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Java并发编程（9）：死锁（含代码）

当线程需要同时持有多个锁时，有可能产生死锁。考虑如下情形：

线程A当前持有互斥所锁lock1，线程B当前持有互斥锁lock2。接下来，当线程A仍然持有lock1时，它试图获取lock2，因为线程B正持有lock2，因此线程A会阻塞等待线程B对lock2的释放。如果此时线程B在持有lock2的时候，也在试图获取lock1，因为线程A正持有lock1，因此线程B会阻塞等待A对lock1的释放。二者都在等待对方所持有锁的释放，而二者却又都没释放自己所持有的锁，这时二者便会一直阻塞下去。这种情形称为死锁。

下面给出一个两个线程间产生死锁的示例，如下：

public class Deadlock extends Object {  private String objID;   public Deadlock(String id) {   objID = id;  }   public synchronized void checkOther(Deadlock other) {   print("entering checkOther()");   try { Thread.sleep(2000); }    catch ( InterruptedException x ) { }   print("in checkOther() - about to " + "invoke 'other.action()'");    //调用other对象的action方法，由于该方法是同步方法，因此会试图获取other对象的对象锁   other.action();   print("leaving checkOther()");  }   public synchronized void action() {   print("entering action()");   try { Thread.sleep(500); }    catch ( InterruptedException x ) { }   print("leaving action()");  }   public void print(String msg) {   threadPrint("objID=" + objID + " - " + msg);  }   public static void threadPrint(String msg) {   String threadName = Thread.currentThread().getName();   System.out.println(threadName + ": " + msg);  }   public static void main(String[] args) {   final Deadlock obj1 = new Deadlock("obj1");   final Deadlock obj2 = new Deadlock("obj2");    Runnable runA = new Runnable() {     public void run() {      obj1.checkOther(obj2);     }    };    Thread threadA = new Thread(runA, "threadA");   threadA.start();    try { Thread.sleep(200); }    catch ( InterruptedException x ) { }    Runnable runB = new Runnable() {     public void run() {      obj2.checkOther(obj1);     }    };    Thread threadB = new Thread(runB, "threadB");   threadB.start();    try { Thread.sleep(5000); }    catch ( InterruptedException x ) { }    threadPrint("finished sleeping");    threadPrint("about to interrupt() threadA");   threadA.interrupt();    try { Thread.sleep(1000); }    catch ( InterruptedException x ) { }    threadPrint("about to interrupt() threadB");   threadB.interrupt();    try { Thread.sleep(1000); }    catch ( InterruptedException x ) { }    threadPrint("did that break the deadlock?");  } }

运行结果如下：

Java并发编程（9）：死锁（含代码）