jmap 命令如下：
[root@admin ~]# jmap --help 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常用的 options 参数： - heap 显示 Java 堆详细信息 - histo 显示堆中对象的统计信息 - permstat 显示堆永久区的类加载器的统计信息 - finalizerinfo 显示在 F-Queue 队列等待 Finalizer 线程执行 finalize 方法的对象 - dump 生成堆转储快照
例如，要生成堆快照文件：
[root@admin ~]# jmap -dump:live,format=b,file=/home/dump.hprof 43616 Dumping heap to /home/dump.hprof ... Heap dump file created如果要打印 heap 的概要信息，GC 算法等，可以使用 -heap ：
[root@admin ~]# jmap -heap 43616 Attaching to process ID 43616, please wait... Debugger attached successfully. Server compiler detected. JVM version is 24.45-b08 using thread-local object allocation. Parallel GC with 4 thread(s) Heap Configuration: MinHeapFreeRatio = 40 MaxHeapFreeRatio = 70 MaxHeapSize = 3191865344 (3044.0MB) 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 = 50855936 (48.5MB) used = 2034448 (1.9402008056640625MB) free = 48821488 (46.55979919433594MB) 4.000414032297036% used From Space: capacity = 7864320 (7.5MB) used = 0 (0.0MB) free = 7864320 (7.5MB) 0.0% used To Space: capacity = 7864320 (7.5MB) used = 0 (0.0MB) free = 7864320 (7.5MB) 0.0% used PS Old Generation capacity = 51380224 (49.0MB) used = 473272 (0.45134735107421875MB) free = 50906952 (48.54865264892578MB) 0.9211170430086097% used PS Perm Generation capacity = 22020096 (21.0MB) used = 2571712 (2.45257568359375MB) free = 19448384 (18.54742431640625MB) 11.678931826636905% used 1453 interned Strings occupying 135688 bytes.-finalizerinfo:
[root@admin ~]# jmap -finalizerinfo 43616 Attaching to process ID 43616, please wait... Debugger attached successfully. Server compiler detected. JVM version is 24.45-b08 Number of objects pending for finalization: 0看到当前并没有等待 finalize 的对象。
-histo： 显示堆中对象的统计信息。 -histo:live 这个命令执行，JVM会先触发gc，然后再统计信息。
[root@admin ~]# jmap -histo:live 43616 | more num #instances #bytes class name ---------------------------------------------- 1: 5734 739488 <methodKlass> 2: 5734 662424 <constMethodKlass> 3: 380 453824 <constantPoolKlass> 4: 349 279520 <constantPoolCacheKlass> 5: 1834 264656 [C 6: 380 257248 <instanceKlassKlass> 7: 528 89504 [B 8: 443 54200 java.lang.Class 9: 627 41648 [[I 10: 1683 40392 java.lang.String 11: 580 35536 [S 12: 785 31400 java.util.TreeMap$Entry 13: 46 24656 <objArrayKlassKlass> 14: 314 13184 [Ljava.lang.Object; 15: 186 7584 [Ljava.lang.String; 16: 13 6152 <methodDataKlass> 17: 8 4288 <typeArrayKlassKlass> 18: 112 3584 java.util.Hashtable$Entry 19: 11 2288 <klassKlass> 20: 61 1952 java.util.concurrent.ConcurrentHashMap$HashEntry 21: 38 1824 sun.util.locale.LocaleObjectCache$CacheEntry-permstat
[root@admin ~]# jmap -permstat 43616 Attaching to process ID 43616, please wait... Debugger attached successfully. Server compiler detected. JVM version is 24.45-b08 finding class loader instances ..done. computing per loader stat ..done. please wait.. computing liveness...............................................................................done. class_loader classes bytes parent_loader alive? type <bootstrap> 377 2522712 null live <internal> 0x0000000741c00d98 0 0 null live sun/misc/Launcher$ExtClassLoader@0x000000073cbc1478 0x0000000741c00738 11 123552 0x0000000741c00d98 live sun/misc/Launcher$AppClassLoader@0x000000073cc24c50 total = 3 388 2646264 N/A alive=3, dead=0 N/A