Interrupt, Interrupted and isInterrupted

[Kaur Manpreet]

Hi All,

I need some help to understand the output of the code below:

class Test extends Thread
  {
	Thread t;
	Test()
	{}
	Test(String s)
	{
		t = new Test();
		t.setName("A");
		setName(s);
		t.start();
	}
   public synchronized void run() 
       {
         System.out.println("Current Thread Before Yield " + Thread.currentThread().getName());
		 try
          {
            interrupt();        // Line 1
		    interrupted();      // Line 2 
		    isInterrupted();    // Line 3

			Thread.yield();

            System.out.println("Current Thread after Yield " + Thread.currentThread().getName());

			if( Thread.currentThread().getName().length()>1)
			{
     			System.out.println("Joining Thread A ...");
				t.join();
			}
			System.out.println("Current Thread before Sleep " + Thread.currentThread().getName());
			Thread.sleep(1000);
          }
          catch(InterruptedException e)
          {
            System.out.println("Interrupted... " + interrupted() + " " + isInterrupted());
          }
       }
 }
  
 public class Ticker
 {
     public static void main(String[] args)
      {
       new Test("B").start();
      }
 }

As such the code compiles and runs normally as if none of the interrupt methods were ever there.
1. If we comment only Line 1 then both the threads run simultaneously and they complete normally.
2. If we comment only Line 2 then, both Thread A and B would throw interrupted exception. Thread A when sleep method is invoked and Thread B when join method is invoked.
3. If we comment only Line 3 then both the threads run simultaneously and they complete normally.
4. If we comment Line 1 and Line 2 both the threads run simultaneously and they complete normally.
5. If we comment Line 1 and Line 3 both the threads run simultaneously and they complete normally.
6. If we comment Line 2 and Line 3 both Thread A and B would throw interrupted exception. Thread A when sleep method is invoked and Thread B when join method is invoked.

These are the results I have arrived during my testing on this piece of code.
On this forum I searched on interrupt methods and found that we can assume that interrupt method sets a flag that the thread has been interrupted and any method (wait, join or sleep) that blocks the thread on which interrupt method is invoked would result in InterruptedException. This helps in explaining the result 6 above. But I am not sure of the rest.

Please advise.

Thanks

[Kaur Manpreet]

Thanks all for taking the time,

I was going through the API and I found the following

1. interrupt method set the interrupted status of the thread on which this method is invoked
2. the status is cleared if on the same thread wait, join or sleep are invoked and it receives an InterruptedException
3. interrupted method verifies whether the thread is interrupted and after that it reset the interrupted status of the thread on which interrupt was invoked (I checked this by printing the value of interrupted method). So second invocation returns false.
4. isInterrupted method verifies whether the thread has been interrupted but does not clear the status of the interrupted thread.

wow... its sometimes good to read the API thoroughly

[Bharath Ayyagari Mylavarapu]

Hello Manpreet,

The following points mentioned by you are highly helpful:
1. interrupt method set the interrupted status of the thread on which this method is invoked
2. the status is cleared if on the same thread wait, join or sleep are invoked and it receives an InterruptedException
3. interrupted method verifies whether the thread is interrupted and after that it reset the interrupted status of the thread on which interrupt was invoked (I checked this by printing the value of interrupted method). So second invocation returns false.
4. isInterrupted method verifies whether the thread has been interrupted but does not clear the status of the interrupted thread.

However, I have a question related to the same topic, can you please help?

Let us say I have the following 2 classes:

// An instance of this class is interrupted by an instance of the other class, "Interrupter"

public class Interruptee implements Runnable {

	@Override
	public void run() {
		try {
			System.out.println(Thread.currentThread().getName()+" is Sleeping");
			Thread.sleep(20000);
			System.out.println(Thread.currentThread().getName()+" has finished Sleeping");
		} catch (InterruptedException e) {
			System.out.println(Thread.currentThread().getName()+ Thread.currentThread().isInterrupted());
			e.printStackTrace();
		}
	}
}

// An instance of this class is used to interrupt an instance of the, "Interruptee" class written above
 
public class Interrupter implements Runnable {
	private Thread t;
	
	public Interrupter(Thread targetThread){
		t = targetThread;
	}
	
	public void run() {
		System.out.println(t.getName() + " is being interrupted by the interrupter");
		System.out.println(t.getName() + " 's interrupted status before interruption is: " +  t.isInterrupted());
		if (t.isInterrupted() == false){
			t.interrupt();
		}
		System.out.println(t.getName() + " 's interrupted status after interruption is: " +  t.isInterrupted());
	}
}

// This is the main class that creates and launches instance of the Interruptee and the Interrupter

public class InterruptLauncher {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Thread interruptee1 = new Thread(new Interruptee());
		interruptee1.setName("Interruptee 1");
		interruptee1.start();
				
		Thread interrupter = new Thread(new Interrupter(interruptee1));
		
		System.out.println("Interrupter will launch in 5 seconds");
		try {
			Thread.sleep(5000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		
		interrupter.start();
	}
}

In this case, like you mentioned in your 4 points,
1) Yes, the interrupt method sets the interrupted status of the thread on which this method is invoked. In this case, I believe it is interruptee1.
2. Yes, the status is cleared if on the same thread wait, join or sleep are invoked and it receives an InterruptedException. This is justified because the run() method inside the Interruptee class is is only doing the task of sleep() and when sleep() is interrupted, it throws an InterruptedException.
3) Yes, interrupted method verifies whether the thread is interrupted and after that it reset the interrupted status of the thread on which interrupt was invoked (I checked this by printing the value of interrupted method). So second invocation returns false.

However, point 4) is not clear to me -
4. isInterrupted method verifies whether the thread has been interrupted but does not clear the status of the interrupted thread.

Inside the Interrupter class, I'm trying to invoke this-> System.out.println(t.getName() + " 's interrupted status after interruption is: " + t.isInterrupted());
My expectation is that t.isInterrupted() has to print true since the thread got interrupted (by virtue of the sleep() method getting interrupted and throwing an InterruptedException) but in real, I see that the output is false!! According to point 4) that you mentioned, isInterrupted() does NOT clear the status of the interrupted thread right? Then, why is the result of this method false (instead of true)?

Thanks,
A M Bharth