What is Exception Occurs in a thread?

When an exception occurs in a thread, the normal flow of execution is disrupted, and the thread may terminate if the exception is not caught and handled within the thread’s run method. Uncaught exceptions in a thread will cause the thread to stop executing, but the main program or other threads will continue to run.

Key Points Exception Handling in Threads:

• If an exception is thrown within the run method of a thread and is not caught, the thread will terminate.
• The stack trace of the exception is printed to the console.
• The uncaught exception handler for the thread (if set) is invoked

2. Uncaught Exception Handler

•  Java provides a way to set an uncaught exception handler for a thread or for all threads.
•  The uncaught exception handler allows the application to handle the exception in a custom way, such as logging the exception or taking corrective actions.

3. Thread Termination

•  If the exception is caught and handled within the run method, the thread can continue running.
•  If the exception is not handled, the thread terminates immediately after the exception is thrown.

Below is an example demonstrating what happens when an exception occurs in a thread:
Example with Uncaught Exception

java
class ExceptionThread extends Thread {
    public void run() {
        try {
            System.out.println("Thread is running...");
            throw new RuntimeException("An exception occurred in the thread.");
        } catch (Exception e) {
            System.out.println("Exception caught in thread: " + e.getMessage());
        }
        System.out.println("Thread is ending...");
    }

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

        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Main thread ends.");
    }
}


Example with Uncaught Exception Handler

java
class ExceptionThreadWithHandler extends Thread {
    public void run() {
        System.out.println("Thread is running...");
        throw new RuntimeException("An exception occurred in the thread.");
    }

    public static void main(String[] args) {
        Thread thread = new ExceptionThreadWithHandler();
        thread.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
            @Override
            public void uncaughtException(Thread t, Throwable e) {
                System.out.println("Uncaught exception in thread: " + t.getName() + " - " + e.getMessage());
            }
        });
        thread.start();

        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Main thread ends.");
    }
}

Example with Uncaught Exception:

• ExceptionThread class extends Thread.
• The run() method throws a RuntimeException.
• The exception is caught within the run() method, and a message is printed.
• The thread continues running after handling the exception.
• In the main method, the thread is started, and the main thread waits for the thread to finish using join().

Example with Uncaught Exception Handler

• ExceptionThreadWithHandler class extends Thread.
• The run() method throws a RuntimeException without catching it.
• An uncaught exception handler is set for the thread using setUncaughtExceptionHandler().
• The handler prints a custom message when the exception occurs.
• In the main method, the thread is started, and the main thread waits for the thread to finish using join().

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