簡介

jmap是JDK中提供的一個用來監視程序執行中的jvm實體記憶體的佔用情況的工具。該程序記憶體內，所有物件的情況，例如產生了哪些物件，物件數量。當系統崩潰時，jmap 可以從core檔案或程序中獲得記憶體的具體匹配情況，包括Heap size, Perm size等。

使用jmap會影響線上執行的應用，所以儘量不要在線上執行此命令。如果想dump堆資訊，可以使用gcore命令，比jmap -dump快。

使用

jmap的使用很簡單，可以通過以下命令檢視。

jmap -h
Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable
 
 <core>
        (to connect to a core file)
    jmap [option] [[email protected]]<remote server IP or hostname>
        (to connect to remote debug server)
.......

案例

檢視堆heap佔用情況

jmap -heap 90099
Attaching to process ID 90099, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is
 
 24.71-b01

using thread-local object allocation.
Parallel GC with 4 thread(s)  #垃圾回收的方式

Heap Configuration: #堆配置資訊
   MinHeapFreeRatio = 0
   MaxHeapFreeRatio = 100
   MaxHeapSize      = 2147483648 (2048.0MB)
   NewSize          = 1310720 (1.25MB)
   MaxNewSize       = 17592186044415 MB
   OldSize          = 5439488
 
 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 21757952 (20.75MB)
   MaxPermSize      = 85983232 (82.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 34603008 (33.0MB)
   used     = 9773032 (9.320289611816406MB)
   free     = 24829976 (23.679710388183594MB)
   28.24330185398911% used
From Space:
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
To Space:
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
PS Old Generation
   capacity = 89128960 (85.0MB)
   used     = 0 (0.0MB)
   free     = 89128960 (85.0MB)
   0.0% used
PS Perm Generation
   capacity = 22020096 (21.0MB)
   used     = 2666808 (2.5432662963867188MB)
   free     = 19353288 (18.45673370361328MB)
   12.110791887555804% used

677 interned Strings occupying 44048 bytes.

檢視物件佔用記憶體情況

$ jmap -histo:live 96176

 num     #instances         #bytes  class name
----------------------------------------------
   1:          5948         766656  <methodKlass>
   2:          5948         679168  <constMethodKlass>
   3:           401         474448  <constantPoolKlass>
   4:           365         289664  <constantPoolCacheKlass>
   5:           401         276112  <instanceKlassKlass>
   6:           562          93176  [B
   7:           900          86256  [C
   8:           463          45328  java.lang.Class
   9:           653          43216  [[I
  10:           608          37144  [S
  11:            45          24480  <objArrayKlassKlass>

輸出內容的一些說明：

[C is a char[]

[S is a short[]

[I is a int[]

[B is a byte[]

[[I is a int[][]

上面的輸出中[C物件佔用Heap這麼多，往往跟String有關，String其內部使用final char[]陣列來儲存資料的。

constMethodKlass/ methodKlass/ constantPoolKlass/ constantPoolCacheKlass/ instanceKlassKlass/ methodDataKlass

與Classloader相關，常駐與Perm區。

匯出堆資訊

可以使用jmap把堆資訊匯出，並使用視覺化工具mat分析。匯出的命令如下：

$ jmap -dump:format=b,file=a.dmp 96185
Dumping heap to /Users/canglong/dev/test/a.dmp ...
Heap dump file created