Java~使用JDK自带的工具来检测是否有死锁的现象

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

/**
 * Created with IntelliJ IDEA.
 * Description: If you don't work hard, you will a loser.
 * User: Listen-Y.
 * Date: 2020-10-03
 * Time: 20:37
 */
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

/**
 * Created with IntelliJ IDEA.
 * Description: If you don't work hard, you will a loser.
 * User: Listen-Y.
 * Date: 2020-10-03
 * Time: 20:42
 */
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

使用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
   

• 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.