java自带命令工具

jstat，这个工具很强大，可以监测Java虚拟机GC多方面的状态，具体参数含义参见此链接：

1. ./jstat-gc8401210003
3. S0CS1CS0US1UECEUOCOUPCPUYGCYGCTFGCFGCTGCT
4. 2112.02112.00.00.017024.00.063872.01319.921248.04728.1743416209.64650.046209.692
5. 2112.02112.00.00.017024.00.063872.01319.921248.04728.1743849209.75550.046209.801
6. 2112.02112.00.00.017024.00.063872.01319.

jmap，这大概是最常用的命令。下面这个命令不但可以列出类的实例数量，还有强制进行一次full GC的“副作用”，这样的副作用对于定位某些问题很有帮助，参见《使用堆外内存》：

1. ./jmap-histo:live84012
3. num#instances#bytesclassname
4. ----------------------------------------------
5. 1:8241177656[B
6. 2:80961106672
7. ...

使用-heap参数，则可以打印堆的使用情况：

1. ./jmap-heap84012
3. AttachingtoPRocessID84012,pleasewait...
4. Debuggerattachedsuccessfully.
5. Servercompilerdetected.
6. JVMversionis20.65-b04-462
8. usingparallelthreadsinthenewgeneration.
9. usingthread-localobjectallocation.
10. ConcurrentMark-SweepGC
12. HeapConfiguration:
13. MinHeapFreeRatio=40
14. MaxHeapFreeRatio=70
15. MaxHeapSize=132120576(126.0MB)
16. NewSize=21757952(20.75MB)
17. MaxNewSize=174456832(166.375MB)
18. OldSize=65404928(62.375MB)
19. NewRatio=7
20. SurvivorRatio=8
21. PermSize=21757952(20.75MB)
22. MaxPermSize=85983232(82.0MB)
24. HeapUsage:
25. NewGeneration(Eden+1SurvivorSpace):
26. capacity=19595264(18.6875MB)
27. used=16785688(16.008079528808594MB)
28. free=2809576(2.6794204711914062MB)
29. 85.66196403375837%used
30. EdenSpace:
31. capacity=17432576(16.625MB)
32. used=16785688(16.008079528808594MB)
33. free=646888(0.6169204711914062MB)
34. 96.28920017328477%used
35. FromSpace:
36. capacity=2162688(2.0625MB)
37. used=0(0.0MB)
38. free=2162688(2.0625MB)
39. 0.0%used
40. ToSpace:
41. capacity=2162688(2.0625MB)
42. used=0(0.0MB)
43. free=2162688(2.0625MB)
44. 0.0%used
45. concurrentmark-sweepgeneration:
46. capacity=65404928(62.375MB)
47. used=1390576(1.3261566162109375MB)
48. free=64014352(61.04884338378906MB)
49. 2.126102791520541%used
50. PermGeneration:
51. capacity=21757952(20.75MB)
52. used=4852336(4.6275482177734375MB)
53. free=16905616(16.122451782226562MB)
54. 22.301437194088855%used

使用-permstat参数，查看永久区：

1. ./jmap-permstat84012
3. AttachingtoprocessID84012,pleasewait...
4. Debuggerattachedsuccessfully.
5. Servercompilerdetected.
6. JVMversionis20.65-b04-462
7. 1239internStringsoccupying104312bytes.
8. findingclassloaderinstances..Warning:skippinginvalidTLABforthreadt@59779
9. Warning:skippinginvalidTLABforthreadt@59527
10. Warning:skippinginvalidTLABforthreadt@59907
11. Warning:skippinginvalidTLABforthreadt@60163
12. Warning:skippinginvalidTLABforthreadt@60419
13. Warning:skippinginvalidTLABforthreadt@60675
14. FindingobjectsizeusingPrintezisbitsandskippingover...
15. done.
16. computingperloaderstat..done.
17. pleasewait..computingliveness...done.
18. class_loaderclassesbytesparent_loaderalive?type
20. 5903973048nulllive
21. 0x00000007f44cace000nulllivesun/misc/Launcher$ExtClassLoader@0x00000007faff8a40
22. 0x00000007f44c147881439280x00000007f44cace0livesun/misc/Launcher$AppClassLoader@0x00000007fb056e88
24. total=35984116976N/Aalive=3,dead=0N/A

