We can get the concept of deadlock in wikipedia.

The picture below gives a common scenario which leads to deadlock.

In this blog, I will share how to detect deadlock situation using JDK standard tool jstack. First we have to write a Java program which will lead to Deadlock:

package thread;

public class DeadLockExample {
/*

  * Thread 1: locked resource 1

    Thread 2: locked resource 2

  */

public static void main(String[] args) {

  final String resource1 = "ABAP";

  final String resource2 = "Java";

  // t1 tries to lock resource1 then resource2

  Thread t1 = new Thread() {

   public void run() {

    synchronized (resource1) {

     System.out.println("Thread 1: locked resource 1");

     try {

      Thread.sleep(100);

     } catch (Exception e) {

     }

     synchronized (resource2) {

      System.out.println("Thread 1: locked resource 2");

     }

    }

   }

  };

  Thread t2 = new Thread() {

   public void run() {

    synchronized (resource2) {

     System.out.println("Thread 2: locked resource 2");

     try {

      Thread.sleep(100);

     } catch (Exception e) {

     }

     synchronized (resource1) {

      System.out.println("Thread 2: locked resource 1");

     }

    }

   }

  };

  t1.start();

  t2.start();

}

}

Execute this program, you will get output: Thread 1: locked resource 1 Thread 2: locked resource 2 Then use command jps -l -m to list the process id of this deadlock program. In my example it is 51476:

Just type jstack + process id, and it will display all detailed information about deadlock:

Here the object 0x00000000d6f64988 and 0x00000000d6f649b8represent the two resource String “ABAP” and “Java”.

Update on 2017-03-04 Saturday 10:35PM

how to get the thread state of a long-running application

Suppose you have found a long-running application which has high CPU utilization rate and you would like to know which exactly line is relevant. Use the following code to simulate the long running situation:

package thread;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;


class MyThread implements Runnable{

	private List<Integer> myList;
	private Object host;
	public MyThread(List<Integer> list, Object object){
		this.myList = list;
		this.host = object;
	}
	
	public void updateListSafe(){
		synchronized(this.host){
			ArrayList<Integer> safe = new ArrayList<Integer>();
			safe.add(1);
		}
	}
	
	private void updateList(int i){
		synchronized(this.host){
			myList.add(i);
		}
	}

	@Override
	public void run() {
		while(true){
			updateList(1);
		}
	}
	
}
public class MyExecutor {

	private ArrayList<Integer> taskList = new ArrayList<Integer>();
	private Object object = new Object();
	private void launch(){
		
        ExecutorService executorService= Executors.newFixedThreadPool(10);

        executorService.execute(new MyThread(taskList, object));
        executorService.execute(new MyThread(taskList, object));
	}
	public static void main(String[] args) {
		MyExecutor test = new MyExecutor();
		test.launch();
	}
}

first find the process id of running application:

then use command jstack 23520 to get the stack trace:

In line 9 and line 18 our application method MyThread.updateList is listed there.

使用jstack检测Java应用的死锁(deadlock)状态

Update on 2017-03-04 Saturday 10:35PM

how to get the thread state of a long-running application