Java – How to Handle InterruptedException

In this tutorial, we will learn InterruptedException and when it is thrown with an example. We will also see how we can handle InterruptedException in our code when working in a multithreaded application.

1. Java InterruptedException

1.1. What are Interrupts?

In concurrency, an interrupt is a signal to Thread to stop itself and figure out what to do next. Generally, it asks the Thread to terminate itself gracefully.

The InterruptedException is thrown when a thread is waiting, or sleeping, or otherwise occupied, and another thread interrupts it by using the interrupt() method present in the Thread class.

Internally, the interrupt mechanism is implemented using an internal flag known as interrupt status. When a Thread invokes interrupt() method then this flag is set, and we can check the status of this flag by using isInterrupted() method. Another method present in Thread class, Thread.interrupted() can clear out this flag again to respond to other interrupt requests.

Note that Thread class contains the following methods used for creating and checking interrupts:

• void interrupt(): interrupts a Thread
• boolean isInterrupted(): checks whether the current Thread has been interrupted or not. The interrupted status of the Thread is unaffected by this method.
• boolean interrupted(): checks whether the current Thread has been interrupted. The interrupted status of the Thread is cleared by this method.

1.2. Thread.interrupt() Method

The interrupt() method interrupts the thread on which it is invoked.

• If this thread is blocked in an invocation of the sleep(), wait() or join() methods then its interrupt status will be cleared, and it will receive an InterruptedException.
• If this thread is blocked in an I/O operation upon an InterruptibleChannel then the channel will be closed, the thread’s interrupt status will be set, and the thread will receive a ClosedByInterruptException.
• If this thread is blocked in a java.nio.channels.Selector then the thread’s interrupt status will be set, and it will return immediately from the selection operation.
• If none of the previous conditions hold then this thread’s interrupt status will be set, and we can check it using isInterrupted() or interrupted() methods.

2. How to Interrupt a Thread

A Thread can interrupt another waiting or sleeping Thread by using the interrupt() method of Thread class.

In the following example, TestThread works periodically after every 2 seconds. It checks if it has been interrupted or not. If not, it continues working, finishes the processing and returns. If it is interrupted in between by another Thread, it can terminate gracefully by throwing InterruptedException to the caller Thread.

public class TestThread extends Thread {  
 
	public void run() {  
		try{

			while(true) {  
                                // Check if it is interrupted, if so then throw InterruptedException
				if(Thread.interrupted()) {  
					throw new InterruptedException();
				}  
                                // else continue working
				else {  
					System.out.println("Continue working");  
				}  
                                Thread.sleep(2000L);
			}
		} catch (InterruptedException e) {
                         // Handling InterruptedException and Graceful shutdown of the Thread
			System.out.println("Graceful shutdown"); 
		}
	}  
}

Another Thread (Eg: main Thread) that started it, sometimes later decides that the task performed by TestThread is not necessary anymore, so it interrupts it.

// TestThread Instantiation
TestThread t1 = new TestThread(); 
 
// Starting TestThread
t1.start(); 

// main Thread enters into sleeping state 
Thread.sleep(5000);

// main Thread decided that TestThread is no longer needed, so interrupting it
t1.interrupt(); 

When we execute this program, we will get the following output,

Output
Continue working
Continue working
Continue working

Graceful Termination

3. Handling InterruptedException

When working in a multithreaded application, it is important to handle InterruptedException gracefully, and the threads should respond to interrupts promptly to avoid the application from entering into a deadlock situation.

• As InterruptedException is a checked exception so we have to handle it either by using try-catch or throws keyword.
• We can propagate it up in the stack to the caller method by using throws keyword. In this case, the caller method needs to handle this exception.
• There are some scenarios where throwing an exception is not possible like in case of Runnable interface run() method, it won’t allow throwing an exception, so for these cases we can use try-catch block and handle this exception explicitly in the calling Thread itself instead of throwing it.

Let’s consider the first case when an Interrupted Thread throws InterruptedException and ask the caller method to handle it instead of handling it by itself.

// method throwing InterruptedException so that it is propagated to the caller method
public static void throwInterruptedException() throws InterruptedException
{
     // Thread enters into sleeping state
     Thread.sleep(1000);

     // Thread interrupting itself
     Thread.currentThread().interrupt();

     // check if the Thread is interrupted
     if (Thread.interrupted()) {
        // Throwing InterruptedException
        throw new InterruptedException();
     } 
}

// main method
public static void main(String[] args)
{
    try{
             // calling a method that throws InterruptedException  
             throwInterruptedException();
         } catch(InterruptedException e){
             System.out.println("Thread interrupted by throwing the exception")
         } 
} 

Let’s now consider the case where an interrupted Thread itself handling the InterruptedException using try-catch block instead of propagating it up to the caller method using throws keyword.

// method handling InterruptedException using try-catch block
public void run()
{
     try{
             // Thread enters into sleeping state
             Thread.sleep(1000);
             // Thread interrupting itself
             Thread.currentThread().interrupt(); 
          } catch(InterruptedException e){
               System.out.println("Thread interrupted and handling exception by itself")
          }
}

// main method
public static void main(String[] args)
{
    // Creating new Thread
    Thread thread = new Thread();
    // Starting a Thread
     thread.start();
}

We can also create user-defined exception (our own custom InterruptedException) by extending Exception class and throw our custom exception from the code if required.

// Creating Custom InterruptedException
public class CustomInterruptedException extends Exception 
{ 
     CustomInterruptedException(String message) { 
       super(message); 
     } 
}

// method throwing our CustomInterruptedException 
public void throwCustomException() throws CustomInterruptedException
{
    // Thread enters into sleeping state
    Thread.sleep(1000);
    // Thread interrupting itself
    Thread.currentThread().interrupt();
    // check if the Thread is interrupted
    if (Thread.interrupted()) 
   {
       // Throwing CustomInterruptedException
       throw new CustomInterruptedException("Thread interrupted by throwing custom exception"); 
   } 
}

3. Conclusion

This article taught us about InterruptedException, its constructors, methods and causes for the same. We have also seen various ways by which we can handle InterruptedException in our code.

Happy Learning !!