:12
EXECUTE COMMAND：/export/servers/jdk1.6.0_25/bin/jmap -histo 44133

 num     #instances         #bytes  class name
----------------------------------------------
   1:        244046      849851696  [I
   2:        417548      419858768  [B
   3:       1347221      127915792  [C
   4:        192112       22450864  [Ljava.lang.Object;
   5:        690346       22091072  java.lang.String
   6:        402121       12867872  java.util.HashMap$Entry
   7:         78652        9438240  java.net.SocksSocketImpl
   8:        118128        9404576  [Ljava.util.HashMap$Entry;
   9:        443640        7098240  java.lang.Object
  10:         48045        6960520  <constMethodKlass>
  11:         48045        6542488  <methodKlass>
  12:        155208        6208320  java.util.HashMap$KeyIterator
  13:        128452        6165696  java.nio.HeapByteBuffer
  14:        251224        6029376  java.lang.StringBuilder
  15:        117438        5637024  java.util.HashMap
  16:          4544        4837032  <constantPoolKlass>
  17:        106570        4262800  java.util.HashMap$EntryIterator
  18:        176445        4234680  java.io.FileDescriptor
  19:         71513        3984000  <symbolKlass>
  20:         78823        3783504  java.nio.HeapCharBuffer
  21:         76481        3671088  java.net.SocketInputStream
  22:          4544        3438000  <instanceKlassKlass>
  23:          3940        2957856  <constantPoolCacheKlass>
  24:         92203        2950496  java.io.FileInputStream
  25:         52028        2913568  org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation
  26:        179818        2877088  java.util.concurrent.atomic.AtomicInteger
  27:         35194        2533968  java.lang.reflect.Field
  28:        128507        2528336  [Ljava.lang.Class;
  29:        104205        2500920  java.lang.Long
  30:         76481        2447392  java.net.Socket

其中：

[C is a char[]
[S is a short[]
[I is a int[]
[B is a byte[]
[[I is a int[][]

constMethodKlass/ methodKlass/ constantPoolKlass/ constantPoolCacheKlass/ instanceKlassKlass/ methodDataKlass

与Classloader相关，常驻与Perm区。

-permstat

使用jmap -permstat pid 打印java堆perm区的classloader统计。如下：

PROCESS PATH：/export/App/...
DATETIME: 2015-08-13 12:17:44
EXECUTE COMMAND：/export/servers/jdk1.6.0_25/bin/jmap -permstat 30119
10404 intern Strings occupying 1025240 bytes.
class_loader	classes	bytes	parent_loader	alive?	type

<bootstrap>	2031	11679192	  null  	live	<internal>
0x00000007b29137a8	1	3112	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800124a0	1	3112	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012350	1	3120	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800123f8	1	3088	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780740080	1	3088	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800122a8	1	3088	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012468	1	3088	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012200	1	1960	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800122e0	1	3112	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012190	1	3312	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012158	1	1944	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012318	1	3112	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007804918b0	0	0	0x0000000780001cf8	dead	java/net/URLClassLoader@0x00000007781cf940
0x0000000780012270	1	3112	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012120	1	3112	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012388	1	3112	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800121c8	1	1944	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780012430	1	3136	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800123c0	1	1944	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007801efb78	0	0	0x0000000780001cf8	dead	java/util/ResourceBundle$RBClassLoader@0x00000007785701c0
0x0000000780012238	1	3112	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x00000007800124d8	1	3112	0x0000000780001cf8	dead	sun/reflect/DelegatingClassLoader@0x00000007780675e8
0x0000000780001cf8	2838	16981304	0x0000000780001d40	live	sun/misc/Launcher$AppClassLoader@0x000000077823e940
0x0000000780001d40	8	49544	  null  	live	sun/misc/Launcher$ExtClassLoader@0x00000007781cfdd8

total = 25	4897	28767760	    N/A    	alive=3, dead=22	    N/A

转载于:https://my.oschina.net/xionghui/blog/491774