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.

要获取更多 Jerry 的原创文章，请关注公众号 ” 汪子熙 ”: