在这里可以找得到这些工具的列表，虽然官网上免责声明为 “The tools described in this section are unsupported and experimental in nature and should be used with that in mind. They might not be available in future JDK versions.”，但实际上这些工具还是非常有用的，尤其可以站在 JVM 的角度来定位问题。我在此简单罗列一些常用的工具和命令，以及相应的执行结果示例。如果你经常和 JVM 打交道，最好对这些工具的常用命令熟记。
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：
./jstat -gc 84012 1000 3 S0C S1C S0U S1U EC EU OC OU PC PU YGC YGCT FGC FGCT GCT 2112.0 2112.0 0.0 0.0 17024.0 0.0 63872.0 1319.9 21248.0 4728.1 743416 209.646 5 0.046 209.692 2112.0 2112.0 0.0 0.0 17024.0 0.0 63872.0 1319.9 21248.0 4728.1 743849 209.755 5 0.046 209.801 2112.0 2112.0 0.0 0.0 17024.0 0.0 63872.0 1319.9 21248.0 4728.1 744282 209.864 5 0.046 209.910
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》：
./jmap -histo:live 84012 num #instances #bytes class name ---------------------------------------------- 1: 824 1177656 [B 2: 8096 1106672 ...
使用-heap 参数，则可以打印堆的使用情况：
./jmap -heap 84012 Attaching to process ID 84012, please wait... Debugger attached successfully. Server compiler detected. JVM version is 20.65-b04-462 using parallel threads in the new generation. using thread-local object allocation. Concurrent Mark-Sweep GC Heap Configuration: MinHeapFreeRatio = 40 MaxHeapFreeRatio = 70 MaxHeapSize = 132120576 (126.0MB) NewSize = 21757952 (20.75MB) MaxNewSize = 174456832 (166.375MB) OldSize = 65404928 (62.375MB) NewRatio = 7 SurvivorRatio = 8 PermSize = 21757952 (20.75MB) MaxPermSize = 85983232 (82.0MB) Heap Usage: New Generation (Eden + 1 Survivor Space): capacity = 19595264 (18.6875MB) used = 16785688 (16.008079528808594MB) free = 2809576 (2.6794204711914062MB) 85.66196403375837% used Eden Space: capacity = 17432576 (16.625MB) used = 16785688 (16.008079528808594MB) free = 646888 (0.6169204711914062MB) 96.28920017328477% used From Space: capacity = 2162688 (2.0625MB) used = 0 (0.0MB) free = 2162688 (2.0625MB) 0.0% used To Space: capacity = 2162688 (2.0625MB) used = 0 (0.0MB) free = 2162688 (2.0625MB) 0.0% used concurrent mark-sweep generation: capacity = 65404928 (62.375MB) used = 1390576 (1.3261566162109375MB) free = 64014352 (61.04884338378906MB) 2.126102791520541% used Perm Generation: capacity = 21757952 (20.75MB) used = 4852336 (4.6275482177734375MB) free = 16905616 (16.122451782226562MB) 22.301437194088855% used
使用-permstat 参数，查看永久区：
./jmap -permstat 84012 Attaching to process ID 84012, please wait... Debugger attached successfully. Server compiler detected. JVM version is 20.65-b04-462 1239 intern Strings occupying 104312 bytes. finding class loader instances ..Warning: skipping invalid TLAB for thread t@59779 Warning: skipping invalid TLAB for thread t@59527 Warning: skipping invalid TLAB for thread t@59907 Warning: skipping invalid TLAB for thread t@60163 Warning: skipping invalid TLAB for thread t@60419 Warning: skipping invalid TLAB for thread t@60675 Finding object size using Printezis bits and skipping over... done. computing per loader stat ..done. please wait.. computing liveness...done. class_loader classes bytes parent_loader alive? type 590 3973048 null live 0x00000007f44cace0 0 0 null live sun/misc/Launcher$ExtClassLoader@0x00000007faff8a40 0x00000007f44c1478 8 143928 0x00000007f44cace0 live sun/misc/Launcher$AppClassLoader@0x00000007fb056e88 total = 3 598 4116976 N/A alive=3, dead=0 N/A
把内存中的堆 dump 成一个镜像文件：
./jmap -dump:live,format=b,file=/xxx/dump.core 84012
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：
./jstack 84012 2013-11-11 18:30:35 Full thread dump Java HotSpot(TM) 64-Bit Server VM (20.65-b04-462 mixed mode): "Attach Listener" daemon prio=9 tid=7ff64e206800 nid=0x117782000 waiting on condition [00000000] java.lang.Thread.State: RUNNABLE "Low Memory Detector" daemon prio=5 tid=7ff64c80f000 nid=0x117c96000 runnable [00000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread1" daemon prio=9 tid=7ff64c80e800 nid=0x117b93000 waiting on condition [00000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread0" daemon prio=9 tid=7ff64c80d800 nid=0x117a90000 waiting on condition [00000000] java.lang.Thread.State: