jstack是一個很實用的工具，不僅能定位到死迴圈的位置，還能找到死鎖的位置，這個工具對我們直接在生產機上定位錯誤帶來了極大的方便，尤其是那些不容易復現的錯誤。

首先呢，先寫一個能產生死鎖的程式碼。程式碼邏輯很簡單，咱就產生兩個執行緒，兩個執行緒分別執行加鎖與放鎖兩個動作，啥時候產生死鎖呢，就是在兩個執行緒都釋放鎖以後，又同時加鎖，於是互相僵持，互相等待，產生死鎖。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * Created by Kevin on 11/4/16.
 */
 

class MultiThread implements Runnable{
    private Object o1;
    private Object o2;
    private int flag;

    public MultiThread(Object o1, Object o2, int flag) {
        this.o1 = o1;
        this.o2 = o2;
        this.flag = flag;
    }

    public void task1() throws InterruptedException{
        synchronized
 
 (o1){
            synchronized (o2){
                System.out.println(Thread.currentThread().getName());
            }
        }
    }

    public void task2() throws InterruptedException{
        synchronized (o2){
            Thread.yield();
            synchronized (o1) {
                System.out.println(Thread.currentThread().getName());
            }
        }
    }


    @Override
 

    public void run() {
        while (true) {
            try {
                if (this.flag == 1) {
                    this.task1();
                } else {
                    this.task2();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class DeadLock {
    public static void main(String[] args){
        Object o1 = new Object();
        Object o2 = new Object();

        ExecutorService executorService = Executors.newFixedThreadPool(10);

        for (int flag = 0; flag < 2; flag++) {
            executorService.execute(new MultiThread(o1, o2, flag));
        }
    }
}

執行程式碼，你會發現控制檯過一會兒就不動了，但是程式執行沒有終止，下面用jstack看看到底啥問題。

首先要找到該程序的pid

$ jps
11523 Main
27894 AppMain
27912 Launcher
27933 Jps
12046 JConsole

然後可以看到pid=27894，辣麼用jstack檢視一下27894：

$ jstack 27894
2016-11-07 14:59:55
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.101-b13 mixed mode):

"Attach Listener" #13 daemon prio=9 os_prio=0 tid=0x00007f5660001000 nid=0x6d3b runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"DestroyJavaVM" #12 prio=5 os_prio=0 tid=0x00007f569800a800 nid=0x6cf7 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"pool-1-thread-2" #11 prio=5 os_prio=0 tid=0x00007f5698156800 nid=0x6d07 waiting for monitor entry [0x00007f5684175000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at MultiThread.task1(DeadLock.java:21)
    - waiting to lock <0x00000000ec215b70> (a java.lang.Object)
    - locked <0x00000000ec215b60> (a java.lang.Object)
    at MultiThread.run(DeadLock.java:41)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

"pool-1-thread-1" #10 prio=5 os_prio=0 tid=0x00007f5698155000 nid=0x6d06 waiting for monitor entry [0x00007f5684276000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at MultiThread.task2(DeadLock.java:30)
    - waiting to lock <0x00000000ec215b60> (a java.lang.Object)
    - locked <0x00000000ec215b70> (a java.lang.Object)
    at MultiThread.run(DeadLock.java:43)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

"Monitor Ctrl-Break" #9 daemon prio=5 os_prio=0 tid=0x00007f5698152800 nid=0x6d05 runnable [0x00007f5684377000]
   java.lang.Thread.State: RUNNABLE
    at java.net.PlainSocketImpl.socketAccept(Native Method)
    at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)
    at java.net.ServerSocket.implAccept(ServerSocket.java:545)
    at java.net.ServerSocket.accept(ServerSocket.java:513)
    at com.intellij.rt.execution.application.AppMain$1.run(AppMain.java:90)
    at java.lang.Thread.run(Thread.java:745)

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x00007f56980da800 nid=0x6d03 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00007f56980bd000 nid=0x6d02 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00007f56980bb800 nid=0x6d01 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00007f56980b8800 nid=0x6d00 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007f56980b7000 nid=0x6cff runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f5698084000 nid=0x6cfe in Object.wait() [0x00007f5685682000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000ec188ee0> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
    - locked <0x00000000ec188ee0> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f569807f800 nid=0x6cfd in Object.wait() [0x00007f5685783000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000ec186b50> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:502)
    at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
    - locked <0x00000000ec186b50> (a java.lang.ref.Reference$Lock)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=0 tid=0x00007f5698078000 nid=0x6cfc runnable 

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f569801f800 nid=0x6cf8 runnable 

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f5698021000 nid=0x6cf9 runnable 

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f5698023000 nid=0x6cfa runnable 

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f5698024800 nid=0x6cfb runnable 

"VM Periodic Task Thread" os_prio=0 tid=0x00007f56980dd000 nid=0x6d04 waiting on condition 

JNI global references: 15


Found one Java-level deadlock:
=============================
"pool-1-thread-2":
  waiting to lock monitor 0x00007f56680062c8 (object 0x00000000ec215b70, a java.lang.Object),
  which is held by "pool-1-thread-1"
"pool-1-thread-1":
  waiting to lock monitor 0x00007f5668004e28 (object 0x00000000ec215b60, a java.lang.Object),
  which is held by "pool-1-thread-2"

Java stack information for the threads listed above:
===================================================
"pool-1-thread-2":
    at MultiThread.task1(DeadLock.java:21)
    - waiting to lock <0x00000000ec215b70> (a java.lang.Object)
    - locked <0x00000000ec215b60> (a java.lang.Object)
    at MultiThread.run(DeadLock.java:41)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
"pool-1-thread-1":
    at MultiThread.task2(DeadLock.java:30)
    - waiting to lock <0x00000000ec215b60> (a java.lang.Object)
    - locked <0x00000000ec215b70> (a java.lang.Object)
    at MultiThread.run(DeadLock.java:43)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

Found 1 deadlock.

看到沒，其中有一段長這個模樣：

Found one Java-level deadlock:
=============================
"pool-1-thread-2":
  waiting to lock monitor 0x00007f56680062c8 (object 0x00000000ec215b70, a java.lang.Object),
  which is held by "pool-1-thread-1"
"pool-1-thread-1":
  waiting to lock monitor 0x00007f5668004e28 (object 0x00000000ec215b60, a java.lang.Object),
  which is held by "pool-1-thread-2"

Java stack information for the threads listed above:
===================================================
"pool-1-thread-2":
    at MultiThread.task1(DeadLock.java:21)
    ......
    at MultiThread.run(DeadLock.java:41)
    ......
    at MultiThread.task2(DeadLock.java:30)
    ......
    at MultiThread.run(DeadLock.java:43)
    ......

Found 1 deadlock.

人家說了這裡有死鎖，哪個檔案，第幾行都告你了，剩下的就自己去改吧。