Java死锁程序(一)

àì夳堔傛蜴生んèń 2022-06-18 09:51 169阅读 0赞

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：

![复制代码][copycode.gif]

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{ @Override public 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obj2 synchronized(DeadLock.obj2){ System.out.println("Lock1 lock obj2"); } } } }catch(Exception e){ e.printStackTrace(); } } } class Lock2 implements Runnable{ @Override public 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(); } } }

![复制代码][copycode.gif]

运行的结果如图所示：

![933805-20160913075511945-266912857.jpg][]

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。

![复制代码][copycode.gif]

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{ @Override public 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{ @Override public 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(); } } }

![复制代码][copycode.gif]

运行结果为：

![933805-20160913075859039-460999224.jpg][]

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。

[copycode.gif]: /images/20220618/68a3f0b6897147c189e881f18607e4f7.png
[933805-20160913075511945-266912857.jpg]: /images/20220618/e16b7ebc3bf249dab0451aa35db16ff6.png
[933805-20160913075859039-460999224.jpg]: /images/20220618/76a6001d412e435896845504b2858ed2.png