死锁就是:一个宝藏需要两把钥匙来打开,同时间正好来了两个人,他们一人一把钥匙,但是双方都再等对方能交出钥匙来打开宝藏,谁都没交出自己的那把钥匙.就这样这俩人一直僵持下去,直到开发人员发现这个局面.

public class ThreadTest implements Runnable {

    private int trainCount = 100;
    private final Object lock = new Object();
    public boolean flag;

    @Override
    public void run() {
        while (true) {
            if (flag) {
                synchronized (lock) {
                    sell1();
                    System.out.println(Thread.currentThread().getName() + " 拿到lock锁");
                }
            } else {
                sell1();
            }
        }
    }

    public synchronized void sell1() {
        System.out.println(Thread.currentThread().getName() + " 拿到this锁");
        synchronized (lock) {
            System.out.println(Thread.currentThread().getName() + " 拿到lock锁");
            if (trainCount > 0) {
                System.out.println(Thread.currentThread().getName() + " Sell a ticket:" + (100 - trainCount + 1));
                trainCount--;
            }
        }
    }


    public static void main(String[] args) {
        ThreadTest threadTest = new ThreadTest();
        Thread thread = new Thread(threadTest, "窗口①");
        Thread thread2 = new Thread(threadTest, "窗口②");
        thread.start();
        threadTest.flag = true;
        thread2.start();

    }
}

上面代码就是一个死锁
导致死锁的原因在于
线程①先拿到同步函数的this锁，在拿到同步代码块中的lock锁
线程②先拿到同步代码块中的lock锁，在拿到同步函数的this锁
互补释放锁导致的

解决死锁的方法：
不要嵌套同步代码块