Java死锁范例以及如何分析死锁

来源：互联网发布：达芬奇调色软件语言编辑：程序博客网时间：2024/04/29 13:07

转载说明：
本文由 ImportNew - 范琦琦翻译自 journaldev。欢迎加入翻译小组。转载请见文末要求。

死锁是两个甚至多个线程，由于彼此等待获取对方资源，被永久阻塞时的一种运行局面，这种局面的生成伴随着至少两个线程和两个或者多个资源。在这里我已写好一个简单的程序，它在运行时将会引起死锁，然后我们就会明白如何分析它。

Java死锁范例

ThreadDeadlock.java

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(5*1000);        t2.start();        Thread.sleep(5*1000);        t3.start();    }}class SyncThread implements Runnable {    private Object obj1;    private Object obj2;    public SyncThread(Object obj1, Object obj2) {        this.obj1 = obj1;        this.obj2 = obj2;    }    @Override    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(name);            System.out.println(name + " attempt to acquire lock on " + obj2);            synchronized (obj2){                System.out.println(name + " acquired lock on " + obj2);                work(name);            }            System.out.println(name + " release lock on " + obj2);        }        System.out.println(name + " release lock on " + obj1);        System.out.println(name + " finished execution");    }    private void work(String name) {        try {            Thread.sleep(10*1000);            System.out.println(name + " is working ...");        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

在上面的程序中同步线程正完成Runnable的接口，它工作的是两个对象，这两个对象向对方寻求死锁而且都在使用同步阻塞。

在主函数中，我使用了三个为同步线程运行的线程，而且在其中每个线程中都有一个可共享的资源。

这些线程以向第一个对象获取封锁这种方式运行。但是当它试着像第二个对象获取封锁时，它就会进入等待状态，因为它已经被另一个线程封锁住了。这样，在线程引起死锁的过程中，就形成了一个依赖于资源的循环。

当我执行上面的程序时，就产生如下了输出，但是程序却因为死锁无法停止。

t1 acquiring lock on java.lang.Object@6b8cebb0t1 acquired lock on java.lang.Object@6b8cebb0t2 acquiring lock on java.lang.Object@441ded1et2 acquired lock on java.lang.Object@441ded1et1 is working ...t1 attempt to acquire lock on java.lang.Object@441ded1et3 acquiring lock on java.lang.Object@724d9353t3 acquired lock on java.lang.Object@724d9353t2 is working ...t2 attempt to acquire lock on java.lang.Object@724d9353t3 is working ...t3 attempt to acquire lock on java.lang.Object@6b8cebb0

在此我们可以清楚地在输出结果中辨认出死锁局面，但是在我们实际生活所用的应用中，发现死锁并将它排除是非常难的。

分析死锁

为了分析一个死锁，我们需要关注下应用中的Java线程转存，例如可以使用VisualVM收集资料，或者jstack应用程序产生线程转存。

以下就是上述程序的线程转存内容。

2012-12-27 19:08:34Full 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: 114Found 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.

这三个线程转存的输出清楚地说明了死锁环境和线程，以及包含死锁环境的资源。

为了分析死锁，我们需要关注死锁状态的线程，然后资源再等待去封锁，每一个资源都有一个独特的ID，有了这个ID我们就能发现是哪一个进程已经封锁住对象。举个例子，线程“t3”正在等待封锁0x000000013df2f658,但是它已经被线程“t1”封锁住了。

当我们分析死锁环境的时候，如果发现线程正在引起死锁，这是我们就要改变代码来避免死锁的产生。
避免死锁

有很多方针可供我们使用来避免死锁的局面。

①。避免嵌套封锁：这是死锁最主要的原因的，如果你已经有一个资源了就要避免封锁另一个资源。如果你运行时只有一个对象封锁，那是几乎不可能出现一个死锁局面的。例如，这里是另一个运行中没有嵌套封锁的run()方法，而且程序运行没有死锁局面，运行得很成功。

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.");}

②。只对有请求的资源进行封锁：你应当只想你要运行的资源获取封锁，比如在上述程序中我在封锁的完全的对象资源。但是如果我们只对它所属领域中的一个感兴趣，那我们应当封锁住那个特殊的领域而并非完全的对象。

③。避免无限期的等待：如果两个线程正在等待对象结束，无限期的使用线程加入，如果你的线程必须要等待另一个线程的结束，若是等待进程的结束加入最好准备最长时间。

原文链接： journaldev 翻译： ImportNew.com - 范琦琦
译文链接： http://www.importnew.com/9668.html
[ 转载请保留原文出处、译者和译文链接。]

死锁检测方法补充：

除了上述使用线程转储文件分析死锁外，jdk自带了GUI可视化死锁检测工具jconsole（内置了内存/线程/类/VM概况/MBeans等项目的分析）

以Linux系统上的使用为例，具体操作步骤为：
①。Terminal中打开jconsole，启动jconsole：

②。选择程序所在的java进程：
这里写图片描述

③。点击“Detect Deadlock”按钮:
这里写图片描述

④。选择jconsole检测出来的具体死锁线程，定位死锁位置：
这里写图片描述

标红处提示我们，线程t2请求公共资源，而此时却被线程t3持有，并被阻塞住。通过查看线程t3，t1，可以发现t1被t2阻塞，t2被t3阻塞，t3被t1阻塞，任一线程均无法释放自己，故形成死锁。

0 0