java中死锁的问题

死锁。

死锁的原因：同步中嵌套同步,多个进程同时抢到不同的锁，都在等待，互不相让

在该程序中, 假如线程0在run方法中打开obj锁，同时1线程在show方法打开this锁，

此时下一步 ，线程0和线程1都需要双方的锁，故都在等待，从而出现了死锁

class Ticket implements Runnable {
    
    private  int tick = 100;
    Object obj = new Object();
    boolean flag = true;
    public void run() {  
        if (flag) {
            while(true) {
                synchronized(obj) {//这个obj锁与28行的obj是同一个锁，虽然锁的位置不一样
                    show();
                }
            }
        }
        else                                  
            while(true)
                this.show();
        
    }
    
    public synchronized void show() {  //this锁
        synchronized(obj) {
            if (tick>0) {
                try{Thread.sleep(10);}catch(Exception e){}
                System.out.println(Thread.currentThread().getName()+"code: "+tick--);
            }
        }
        
    }
    
}

class DeadLockDemo {

    public static void main(String[] args) {
        Ticket t = new Ticket();
        
        Thread t1 = new Thread(t);
        Thread t2 = new Thread(t);

        t1.start();
        try{Thread.sleep(10);}catch(Exception e){}  
        t.flag = false;
        t2.start();
        
    }
}

练习

if和else中，如果两个线程各自都抢到了一把锁，则死锁，

若某个线程提前抢到了两把锁 则不会出现死锁了

class Test implements Runnable {
    
    private boolean flag;
    Test(boolean flag) {
        this.flag = flag;
    }
    public void run() {
        if (flag) {
            synchronized(MyLock.locka) {
                System.out.println("if locka");
                synchronized(MyLock.lockb) {
                    System.out.println("if lockb");
                }
            }
        }
        else {
            synchronized(MyLock.lockb) {
                System.out.println("else lockb");
                synchronized(MyLock.locka) {
                    System.out.println("else lockb");
                }
            }
        }
    }
    
}

class MyLock {
    static Object locka = new Object();  //静态后方便直接调用
    static Object lockb = new Object();
}

class DeadLockTest {

    public static void main(String[] args) {
        Thread t1 = new Thread(new Test(true));
        Thread t2 = new Thread(new Test(false));
        t1.start();
        try{Thread.sleep(10);}catch(Exception e){}  //加上就不死锁了，不加可能死锁
        t2.start();
    }
}