Java死锁示例–如何分析死锁情况

死锁是两个或多个线程永远被阻塞的编程情况，这种情况发生在至少两个线程和两个或更多资源的情况下。在这里，我编写了一个简单的程序，该程序将导致死锁情况，然后我们将看到如何对其进行分析。

Java死锁示例

package com.journaldev.threads;
public class ThreadDeadlock {public static void main(String[] args) throws InterruptedException {Object obj1 = new Object();Object obj2 = new Object();Object obj3 = new Object();Thread t1 = new Thread(new SyncThread(obj1, obj2), 't1');Thread t2 = new Thread(new SyncThread(obj2, obj3), 't2');Thread t3 = new Thread(new SyncThread(obj3, obj1), 't3');t1.start();Thread.sleep(5000);t2.start();Thread.sleep(5000);t3.start();}
}
class SyncThread implements Runnable{private Object obj1;private Object obj2;public SyncThread(Object o1, Object o2){this.obj1=o1;this.obj2=o2;}@Overridepublic void run() {String name = Thread.currentThread().getName();System.out.println(name + ' acquiring lock on '+obj1);synchronized (obj1) {System.out.println(name + ' acquired lock on '+obj1);work();System.out.println(name + ' acquiring lock on '+obj2);synchronized (obj2) {System.out.println(name + ' acquired lock on '+obj2);work();}System.out.println(name + ' released lock on '+obj2);}System.out.println(name + ' released lock on '+obj1);System.out.println(name + ' finished execution.');}private void work() {try {Thread.sleep(30000);} catch (InterruptedException e) {e.printStackTrace();}}
}

在上面的程序中，SyncThread实现了Runnable接口，并且通过使用同步块逐个获取每个对象上的锁，从而对两个对象起作用。

在main方法中，我为SyncThread运行了三个线程，并且每个线程之间都有一个共享资源。线程以这样的方式运行：它将能够获取第一个对象的锁定，但是当它试图获取第二个对象的锁定时，它将进入等待状态，因为它已经被另一个线程锁定了。这对导致死锁的线程之间的资源形成了循环依赖性。

当我执行上面的程序时，这是生成的输出，但是由于死锁，程序永远不会终止。

t1 acquiring lock on java.lang.Object@6d9dd520
t1 acquired lock on java.lang.Object@6d9dd520
t2 acquiring lock on java.lang.Object@22aed3a5
t2 acquired lock on java.lang.Object@22aed3a5
t3 acquiring lock on java.lang.Object@218c2661
t3 acquired lock on java.lang.Object@218c2661
t1 acquiring lock on java.lang.Object@22aed3a5
t2 acquiring lock on java.lang.Object@218c2661
t3 acquiring lock on java.lang.Object@6d9dd520

