98 Java - Interrupting Thread

Interrupting a Thread in Java

A thread can send an interrupt signal to JVM to interrupt by invoking interrupt on the Thread object for the thread to be interrupted. This means interruption of a thread is caused by any other thread calling the interrupt() method.

Methods for Interrupting a Thread

The Thread class provides three methods for interrupting a thread -

• void interrupt() − Interrupts the thread.

• static boolean interrupted() − Tests whether the current thread has been interrupted.

• boolean isInterrupted() − Tests whether the thread has been interrupted.

Example of Interrupting a Java Thread

In this example, we're creating a Task object which has implemented Runnable interface to act as a thread. Using start() method, we've started the thread. As next statement, we're interrupting the thread using interrupt() method and we're printing thread properties in run() method.

package com.codeheap;

public class TestThread {
   public static void main(String[] args) {
      System.out.println("Thread main started");
      final Task task = new Task();
      final Thread thread = new Thread(task);
      thread.start();
      thread.interrupt(); // calling interrupt() method
      System.out.println("Main Thread finished");
   }
}
class Task implements Runnable {
   @Override
   public void run() {
      for (int i = 0; i < 5; i++) {
         System.out.println("[" + Thread.currentThread().getName() + "] Message " + i);
         if(Thread.interrupted()) {
            System.out.println("This thread was interruped by someone calling this Thread.interrupt()");
            System.out.println("Cancelling task running in thread " + Thread.currentThread().getName());
            System.out.println("After Thread.interrupted() call, JVM reset the interrupted value to: " + Thread.interrupted());
            break;
         }
      }
   }
}

Output

Thread main started
Main Thread finished
[Thread-0] Message 0
This thread was interruped by someone calling this Thread.interrupt()
Cancelling task running in thread Thread-0
After Thread.interrupted() call, JVM reset the interrupted value to: false

Example of Interrupting a Java Thread with InterruptedException Handling

In this example, we're handling the interruption and continuing the thread. We're creating a Task object which has implemented Runnable interface to act as a thread. Using start() method, we've started the thread. As next statement, we're interrupting the thread using interrupt() method and we're handling InterruptedException in run() method and printing the details.

package com.codeheap;

public class TestThread {
   public static void main(String[] args) {
      System.out.println("Thread main started");
      final Task task = new Task();
      final Thread thread = new Thread(task);
      thread.start();
      thread.interrupt(); // calling interrupt() method
      System.out.println("Main Thread finished");
   }
}
class Task implements Runnable {
   @Override
   public void run() {
      for (int i = 0; i < 5; i++) {
         System.out.println("[" + Thread.currentThread().getName() + "] Message " + i);
         try {
			Thread.sleep(50);
		} catch (InterruptedException e) {
			System.out.println("This thread was interruped by someone calling this Thread.interrupt()");
		}        
      }
   }
}

Output

Thread main started
Main Thread finished
[Thread-0] Message 0
This thread was interruped by someone calling this Thread.interrupt()
[Thread-0] Message 1
[Thread-0] Message 2
[Thread-0] Message 3
[Thread-0] Message 4 

Example of Checking Whether Thread is Interrupted or Not

In this example, we're showcasing multiple threads and use of interrupted() method to check if a thread is interrupted or not. We're creating multiple Task objects which has implemented Runnable interface to act as a thread. Using start() method, we've starting the threads. Then, we're interrupting a thread using interrupt() method and using interrupted in run() method, we're printing the details.

package com.codeheap;

public class TestThread {
   public static void main(String[] args) {
      System.out.println("Thread main started");
      final Task task1 = new Task();
      final Thread thread1 = new Thread(task1);
      final Task task2 = new Task();
      final Thread thread2 = new Thread(task2);
      final Task task3 = new Task();
      final Thread thread3 = new Thread(task3);
      thread1.start();
      thread2.start();
      thread3.start();
      thread1.interrupt(); // calling interrupt() method
      System.out.println("Main Thread finished");
   }
}
class Task implements Runnable {
   @Override
   public void run() {
      if(Thread.interrupted()) {
         System.out.println("[" + Thread.currentThread().getName() + "] Interrupted ");
      }else {
         System.out.println("[" + Thread.currentThread().getName() + "] Uninterrupted ");
      }
   }
}

Output

Thread main started
Main Thread finished
[Thread-2] Uninterrupted 
[Thread-0] Interrupted 
[Thread-1] Uninterrupted