jmap查看内存使用情况与生成heapdump

例：jmap -heap 12345输出：Attaching to process ID 2657,please wait...Debugger attached successfully.Client compiler detected.JVM version is 1.5.0_16-b02?using thread-local object allocation.Mark Sweep Compact GC?Heap Configuration:?? MinHeapFreeRatio = 40?? MaxHeapFreeRatio = 70?? MaxHeapSize????? = 67108864 (64.0MB)?? NewSize????????? = 655360 (0.625MB)?? MaxNewSize?????? = 4294901760 (4095.9375MB)?? OldSize????????? = 1441792 (1.375MB)?? NewRatio???????? = 12?? SurvivorRatio??? = 8?? PermSize???????? = 8388608 (8.0MB)?? MaxPermSize????? = 67108864 (64.0MB)?Heap Usage:New Generation (Eden + 1 Survivor Space):----------------------------------------新生代区?? capacity = 4521984 (4.3125MB)?? used???? = 1510200 (1.4402389526367188MB)?? free???? = 3011784 (2.8722610473632812MB)?? 33.39684527853261% usedEden Space:--------------------------------------------------------------------伊甸园区

?? capacity = 4063232 (3.875MB)?? used???? = 1495992 (1.4266891479492188MB)?? free???? = 2567240 (2.4483108520507812MB)?? 36.81778446320565% usedFrom Space:-------------------------------------------------------------------年轻代（幸存者乐园1）

?? capacity = 458752 (0.4375MB)?? used???? = 14208 (0.0135498046875MB)?? free???? = 444544 (0.4239501953125MB)?? 3.0970982142857144% usedTo Space:-----------------------------------------------------------------------------年轻代（幸存者乐园2）?? capacity = 458752 (0.4375MB)?? used???? = 0 (0.0MB)?? free???? = 458752 (0.4375MB)?? 0.0% used

concurrent mark-sweep generation:-------------------------------------------------老年代 capacity = 8589934592 (8192.0MB) used = 0 (0.0MB) free = 8589934592 (8192.0MB) 0.0% used

Perm Generation:----------------------------------------------------------------------永久代?? capacity = 11796480 (11.25MB)?? used???? = 11712040 (11.169471740722656MB)?? free???? = 84440 (0.08052825927734375MB)?? 99.28419325086806% used以上的输出很简单，第四行起开始输出此进程我们的JAVA使用的环境。Heap Configuration，指在我们启动时设置的一些JVM参数。像最大使用内存大小，年老代，年青代，持久代大小等。有这个可以很简单的查看本进程的内存使用情况。也许进程占用的总内存比较多，但我们在这里可以看到真正用到的并没有多少，很多都是"Free"。内存使用的堆积大多在老年代，内存池露始于此，所以要格外关心“tenured generation”。例：jmap -heap:format=b 2657会产生一个heap.bin的heapdump文件。需要注意的是，此生成heapdump的参数为JDK1.5，在1.6中的格式为:jmap -dump:live,format=b,file=xxx 2657这里更加强大一些，可以指定是存活的对象，还有生成heapdump的文件名。Size??? Count?? Class description-------------------------------------------------------8394352 105???? long[]8293192 57202?? char[]7834776 14157?? byte[]6713592 53743?? * ConstMethodKlass4194320 1?????? com.xjawa.cms5server.Kontent[]4055072 12319?? int[]3291104 85082?? * SymbolKlass3016040 53743?? * MethodKlass2774936 4253??? * ConstantPoolKlass1871480 4253??? * InstanceKlassKlass1811808 3990??? * ConstantPoolCacheKlass1488672 62028?? java.lang.String1203280 13258?? java.lang.Object[]

-h? 或者? ?-help：查看jmap所有可用命令及作用。