在这里，我们可以从输出中清楚地确定死锁情况，但是在实际应用中，很难找到死锁情况并对其进行调试。

分析死锁

要分析死锁，我们需要查看应用程序的Java线程转储，在上一篇文章中，我解释了如何使用VisualVM Profiler或jstack实用程序生成线程转储。

这是上面程序的线程转储。

2012-12-27 19:08:34
Full thread dump Java HotSpot(TM) 64-Bit Server VM (23.5-b02 mixed mode):
'Attach Listener' daemon prio=5 tid=0x00007fb0a2814000 nid=0x4007 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE
'DestroyJavaVM' prio=5 tid=0x00007fb0a2801000 nid=0x1703 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE
't3' prio=5 tid=0x00007fb0a204b000 nid=0x4d07 waiting for monitor entry [0x000000015d971000]java.lang.Thread.State: BLOCKED (on object monitor)at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)- waiting to lock <0x000000013df2f658> (a java.lang.Object)- locked <0x000000013df2f678> (a java.lang.Object)at java.lang.Thread.run(Thread.java:722)
't2' prio=5 tid=0x00007fb0a1073000 nid=0x4207 waiting for monitor entry [0x000000015d209000]java.lang.Thread.State: BLOCKED (on object monitor)at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)- waiting to lock <0x000000013df2f678> (a java.lang.Object)- locked <0x000000013df2f668> (a java.lang.Object)at java.lang.Thread.run(Thread.java:722)
't1' prio=5 tid=0x00007fb0a1072000 nid=0x5503 waiting for monitor entry [0x000000015d86e000]java.lang.Thread.State: BLOCKED (on object monitor)at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)- waiting to lock <0x000000013df2f668> (a java.lang.Object)- locked <0x000000013df2f658> (a java.lang.Object)at java.lang.Thread.run(Thread.java:722)
'Service Thread' daemon prio=5 tid=0x00007fb0a1038000 nid=0x5303 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE
'C2 CompilerThread1' daemon prio=5 tid=0x00007fb0a1037000 nid=0x5203 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE
'C2 CompilerThread0' daemon prio=5 tid=0x00007fb0a1016000 nid=0x5103 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE
'Signal Dispatcher' daemon prio=5 tid=0x00007fb0a4003000 nid=0x5003 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE
'Finalizer' daemon prio=5 tid=0x00007fb0a4800000 nid=0x3f03 in Object.wait() [0x000000015d0c0000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)- locked <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:177)
'Reference Handler' daemon prio=5 tid=0x00007fb0a4002000 nid=0x3e03 in Object.wait() [0x000000015cfbd000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x000000013de75320> (a java.lang.ref.Reference$Lock)at java.lang.Object.wait(Object.java:503)at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)- locked <0x000000013de75320> (a java.lang.ref.Reference$Lock)
'VM Thread' prio=5 tid=0x00007fb0a2049800 nid=0x3d03 runnable
'GC task thread#0 (ParallelGC)' prio=5 tid=0x00007fb0a300d800 nid=0x3503 runnable
'GC task thread#1 (ParallelGC)' prio=5 tid=0x00007fb0a2001800 nid=0x3603 runnable
'GC task thread#2 (ParallelGC)' prio=5 tid=0x00007fb0a2003800 nid=0x3703 runnable
'GC task thread#3 (ParallelGC)' prio=5 tid=0x00007fb0a2004000 nid=0x3803 runnable
'GC task thread#4 (ParallelGC)' prio=5 tid=0x00007fb0a2005000 nid=0x3903 runnable
'GC task thread#5 (ParallelGC)' prio=5 tid=0x00007fb0a2005800 nid=0x3a03 runnable
'GC task thread#6 (ParallelGC)' prio=5 tid=0x00007fb0a2006000 nid=0x3b03 runnable
'GC task thread#7 (ParallelGC)' prio=5 tid=0x00007fb0a2006800 nid=0x3c03 runnable
'VM Periodic Task Thread' prio=5 tid=0x00007fb0a1015000 nid=0x5403 waiting on condition
JNI global references: 114
Found one Java-level deadlock:
=============================
't3':waiting to lock monitor 0x00007fb0a1074b08 (object 0x000000013df2f658, a java.lang.Object),which is held by 't1'
't1':waiting to lock monitor 0x00007fb0a1010f08 (object 0x000000013df2f668, a java.lang.Object),which is held by 't2'
't2':waiting to lock monitor 0x00007fb0a1012360 (object 0x000000013df2f678, a java.lang.Object),which is held by 't3'
Java stack information for the threads listed above:
===================================================
't3':at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)- waiting to lock <0x000000013df2f658> (a java.lang.Object)- locked <0x000000013df2f678> (a java.lang.Object)at java.lang.Thread.run(Thread.java:722)
't1':at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)- waiting to lock <0x000000013df2f668> (a java.lang.Object)- locked <0x000000013df2f658> (a java.lang.Object)at java.lang.Thread.run(Thread.java:722)
't2':at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)- waiting to lock <0x000000013df2f678> (a java.lang.Object)- locked <0x000000013df2f668> (a java.lang.Object)at java.lang.Thread.run(Thread.java:722)
Found 1 deadlock.

线程转储输出清楚地显示了死锁情况，并且所涉及的线程和资源导致了死锁情况。

为了分析死锁，我们需要找出状态为BLOCKED的线程，然后寻找它正在等待锁定的资源，每个资源都有一个唯一的ID，通过它我们可以找到哪个线程已经在对象上保持了锁定。例如，线程“ t3”正在等待锁定0x000000013df2f658，但它已被线程“ t1”锁定。

一旦我们分析了死锁情况并找出了导致死锁的线程，就需要进行代码更改以避免死锁情况。

避免死锁

这些是我们可以避免大多数僵局情况的一些准则。

避免嵌套锁 ：这是造成死锁的最常见原因，如果已经持有另一个资源，请避免锁定另一个资源。如果仅使用一个对象锁，则几乎不可能出现死锁情况。例如，这是不带嵌套锁的run（）方法的另一种实现，并且程序成功运行且没有死锁情况。

public void run() {String name = Thread.currentThread().getName();System.out.println(name + ' acquiring lock on ' + obj1);synchronized (obj1) {System.out.println(name + ' acquired lock on ' + obj1);work();}System.out.println(name + ' released lock on ' + obj1);System.out.println(name + ' acquiring lock on ' + obj2);synchronized (obj2) {System.out.println(name + ' acquired lock on ' + obj2);work();}System.out.println(name + ' released lock on ' + obj2);System.out.println(name + ' finished execution.');}

仅锁定所需条件 ：您应该仅对必须处理的资源获得锁定，例如，在上述程序中，我正在锁定完整的Object资源，但是如果我们仅对其中一个字段感兴趣，则应仅锁定该对象具体字段不完整的对象。
避免无限期地等待 ：如果两个线程正在使用线程连接无限期地等待彼此完成，则可能会陷入死锁。如果您的线程必须等待另一个线程完成，则始终最好使用join，并在等待线程完成的最长时间内使用它。

参考： Java死锁示例–如何从开发人员食谱博客的JCG合作伙伴 Pankaj Kumar中分析死锁情况。