Java死锁程序(一)

    xiaoxiao2021-03-25  279

    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{ @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(); } } }

    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{ @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(); } } }

    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。

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