Java~使用JDK自帶的工具來檢測是否有死鎖的現象

多執行緒的死鎖

• Java執行緒死鎖是一個經典的多執行緒問題, 因為不同的執行緒都在等待根本不可能被釋放的鎖, 從而導致執行緒中的任務無法完成
• 下面演示互相等待對方鎖釋放的死鎖

/**
 * 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;
    }

    //倆把鎖
    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 + " :" + "進入lock1");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

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

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

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

    }
}

• 啟動類

/**
 * 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();
        //注意這裡是等待0.1秒 而執行緒是等待三秒
        Thread.sleep(100);

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

}

• 執行結果
  
• 此時我們發現已經進入死鎖

使用JDK檢測死鎖

1. 在你安裝JDK的bin目錄下按住shift加滑鼠右鍵啟動powershell
2. 輸入jps命令
   
3. 知道我們啟動類中死鎖的執行緒的執行緒id是14784
4. 使用jstack命令
   
5. 在最後面獲得資訊
   

• 瀏覽資訊知道出現死鎖的程式碼行在哪, 和在等待什麼鎖導致的死鎖都可以發現.