Interrupting a Thread:

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 behaviour 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.

Methods of Thread class for interrupting a thread

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

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

Example #1

Example of interrupting a thread that stops working

In this example, after interrupting the thread, we are propagating it, so it will stop working. If we don't want to stop the thread, we can handle it where sleep() or wait() method is invoked. Let's first see the example where we are propagating the exception.

snippet

class TestInterruptingThread1 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[]){
TestInterruptingThread1 t1=new TestInterruptingThread1();
t1.start();
try{
t1.interrupt();
}catch(Exception e){System.out.println("Exception handled "+e);}

}
}

Output

Output:Exception in thread-0 java.lang.RuntimeException: Thread interrupted... java.lang.InterruptedException: sleep interrupted at A.run(A.java:7)

Example #2

Example of interrupting a thread that doesn't stop working

In this example, after interrupting the thread, we handle the exception, so it will break out the sleeping but will not stop working.

snippet

class TestInterruptingThread2 extends Thread{
public void run(){
try{
Thread.sleep(1000);
System.out.println("task");
}catch(InterruptedException e){
System.out.println("Exception handled "+e);
}
System.out.println("thread is running...");
}

public static void main(String args[]){
TestInterruptingThread2 t1=new TestInterruptingThread2();
t1.start();

t1.interrupt();

}
}

Output

Output:Exception handled java.lang.InterruptedException: sleep interrupted thread is running...

Example #3

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.

snippet

class TestInterruptingThread3 extends Thread{

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

public static void main(String args[]){
TestInterruptingThread3 t1=new TestInterruptingThread3();
t1.start();

t1.interrupt();

}
}

Output

Output:1 2 3 4 5

isInterrupted and interrupted method

The isInterrupted() method returns the interrupted flag either true or false. The static interrupted() method returns the interrupted flag afterthat it sets the flag to false if it is true.

Example

snippet

public class TestInterruptingThread4 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[]){

TestInterruptingThread4 t1=new TestInterruptingThread4();
TestInterruptingThread4 t2=new TestInterruptingThread4();

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

t2.start();

}
}

Output

Output:Code for interrupted thread code for normal thread code for normal thread code for normal thread