jstack命令可以用来查看运行的Java进程下，多线程的运行情况，语句如“[root@admin ~]# jstack 43616 Full thread dump Java HotSpot(TM) 64-Bit Server...”。
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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
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