Question

this the code in which i am trying to demonstrate nested lock problem,

  import java.util.concurrent.locks.*;


  class SharedResource{
    private static final Lock lock = new ReentrantLock();


    private void methodThree(String name,int x) throws Exception{
        lock.lock();
        while(x <= 15){
            System.out.println("METHOD-THREE / THREAD-NAME : "+name+" NUM-VAL "+x);
            x++;
            Thread.sleep(250);
        }
    }

    private void methodTwo(String name,int x) throws Exception{
        lock.lock();
        while(x <= 10){
            System.out.println("METHOD-TWO / THREAD-NAME : "+name+" NUM-VAL "+x);
            x++;
            Thread.sleep(250);
        }
        methodThree(name,x);
    }

    public void methodOne(String name,int x) throws Exception{
        try{        
            lock.lock();
            while(x <= 5){
                System.out.println("METHOD-ONE / THREAD-NAME : "+name+" NUM-VAL "+x);
                x++;
                Thread.sleep(250);
            }   
            methodTwo(name,x);          
        }finally{
            lock.unlock();
        }       
    }

  }

  class MyRequestREQ extends Thread{

    private SharedResource res;
    private int num = 1;

    MyRequestREQ(SharedResource res,String name){
        super(name);
        this.res = res;
    }

    @Override
    public void run(){      
        try{
            res.methodOne(Thread.currentThread().getName(),num);
        }catch(Exception e){
            System.out.println(e);
        }
    }
  }

  public class LockCountPractise{
    public static void main(String [] args){
        SharedResource resource = new SharedResource();
        MyRequestREQ[] requests = new MyRequestREQ[]{
            new MyRequestREQ(resource,"JACK"),
            new MyRequestREQ(resource,"JILL"),
            new MyRequestREQ(resource,"JASON")
        };

        for(int x=0; x < requests.length;x++){
            requests[x].start();
        }
    }
  }

but the output which i get is all the one run by the thread “JACK”, this thread prints till count 15, and just hung up.

Is the above program face a deadlock issue ?

do i need to unlock the lock in all of the methods of class SharedResource ?

Waiting for the suggestion.

Answer 1

you are not unlocking in method3 so when the first thread is done the others can’t go on since they can’t acquire the lock.

do i need to unlock the lock in all of the methods of class
SharedResource ?

yes because every time you call lock():

If the current thread already holds the lock then the hold count is
incremented by one and the method returns immediately.

see:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/ReentrantLock.html#lock%28%29

as a suggestion: you can acquire the lock in method1 and release it in method3. so there will be 1 lock 1 unlock, you’ll be fine. no need to 3 lock-unlock cycle.

actually it depends what behaviour you want:

• To let different threads acquire the lock between counters (thread1 counts to 5 then thread3 comes and counts then thread1 continues) you need lock-unlock in every method.
• To get one thread start and finish counting without any interfere you need 1 lock-unlock