Java调试

线上load高的问题排查步骤是：

1. 先用top找到耗资源的进程
2. ps+grep找到对应的java进程/线程
3. jstack分析哪些线程阻塞了，阻塞在哪里
4. jstat看看FullGC频率
5. jmap看看有没有内存泄露

使用jstack查看的例子：

C:\Users\Iammeaz>jstack 4696

2016-01-08 10:26:06

Full thread dump Java HotSpot(TM) 64-Bit Server VM (11.3-b02 mixed mode):

"Low Memory Detector" daemon prio=6 tid=0x000000006a63f800 nid=0x1790 runnable [0x0000000000000000..0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

"CompilerThread1" daemon prio=10 tid=0x000000006a63a400 nid=0x1780 waiting on condition [0x0000000000000000..0x000000006

   java.lang.Thread.State: RUNNABLE

"CompilerThread0" daemon prio=10 tid=0x000000006a62d000 nid=0x136c waiting on condition [0x0000000000000000..0x000000000

   java.lang.Thread.State: RUNNABLE

"JDWP Command Reader" daemon prio=6 tid=0x000000006a612c00 nid=0x13c0 runnable [0x0000000000000000..0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

"JDWP Event Helper Thread" daemon prio=6 tid=0x000000006a612000 nid=0x1bf4 runnable [0x0000000000000000..0x0000000000000

   java.lang.Thread.State: RUNNABLE

"JDWP Transport Listener: dt_socket" daemon prio=6 tid=0x000000006a60b000 nid=0x1be0 runnable [0x0000000000000000..0x000

   java.lang.Thread.State: RUNNABLE

"Attach Listener" daemon prio=10 tid=0x000000006a5f7c00 nid=0x1340 waiting on condition [0x0000000000000000..0x000000000

   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=10 tid=0x000000006a5a5800 nid=0x11e0 runnable [0x0000000000000000..0x0000000000000000]

   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=8 tid=0x000000006a594000 nid=0x13c4 in Object.wait() [0x000000006a9ef000..0x000000006a9ef890]

   java.lang.Thread.State: WAITING (on object monitor)

        at java.lang.Object.wait(Native Method)

        - waiting on <0x000000004a071210> (a java.lang.ref.ReferenceQueue$Lock)

        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:116)

        - locked <0x000000004a071210> (a java.lang.ref.ReferenceQueue$Lock)

        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:132)

        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=0x000000006a593400 nid=0x1598 in Object.wait() [0x000000006a8ef000..0x000000006a8

   java.lang.Thread.State: WAITING (on object monitor)

        at java.lang.Object.wait(Native Method)

        - waiting on <0x000000004a071078> (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 <0x000000004a071078> (a java.lang.ref.Reference$Lock)

"main" prio=6 tid=0x000000000062c800 nid=0x1190 at breakpoint[0x00000000025bf000..0x00000000025bfc50]

   java.lang.Thread.State: RUNNABLE

        at com.damonzh.maxsubseqsum.Demo01.MaxSubseqSum1(Demo01.java:15)

        at com.damonzh.maxsubseqsum.Demo01.main(Demo01.java:7)

"VM Thread" prio=10 tid=0x0000000000525000 nid=0x14cc runnable

"GC task thread#0 (ParallelGC)" prio=6 tid=0x0000000000471400 nid=0x105c runnable

"GC task thread#1 (ParallelGC)" prio=6 tid=0x0000000000472800 nid=0xe94 runnable

"GC task thread#2 (ParallelGC)" prio=6 tid=0x0000000000474000 nid=0x180c runnable

"GC task thread#3 (ParallelGC)" prio=6 tid=0x0000000000475400 nid=0xe14 runnable

"VM Periodic Task Thread" prio=10 tid=0x000000000046d400 nid=0x494 waiting on condition

JNI global references: 1062

但这个还不是重点，那么问题来了（blue fly is the strongest）：这些工具如何能获取远程Java进程的信息的？又是如何远程控制Java进程的运行的？

相信有不少人和我一样对这些工具的 实现原理 。

Java调试体系JPDA

Java虚拟机设计了专门的API接口供调试和监控虚拟机使用，被称为Java平台调试体系即Java Platform Debugger Architecture(JPDA)。