jmap工具

jmap命令是一个可以输出所有内存中对象的工具，甚至可以将VM 中的heap，以二进制输出成文本。打印出某个java进程（使用pid）内存内的，所有‘对象’的情况（如：产生那些对象，及其数量）。

语法

C:\Users\shuqi>jmap
Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable <core>
        (to connect to a core file)
    jmap [option] [server_id@]<remote server IP or hostname>
        (to connect to remote debug server)

where <option> is one of:
    <none>               to print same info as Solaris pmap
    -heap                to print java heap summary
    -histo[:live]        to print histogram of java object heap; if the "live"
                         suboption is specified, only count live objects
    -clstats             to print class loader statistics
    -finalizerinfo       to print information on objects awaiting finalization
    -dump:<dump-options> to dump java heap in hprof binary format
                         dump-options:
                           live         dump only live objects; if not specified,
                                        all objects in the heap are dumped.
                           format=b     binary format
                           file=<file>  dump heap to <file>
                         Example: jmap -dump:live,format=b,file=heap.bin <pid>
    -F                   force. Use with -dump:<dump-options> <pid> or -histo
                         to force a heap dump or histogram when <pid> does not
                         respond. The "live" suboption is not supported
                         in this mode.
    -h | -help           to print this help message
    -J<flag>             to pass <flag> directly to the runtime system

生成dump文件

C:\Users\shuqi>jmap -dump:live,format=b,file=d:\\d.bin 9880
Dumping heap to D:\d.bin ...
Heap dump file created

打印Heap信息

C:\Users\shuqi>jmap -heap 9880
Attaching to process ID 9880, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.111-b14

using thread-local object allocation.
Parallel GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 1073741824 (1024.0MB)
   NewSize                  = 44564480 (42.5MB)
   MaxNewSize               = 357564416 (341.0MB)
   OldSize                  = 89653248 (85.5MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 26214400 (25.0MB)
   used     = 1048720 (1.0001373291015625MB)
   free     = 25165680 (23.999862670898438MB)
   4.00054931640625% used
From Space:
   capacity = 524288 (0.5MB)
   used     = 294912 (0.28125MB)
   free     = 229376 (0.21875MB)
   56.25% used
To Space:
   capacity = 524288 (0.5MB)
   used     = 0 (0.0MB)
   free     = 524288 (0.5MB)
   0.0% used
PS Old Generation
   capacity = 54001664 (51.5MB)
   used     = 13028944 (12.425369262695312MB)
   free     = 40972720 (39.07463073730469MB)
   24.126930607175364% used

11332 interned Strings occupying 1040832 bytes.