How Can I Catch Exceptions Thrown by Threads in Java?

Catching Exceptions from Threads in Java

In multithreaded applications, managing exceptions thrown within different threads can be a challenge. Consider a scenario where a main class initiates a new thread and attempts to catch any runtime exceptions generated by it.

// Original Code
public class CatchThreadException {

    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread() {
            @Override
            public void run() {
                throw new RuntimeException("Exception from thread");
            }
        };

        try {
            t.start();
            t.join();
        } catch (RuntimeException e) {
            System.out.println("** RuntimeException from main");
        }

        System.out.println("Main stopped");
    }
}

In this code, the main thread waits for the child thread to complete using the join() method. However, when the child thread throws an exception, the main thread does not catch it.

Uncaught Exception Handler for Threads

To address this issue, Java provides a Thread.UncaughtExceptionHandler interface. By implementing this interface and assigning it to a thread, you can handle uncaught exceptions thrown within that thread.

// Using Uncaught Exception Handler
public class CatchThreadException {

    public static void main(String[] args) throws InterruptedException {
        Thread.UncaughtExceptionHandler h = new Thread.UncaughtExceptionHandler() {
            @Override
            public void uncaughtException(Thread th, Throwable ex) {
                System.out.println("Uncaught exception: " + ex);
            }
        };
        Thread t = new Thread() {
            @Override
            public void run() {
                throw new RuntimeException("Exception from thread");
            }
        };
        t.setUncaughtExceptionHandler(h);
        t.start();
        t.join();
        System.out.println("Main stopped");
    }
}

In this modified code:

1. We create a new instance of an UncaughtExceptionHandler.
2. We assign the UncaughtExceptionHandler to the child thread using the setUncaughtExceptionHandler() method.
3. When an uncaught exception occurs within the child thread, the uncaughtException() method of our handler is invoked, allowing us to handle the exception gracefully.

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