多线程中的互锁现象

互锁现象

　　在多线程中经常会出现线程间的互锁现象。

　　进程间的互锁，假设有两个线程，线程1的完成需要用线程2 的资源，线程2的完成需要用线程1的资源。当两个线程都启动后，线程1 等待线程2 的资源， 线程2 等待线程1 的资源，以致于两个线程都没办法完成，都在等待状态。这种现象就叫做线程间的互锁。

//线程1 
public class LockRunnable1 implements Runnable {
    private String lock1 = "lock1";
    private String lock2 = "lock2";
    @Override
    public void run() {
        synchronized (lock1) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println("我在等你把lock2给我！");
            synchronized (lock2) {              
            }
        }       
    }
}
//线程2
public class LockRunnable2 implements Runnable{
    private String lock1 = "lock1";
    private String lock2 = "lock2";
    @Override
    public void run() {
        synchronized (lock2) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println("我在等你把lock1给我！");
            synchronized (lock1) {              
            }
        }       
    }
}
//测试
public class TestRunnable {
    public static void main(String[] args) {
        //测试互锁
        LockRunnable1 run1 = new LockRunnable1();
        LockRunnable2 run2 = new LockRunnable2();   
        Thread t1 = new Thread(run1, "小红");
        Thread t2 = new Thread(run2,"小名");
        t1.start();
        t2.start();     
    }
}

运行结果：

　　从结果中可以看到，程序一直在运行。因为红色圆圈处一直处于运行状态！

如何解决互锁

　　可以通过调用同步锁的wait()和notify()方法来解决互锁问题。

　　首先说一下wait()方法，wait()方法必须用在同步锁中。调用wait()方法，线程进入等待状态，直到有notify()唤醒它。

　　nitify()方法同样也需要用在同步锁中。作用是将在等待状态的进程唤醒，进入运行状态。

//线程1
public class LockRunnable1 implements Runnable {
    private String lock1 = "lock1";
    private String lock2 = "lock2";
    @Override
    public void run() {
        System.out.println("run1 开始运行！");
        synchronized (lock1) {
            //休眠1秒
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("run1释放lock1， 进入等待状态！");
            try {
                lock1.wait();//进入等待状态。
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("run1在等你把lock2给我！");
            synchronized (lock2) {  
                System.out.println("run1获得lock2，继续运行！");
            }
        }           
    }
}
//线程2
public class LockRunnable2 implements Runnable{
    private String lock1 = "lock1";
    private String lock2 = "lock2";
    @Override
    public void run() {
        System.out.println("run2开始运行！");
        synchronized (lock2) {
            //休眠1秒
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("run2在等run1把lock1给他！");
            synchronized (lock1) {
                System.out.println("run2获得lock1， 继续执行！");
                //run2唤醒lock1
                lock1.notify(); 
                //休眠3S
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }           
                System.out.println("run2 运行完毕！");
            }
        }       
    }
}
//测试
public class TestRunnable {
    public static void main(String[] args) {
    //测试互锁
        LockRunnable1 run1 = new LockRunnable1();
        LockRunnable2 run2 = new LockRunnable2();

        Thread t1 = new Thread(run1, "小红");
        Thread t2 = new Thread(run2,"小名");
        t1.start();
        t2.start();
    }
}

运行结果：