把内存中的堆dump成一个镜像文件：

1. ./jmap-dump:live,format=b,file=/Users/xiongyi/Documents/dump.core84012

jstack，线程堆栈打印。注意waiting to lock <xxx>在等待锁，比如进入临界区时；locked <xxx>表示当前同步操作，线程锁住了某资源；而waiting on <xxx>指的是在同步块内，wait方法的执行中暂时地释放了该锁的占用，等唤醒的时候需要重新获取：

1. ./jstack84012
3. 2013-11-1118:30:35
4. FullthreaddumpJavaHotSpot(TM)64-BitServerVM(20.65-b04-462mixedmode):
6. "AttachListener"daemonprio=9tid=7ff64e206800nid=0x117782000waitingoncondition[00000000]
7. java.lang.Thread.State:RUNNABLE
9. "LowMemoryDetector"daemonprio=5tid=7ff64c80f000nid=0x117c96000runnable[00000000]
10. java.lang.Thread.State:RUNNABLE
12. "C2CompilerThread1"daemonprio=9tid=7ff64c80e800nid=0x117b93000waitingoncondition[00000000]
13. java.lang.Thread.State:RUNNABLE
15. "C2CompilerThread0"daemonprio=9tid=7ff64c80d800nid=0x117a90000waitingoncondition[00000000]
16. java.lang.Thread.State:RUNNABLE
18. "SignalDispatcher"daemonprio=9tid=7ff64c80d000nid=0x11798d000runnable[00000000]
19. java.lang.Thread.State:RUNNABLE
21. "SurrogateLockerThread(ConcurrentGC)"daemonprio=5tid=7ff64c80c000nid=0x11788a000waitingoncondition[00000000]
22. java.lang.Thread.State:RUNNABLE
24. "Finalizer"daemonprio=8tid=7ff64e13d800nid=0x11767f000inObject.wait()[11767e000]
25. java.lang.Thread.State:WAITING(onobjectmonitor)
26. atjava.lang.Object.wait(NativeMethod)
27. -waitingon<7f44c0ed0>(ajava.lang.ref.ReferenceQueue$Lock)
28. atjava.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
29. -locked<7f44c0ed0>(ajava.lang.ref.ReferenceQueue$Lock)
30. atjava.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
31. atjava.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:171)
33. "ReferenceHandler"daemonprio=10tid=7ff64e13c800nid=0x11757c000inObject.wait()[11757b000]
34. java.lang.Thread.State:WAITING(onobjectmonitor)
35. atjava.lang.Object.wait(NativeMethod)
36. -waitingon<7f44c0018>(ajava.lang.ref.Reference$Lock)
37. atjava.lang.Object.wait(Object.java:485)
38. atjava.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
39. -locked<7f44c0018>(ajava.lang.ref.Reference$Lock)
41. "main"prio=5tid=7ff64c800800nid=0x10f709000runnable[10f708000]
42. java.lang.Thread.State:RUNNABLE
43. attest.Test.main(Test.java:7)
45. "VMThread"prio=9tid=7ff64e138000nid=0x117479000runnable
47. "Gangworker#0(ParallelGCThreads)"prio=9tid=7ff64e000000nid=0x112b0f000runnable
49. "Gangworker#1(ParallelGCThreads)"prio=9tid=7ff64e001000nid=0x112c12000runnable
51. "Gangworker#2(ParallelGCThreads)"prio=9tid=7ff64e001800nid=0x112d15000runnable
53. "Gangworker#3(ParallelGCThreads)"prio=9tid=7ff64e002000nid=0x112e18000runnable
55. "Gangworker#4(ParallelGCThreads)"prio=9tid=7ff64e002800nid=0x112f1b000runnable
57. "Gangworker#5(ParallelGCThreads)"prio=9tid=7ff64e003800nid=0x11301e000runnable
59. "Gangworker#6(ParallelGCThreads)"prio=9tid=7ff64e004000nid=0x113121000runnable
61. "Gangworker#7(ParallelGCThreads)"prio=9tid=7ff64e004800nid=0x113224000runnable
63. "ConcurrentMark-SweepGCThread"prio=9tid=7ff64e0e2000nid=0x1170f0000runnable
64. "Gangworker#0(ParallelCMSThreads)"prio=9tid=7ff64e0e0800nid=0x1166ea000runnable
66. "Gangworker#1(ParallelCMSThreads)"prio=9tid=7ff64e0e1800nid=0x1167ed000runnable
68. "VMPeriodicTaskThread"prio=10tid=7ff64c820800nid=0x117d99000waitingoncondition
70. "ExceptionCatcherThread"prio=10tid=7ff64c801800nid=0x10f936000runnable
71. JNIglobalreferences:963

