Java死锁程序(一)

思路是创建两个字符串a和b，再创建两个线程A和B，让每个线程都用synchronized锁住字符串（A先锁a，再去锁b；B先锁b，再锁a），如果A锁住a，B锁住b，A就没办法锁住b，B也没办法锁住a，这时就陷入了死锁。直接贴代码：

public class DeadLock {public static String obj1 = "obj1";public static String obj2 = "obj2";public static void main(String[] args){Thread a = new Thread(new Lock1());Thread b = new Thread(new Lock2());a.start();b.start();}    
}
class Lock1 implements Runnable{@Overridepublic void run(){try{System.out.println("Lock1 running");while(true){synchronized(DeadLock.obj1){System.out.println("Lock1 lock obj1");Thread.sleep(3000);//获取obj1后先等一会儿，让Lock2有足够的时间锁住obj2synchronized(DeadLock.obj2){System.out.println("Lock1 lock obj2");}}}}catch(Exception e){e.printStackTrace();}}
}
class Lock2 implements Runnable{@Overridepublic void run(){try{System.out.println("Lock2 running");while(true){synchronized(DeadLock.obj2){System.out.println("Lock2 lock obj2");Thread.sleep(3000);synchronized(DeadLock.obj1){System.out.println("Lock2 lock obj1");}}}}catch(Exception e){e.printStackTrace();}}
}

运行的结果如图所示：

可以看到，Lock1获取obj1，Lock2获取obj2，但是它们都没有办法再获取另外一个obj，因为它们都在等待对方先释放锁，这时就是死锁。

如果我们只运行Lock1呢？修改一下main函数，把线程b注释掉。

public class DeadLock {public static String obj1 = "obj1";public static String obj2 = "obj2";public static void main(String[] args){Thread a = new Thread(new Lock1());//Thread b = new Thread(new Lock2());
        a.start();//b.start();
    }
}
class Lock1 implements Runnable{@Overridepublic void run(){try{System.out.println("Lock1 running");while(true){synchronized(DeadLock.obj1){System.out.println("Lock1 lock obj1");Thread.sleep(3000);synchronized(DeadLock.obj2){System.out.println("Lock1 lock obj2");}}}}catch(Exception e){e.printStackTrace();}}
}
class Lock2 implements Runnable{@Overridepublic void run(){try{System.out.println("Lock2 running");while(true){synchronized(DeadLock.obj2){System.out.println("Lock2 lock obj2");Thread.sleep(3000);synchronized(DeadLock.obj1){System.out.println("Lock2 lock obj1");}}}}catch(Exception e){e.printStackTrace();}}
}

运行结果为：

由于没有其它线程和Lock1争夺obj1和obj2，Lock1可以不断地循环获取并释放它们，这时没有死锁。