多线程的死锁

• Java线程死锁是一个经典的多线程问题, 因为不同的线程都在等待根本不可能被释放的锁, 从而导致线程中的任务无法完成
• 下面演示互相等待对方锁释放的死锁

public class DealThread implements Runnable {

    private String userName;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    //俩把锁
    private final Object lock1 = new Object();
    private final Object lock2 = new Object();


    @Override
    public void run() {

        if ("a".equals(userName)) {
            synchronized (lock1) {
                System.out.println("userName=" + userName + " :" + "进入lock1");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                synchronized (lock2) {
                    System.out.println("由lock1->进入->lock2");
                }
            }
        }

        if ("b".equals(userName)) {
            synchronized (lock2) {
                System.out.println("userName=" + userName + " :" + "进入lock2");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                synchronized (lock1) {
                    System.out.println("由lock2->进入->lock1");
                }
            }
        }

    }
}

• 启动类

public class Run {

    public static void main(String[] args) throws InterruptedException {
        DealThread dealThread = new DealThread();
        dealThread.setUserName("a");
        Thread thread = new Thread(dealThread);
        thread.start();
        //注意这里是等待0.1秒 而线程是等待三秒
        Thread.sleep(100);

        dealThread.setUserName("b");
        Thread thread1 = new Thread(dealThread);
        thread1.start();
    }

}

• 运行结果

此时我们发现已经进入死锁

用JDK检测死锁

1. 在你安装JDK的bin目录下按住shift加鼠标右键启动powershell

2. 输入jps命令

3. 知道我们启动类中死锁的线程的线程id是14784

4. 使用jstack命令

5. 在最后面获得信息

6. 浏览信息知道出现死锁的代码行在哪, 和在等待什么锁导致的死锁都可以发现.