jinfo，可以打印JVM执行的参数信息，有一个非常大的作用在于，部分JVM参数在执行过程中是可以修改的，请参见这篇《通过jinfo工具在full GC前后做heap dump》，通过jinfo改变参数HeapDumpBeforeFullGC和HeapDumpAfterFullGC，输出heap dump后，再改回来。

jhat，可以比对core文件之间的对象变化，支持对象查询语言（OQL），请参见这里。

1. ./jhat-stacktrue-refstrue-port8080-baseline/xxx/dump-baseline.core-debug1/Users/xiongyi/Documents/dump-newer.core

javap，用于反编译class文件，对于JVM指令集，这里有完整的文档。

1. javap-c-v./Test.class
2. Classfilexxx/Test.class
3. LastmodifiedNov11,2013;size441bytes
4. md5checksum69488187cc8a8f166bc6dd0d517fb4cb
5. Compiledfrom"Test.java"
6. publicclasstest.Test
7. SourceFile:"Test.java"
8. minorversion:0
9. majorversion:50
10. flags:ACC_PUBLIC,ACC_SUPER
11. Constantpool:
12. #1=Methodref#6.#16//java/lang/Object."":()V
13. #2=Integer1048576
14. #3=Fieldref#17.#18//java/lang/System.out:Ljava/io/PrintStream;
15. #4=Methodref#19.#20//java/io/PrintStream.println:(Ljava/lang/Object;)V
16. #5=Class#21//test/Test
17. #6=Class#22//java/lang/Object
18. #7=Utf8
19. #8=Utf8()V
20. #9=Utf8Code
21. #10=Utf8LineNumberTable
22. #11=Utf8main
23. #12=Utf8([Ljava/lang/String;)V
24. #13=Utf8StackMapTable
25. #14=Utf8SourceFile
26. #15=Utf8Test.java
27. #16=NameAndType#7:#8//"":()V
28. #17=Class#23//java/lang/System
29. #18=NameAndType#24:#25//out:Ljava/io/PrintStream;
30. #19=Class#26//java/io/PrintStream
31. #20=NameAndType#27:#28//println:(Ljava/lang/Object;)V
32. #21=Utf8test/Test
33. #22=Utf8java/lang/Object
34. #23=Utf8java/lang/System
35. #24=Utf8out
36. #25=Utf8Ljava/io/PrintStream;
37. #26=Utf8java/io/PrintStream
38. #27=Utf8println
39. #28=Utf8(Ljava/lang/Object;)V
40. {
41. publictest.Test();
42. flags:ACC_PUBLIC
43. Code:
44. stack=1,locals=1,args_size=1
45. 0:aload_0
46. 1:invokespecial#1//Methodjava/lang/Object."":()V
47. 4:return
48. LineNumberTable:
49. line4:0
51. publicstaticvoidmain(java.lang.String[]);
52. flags:ACC_PUBLIC,ACC_STATIC
53. Code:
54. stack=2,locals=2,args_size=1
55. 0:ldc#2//int1048576
56. 2:newarraybyte
57. 4:astore_1
58. 5:getstatic#3//Fieldjava/lang/System.out:Ljava/io/PrintStream;
59. 8:aload_1
60. 9:invokevirtual#4//Methodjava/io/PrintStream.println:(Ljava/lang/Object;)V
61. 12:goto0
62. LineNumberTable:
63. line7:0
64. line8:5
65. line9:12
66. StackMapTable:number_of_entries=1
67. frame_type=0/*same*/
69. }