RUNNABLE "Signal Dispatcher" daemon prio=9 tid=7ff64c80d000 nid=0x11798d000 runnable [00000000] java.lang.Thread.State: RUNNABLE "Surrogate Locker Thread (Concurrent GC)" daemon prio=5 tid=7ff64c80c000 nid=0x11788a000 waiting on condition [00000000] java.lang.Thread.State: RUNNABLE "Finalizer" daemon prio=8 tid=7ff64e13d800 nid=0x11767f000 in Object.wait() [11767e000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <7f44c0ed0> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118) - locked <7f44c0ed0> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:171) "Reference Handler" daemon prio=10 tid=7ff64e13c800 nid=0x11757c000 in Object.wait() [11757b000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <7f44c0018> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:485) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116) - locked <7f44c0018> (a java.lang.ref.Reference$Lock) "main" prio=5 tid=7ff64c800800 nid=0x10f709000 runnable [10f708000] java.lang.Thread.State: RUNNABLE at test.Test.main(Test.java:7) "VM Thread" prio=9 tid=7ff64e138000 nid=0x117479000 runnable "Gang worker#0 (Parallel GC Threads)" prio=9 tid=7ff64e000000 nid=0x112b0f000 runnable "Gang worker#1 (Parallel GC Threads)" prio=9 tid=7ff64e001000 nid=0x112c12000 runnable "Gang worker#2 (Parallel GC Threads)" prio=9 tid=7ff64e001800 nid=0x112d15000 runnable "Gang worker#3 (Parallel GC Threads)" prio=9 tid=7ff64e002000 nid=0x112e18000 runnable "Gang worker#4 (Parallel GC Threads)" prio=9 tid=7ff64e002800 nid=0x112f1b000 runnable "Gang worker#5 (Parallel GC Threads)" prio=9 tid=7ff64e003800 nid=0x11301e000 runnable "Gang worker#6 (Parallel GC Threads)" prio=9 tid=7ff64e004000 nid=0x113121000 runnable "Gang worker#7 (Parallel GC Threads)" prio=9 tid=7ff64e004800 nid=0x113224000 runnable "Concurrent Mark-Sweep GC Thread" prio=9 tid=7ff64e0e2000 nid=0x1170f0000 runnable "Gang worker#0 (Parallel CMS Threads)" prio=9 tid=7ff64e0e0800 nid=0x1166ea000 runnable "Gang worker#1 (Parallel CMS Threads)" prio=9 tid=7ff64e0e1800 nid=0x1167ed000 runnable "VM Periodic Task Thread" prio=10 tid=7ff64c820800 nid=0x117d99000 waiting on condition "Exception Catcher Thread" prio=10 tid=7ff64c801800 nid=0x10f936000 runnable JNI global references: 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。
./jhat -stack true -refs true -port 8080 -baseline /xxx/dump-baseline.core -debug 1 /xxx/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。
javap -c -v ./Test.class
Classfile xxx/Test.class
Last modified Nov 11, 2013; size 441 bytes
MD5 checksum 69488187cc8a8f166bc6dd0d517fb4cb
Compiled from "Test.java"
public class test.Test
SourceFile: "Test.java"
minor version: 0
major version: 50
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#16 // java/lang/Object."":()V
#2 = Integer 1048576
#3 = Fieldref #17.#18 // java/lang/System.out:Ljava/io/PrintStream;
#4 = Methodref #19.#20 // java/io/PrintStream.println:(Ljava/lang/Object;)V
#5 = Class #21 // test/Test
#6 = Class #22 // java/lang/Object
#7 = Utf8
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 main
#12 = Utf8 ([Ljava/lang/String;)V
#13 = Utf8 StackMapTable
#14 = Utf8 SourceFile
#15 = Utf8 Test.java
#16 = NameAndType #7:#8 // "":()V
#17 = Class #23 // java/lang/System
#18 = NameAndType #24:#25 // out:Ljava/io/PrintStream;
#19 = Class #26 // java/io/PrintStream
#20 = NameAndType #27:#28 // println:(Ljava/lang/Object;)V
#21 = Utf8 test/Test
#22 = Utf8 java/lang/Object
#23 = Utf8 java/lang/System
#24 = Utf8 out
#25 = Utf8 Ljava/io/PrintStream;
#26 = Utf8 java/io/PrintStream
#27 = Utf8 println
#28 = Utf8 (Ljava/lang/Object;)V
{
public test.Test();
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 4: 0
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: ldc #2 // int 1048576
2: newarray byte
4: astore_1
5: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
8: aload_1
9: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
12: goto 0
LineNumberTable:
line 7: 0
line 8: 5
line 9: 12
StackMapTable: number_of_entries = 1
frame_type = 0 /* same */
}
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