Java jstack 命令详解

Java 中 jstack 命令可以用来查看运行的 Java 进程下，多线程的运行情况，我们用一个死锁的例子来看：
比如下面这段死锁的代码：

public class DeadLock {

    private static Object objA = new Object();
    private static Object objB = new Object();

    public static void main(String[] args) {
        Thread thread1 = new Thread(new Thread1());
        Thread thread2 = new Thread(new Thread2());
        thread1.start();
        thread2.start();
    }

    private static class Thread1 implements Runnable{

        @Override
        public void run() {
            synchronized (objA) {
                System.out.println("线程1得到A对象的锁");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (objB) {
                    System.out.println("线程1得到B对象的锁");
                }
            }
        }
    }

    private static class Thread2 implements Runnable{

        @Override
        public void run() {
            synchronized (objB) {
                System.out.println("线程2得到B对象的锁");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (objA) {
                    System.out.println("线程2得到A对象的锁");
                }
            }
        }   
    }
}

当然，运行结果只能看到两个线程只拿到了一个锁，在一直等待对方的锁。

线程1得到A对象的锁
线程2得到B对象的锁

我们可以使用 jps 来查看对应的 PID ，然后使用 jstack 来查看其线程情况：

[root@admin ~]# jps
42576 Jps
43616 DeadLock

[root@admin ~]# jstack 43616
Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.45-b08 mixed mode):

"DestroyJavaVM" prio=6 tid=0x00000000047c1000 nid=0x9878 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Thread-1" prio=6 tid=0x0000000010aa3000 nid=0xafa0 waiting for monitor entry [0x000000001105f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.zaimeibian.Test$Thread2.run(Test.java:46)
        - waiting to lock <0x00000007c099cc20> (a java.lang.Object)
        - locked <0x00000007c099cc30> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)

"Thread-0" prio=6 tid=0x0000000010aa2800 nid=0xae74 waiting for monitor entry [0x0000000010f5f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.zaimeibian.Test$Thread1.run(Test.java:27)
        - waiting to lock <0x00000007c099cc30> (a java.lang.Object)
        - locked <0x00000007c099cc20> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)

"Service Thread" daemon prio=6 tid=0x000000000f10a000 nid=0x9a8c runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=10 tid=0x000000000f109800 nid=0xaf28 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=10 tid=0x000000000f105800 nid=0x85dc waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Attach Listener" daemon prio=10 tid=0x000000000f104800 nid=0xac04 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=10 tid=0x000000000f102000 nid=0xa678 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=8 tid=0x000000000f0bd000 nid=0xaed8 in Object.wait() [0x000000001045f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000007c0905568> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
        - locked <0x00000007c0905568> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:189)

"Reference Handler" daemon prio=10 tid=0x000000000f0b2000 nid=0xaedc in Object.wait() [0x000000001035f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000007c09050f0> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:503)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
        - locked <0x00000007c09050f0> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=10 tid=0x000000000f0b0000 nid=0xaef0 runnable

"GC task thread#0 (ParallelGC)" prio=6 tid=0x00000000047d6000 nid=0xacb0 runnable

"GC task thread#1 (ParallelGC)" prio=6 tid=0x00000000047d8000 nid=0xaee0 runnable

"GC task thread#2 (ParallelGC)" prio=6 tid=0x00000000047d9800 nid=0xaed4 runnable

"GC task thread#3 (ParallelGC)" prio=6 tid=0x00000000047db000 nid=0xac54 runnable

"VM Periodic Task Thread" prio=10 tid=0x000000000f132000 nid=0xaff0 waiting on condition

JNI global references: 105


Found one Java-level deadlock:
=============================
"Thread-1":
  waiting to lock monitor 0x000000000f0ba488 (object 0x00000007c099cc20, a java.lang.Object),
  which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x000000000f0bcf28 (object 0x00000007c099cc30, a java.lang.Object),
  which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
        at com.zaimeibian.Test$Thread2.run(Test.java:46)
        - waiting to lock <0x00000007c099cc20> (a java.lang.Object)
        - locked <0x00000007c099cc30> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)
"Thread-0":
        at com.zaimeibian.Test$Thread1.run(Test.java:27)
        - waiting to lock <0x00000007c099cc30> (a java.lang.Object)
        - locked <0x00000007c099cc20> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:744)

Found 1 deadlock.

我们可以看到 jstack 打印出了线程的状态，而且发现一个死锁。
另外，线程状态有以下几种：
- RUNNABLE 线程运行中或 I/O 等待
- BLOCKED 线程在等待 monitor 锁( synchronized 关键字)
- TIMED_WAITING 线程在等待唤醒，但设置了时限
- WAITING 线程在无限等待唤醒