线程同步中的死锁

何为死锁

多线程各自持有不同的锁，并互相试图获取对方已持有的锁，导致无限等待的状况，称为死锁。比如:

public class DeadLock {
    public static void main(String[] args) {
        Object o1 = new ArrayList<>();
        Object o2 = new ArrayList<>();
        Thread t1 = new Thread(new MyRunnable1(o1, o2));
        Thread t2 = new Thread(new MyRunnable2(o1, o2));
        t1.start();
        t2.start();
    }
}

class MyRunnable1 implements Runnable {

    final Object o1;
    final Object o2;

    MyRunnable1(Object o1, Object o2) {
        this.o1 = o1;
        this.o2 = o2;
    }

    @Override
    public void run() {
        synchronized (o1) {
            System.out.println(Thread.currentThread().getName() + "已经获得o1的锁");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (o2) {
                System.out.println("fsf");
            }
        }
    }

}

class MyRunnable2 implements Runnable {

    final Object o1;
    final Object o2;

    MyRunnable2(Object o1, Object o2) {
        this.o1 = o1;
        this.o2 = o2;
    }

    @Override
    public void run() {
        synchronized (o2) {
            System.out.println(Thread.currentThread().getName() + "已经获得o2的锁");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (o1) {
                System.out.println("fsf");
            }
        }
    }

}

避免死锁

避免死锁的方法是线程获取锁的顺序要一致。