Java多线程并发环境下的synchronized死锁实例

Posted 2022-11-26 zhangphil

Java多线程并发环境下的synchronized死锁实例

Java并发多线程环境中，造成死锁的最简单的场景是：多线程中的一个线程T_A持有锁L1并且申请试图获得锁L2；而多线程中另外一个线程T_B持有锁L2并且试图申请获得锁L1。线程的锁申请操作是阻塞的，于是造成线程T_A和线程T_B无法正确获得想要的锁，两个线程被阻塞进入死锁状态。

例如：

public class MainClass 
    private Object lock1 = new Object();

    private Object lock2 = new Object();

    public static void main(String[] args) 
        MainClass mainClass = new MainClass();
        mainClass.demo();
    

    private void demo() 
        Thread ta = new Thread(new DeadA());
        Thread tb = new Thread(new DeadB());
        ta.start();
        tb.start();
    

    private class DeadA implements Runnable 
        private String id = "A";

        @Override
        public void run() 
            System.out.println(id + " 申请锁1...");
            synchronized (lock1) 
                System.out.println(id + " 获得锁1");

                System.out.println(id + " 申请锁2...");
                synchronized (lock2) 
                    //A得不到锁2，死锁
                    System.out.println(id + "获得锁2");
                
            

            System.out.println(id + " 运行结束");
        
    

    private class DeadB implements Runnable 
        private String id = "B";

        @Override
        public void run() 
            System.out.println(id + " 申请锁2...");
            synchronized (lock2) 
                System.out.println(id + " 获得锁2");

                System.out.println(id + " 申请锁1...");
                synchronized (lock1) 
                    //B得不到锁1，死锁
                    System.out.println(id + " 获得锁1");
                
            

            System.out.println(id + " 运行结束");
        
    

输出：

B 申请锁2...
A 申请锁1...
B 获得锁2
A 获得锁1
B 申请锁1...
A 申请锁2...

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