Interrupting Java threads

Interrupting a thread means stopping what it is doing before it has completed its task, effectively aborting its current operation. Whether the thread dies, waits for new tasks, or goes on to the next step depends on the application.

If any thread is in sleeping or waiting state (i.e. sleep() or wait() is invoked), calling the interrupt() method on the thread, breaks out the sleeping or waiting state throwing InterruptedException. If the thread is not in the sleeping or waiting state, calling the interrupt() method performs normal behavior and doesn't interrupt the thread but sets the interrupt flag to true. Let's first see the methods provided by the Thread class for thread interruption.

The 3 methods provided by the Thread class for interrupting a thread

public void interrupt()
public static boolean interrupted()
public boolean isInterrupted()

Example of interrupting a thread that stops working

class InterruptDemo extends Thread{
   public void run(){
     try{
        Thread.sleep(1000);
        System.out.println("task");
     }catch(InterruptedException e){
         throw new RuntimeException("Thread interrupted..."+e);
     }

}

public static void main(String args[]){
   InterruptDemo t1 = new InterruptDemo();
   t1.start();
   try{
       t1.interrupt();
   }catch(Exception e){
     System.out.println("Exception handled "+e);
    }

  }
}

output:

Example of interrupting a thread that doesn't stop working

class InterruptDemo extends Thread{
   public void run(){
     try{
        Thread.sleep(1000);
        System.out.println("task");
     }catch(InterruptedException e){
         System.out.println("Exception handled "+e);
     }

}

public static void main(String args[]){
   InterruptDemo t1 = new InterruptDemo();
   t1.start();
   try{
       t1.interrupt();
   }catch(Exception e){
     System.out.println("Exception handled "+e);
    }
     System.out.println("thread is running...");
  }
}

output:

Example of interrupting thread that behaves normally
If thread is not in sleeping or waiting state, calling the interrupt() method sets the interrupted flag to true that can be used to stop the thread by the java programmer later.

class InterruptThread extends Thread{

   public void run(){
      for(int i=1;i<=5;i++)
         System.out.println(i);
      }

   public static void main(String args[]){
      InterruptThread t1 = new InterruptThread();
      t1.start();

      t1.interrupt();

    }
}

output:

What about isInterrupted and interrupted method?
The isInterrupted() method returns the interrupted flag either true or false. The static interrupted() method returns the interrupted flag after that it sets the flag to false if it is true.

class InterruptedDemo extends Thread{

   public void run(){
      for(int i=1;i<=2;i++){
         if(Thread.interrupted()){
             System.out.println("code for interrupted thread");
          }
          else{
             System.out.println("code for normal thread");
          }

       }//end of for loop
    }

    public static void main(String args[]){

       InterruptedDemo t1=new InterruptedDemo();
       InterruptedDemo t2=new InterruptedDemo();

       t1.start();
       t1.interrupt();

       t2.start();

    }
}

output:

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Labels: multithreading