死鎖產生情況：雙方互相持有對方的鎖的情況

死鎖示例程式碼：

public class DealThread implements Runnable {
	public String username;
	public Object lock1 = new Object();
	public Object lock2 = new Object();
	public void setFlag(String username) {
		this.username = username;
	}
	
	@Override
	public void run() {
		if(username.equals("a")) {
			synchronized(lock1) {
				try {
					System.out.println("username = " + username);
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				synchronized(lock2) {
					System.out.println("lock2執行");
				}
			}
		}
		if(username.equals("b")) {
			synchronized(lock2) {
				try {
					System.out.println("username = " + username);
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				synchronized(lock1) {
					System.out.println("lock1執行");
				}
			}
		}
	}
}

Run方法：
public class Run {
	public static void main(String[] args) {
		try {
			DealThread t1 = new DealThread();
			t1.setFlag("a");
			Thread thread1 = new Thread(t1);
			thread1.start();
			Thread.sleep(100);
			t1.setFlag("b");
			Thread thread2 = new Thread(t1);
			thread2.start();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}
 

執行結果：

排查：

使用jdk自帶工具進行排查
執行jps指令，可以檢視當前執行的執行緒：

可以看到Run執行緒的id值為3684。在執行jstack命令，檢視結果：