threads question in examLab practice test 1

[Prasad Kharkar]

I am really sorry as i am posting such lengthy problems but please help me to understand this code
I know you will have to write lots of explanation but please help me
it is not the case that I have not tried but I am not able to understand
and the ExamLab explanation was not enough for me

here is the code

   
class A{

     synchronized void doTask1(){
          System.out.print(1);
     }

}


class B extends Thread{

     A a; Thread t;

     B(A a, Thread t){
          this.a=a; this.t=t;
     }

     public void run(){
          doTask2();
     }

     public void doTask2(){
          synchronized(a){
               try{
                    t.join();
               }catch(Exception e){}
          }
     }

     public void doTask3(){
          synchronized(a){
               System.out.print(2);
          }
     }

}


public class TDemo{

     public static void main(String args[]) throws Exception{
          A a=new A();
          B b=new B(a,Thread.currentThread());
          b.start();
          Thread.sleep(1000);
          a.doTask1();
          b.doTask3();
     }
}
 

The anser is code compiles but no output and program does not terminate

[Larry Olson]

Prasad Kharkar wrote:I am really sorry as i am posting such lengthy problems but please help me to understand this code
I know you will have to write lots of explanation but please help me
it is not the case that I have not tried but I am not able to understand
and the ExamLab explanation was not enough for me

Hi,

I don't blame you for the question. Don't be apologetic. I assume most people taking ExamLab are in the same boat. These questions are really tough to crack. IMO this particular code is not trivial and it needs a lot of analysis and insight to be able to understand and answer. If I have the time and patience to do the analysis, I shall post my findings. Otherwise I am sure there are experts out here who will help!

Good luck

[dipayan chatterjee]

Hi Prasad,

In this program you have two threads , main thread and b. After starting b thread , main goes to sleep.
Now b thread get chance to execute . It calls run method and doTask2() method , where it acquires lock of object a
and joins main thread . Due to this, thread b goes in block state with lock of object a .
Now when main thread come out of sleep state . It try to call doTask1() method on object a , but because this method is synchronized , it needs lock of object a , but lock is not available, because it is with thread b.

So both threads now in block state.


Hope it will help you

[Abimaran Kugathasan]

dipayan chatterjee wrote:Hi Prasad,

In this program you have two threads , main thread and b. After starting b thread , main goes to sleep.
Now b thread get chance to execute . It calls run method and doTask2() method , where it acquires lock of object a
and joins main thread . Due to this, thread b goes in block state with lock of object a .
Now when main thread come out of sleep state . It try to call doTask1() method on object a , but because this method is synchronized , it needs lock of object a , but lock is not available, because it is with thread b.

So both threads now in block state.


Hope it will help you

Great, Exactly.
Try this code :

class A	{
	synchronized void doTask1()	{
		System.out.println(1);
	}
}

class B extends Thread	{
	A a;
	Thread t;

	B(A a, Thread t)	{
		this.a = a;
		this.t = t;
	}

	public void run()	{
		System.out.println("run");
		doTask2();
	}

	public void doTask2()	{
		System.out.println("Join");
		synchronized(a)	{
			try	{
				t.join();
			} catch(Exception e)	{}
		}
	}

	public void doTask3()	{
		synchronized(a)	{
			System.out.println(2);
		}
	}
}

public class TDemo	{
	public static void main(String[] args) throws Exception	{
		A a = new A();
		B b = new B(a, Thread.currentThread());
		b.start();
		//Thread.sleep(10);
		a.doTask1();
		b.doTask3();
	}
}

[Prasad Kharkar]

very well replied dipayan
nice work
thanks for that
thread resolved