jstack是一个很实用的工具，不仅能定位到死循环的位置，还能找到死锁的位置，这个工具对我们直接在生产机上定位错误带来了极大的方便，尤其是那些不容易复现的错误。
首先呢，先写一个能产生死锁的代码。代码逻辑很简单，咱就产生两个线程，两个线程分别执行加锁与放锁两个动作，啥时候产生死锁呢，就是在两个线程都释放锁以后，又同时加锁，于是互相僵持，互相等待，产生死锁。
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));
}
}
}
运行代码，你会发现控制台过一会儿就不动了，但是程序运行没有终止，下面用jstack看看到底啥问题。
首先要找到该进程的pid
$ jps
11523 Main
27894 AppMain
27912 Launcher
27933 Jps
12046 JConsole然后可以看到pid=27894，辣么用jstack查看一下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.看到没，其中有一段长这个模样：
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.人家说了这里有死锁，哪个文件，第几行都告你了，剩下的就自己去改吧。
