How Can I Catch Unhandled Exceptions from Threads in Java?

Catching Exceptions from Threads Using Thread.UncaughtExceptionHandler

In multithreaded Java applications, it's crucial to handle exceptions thrown by spawned threads. Consider a scenario where a main class launches a thread and waits for it to complete. If the thread throws an exception, the main class may fail to catch it using a standard try-catch block within the join() method.

The reason for this is that when a thread dies due to an unhandled exception, it terminates abruptly without propagating the exception to the calling thread. To address this issue, Java introduced the Thread.UncaughtExceptionHandler interface.

Implementing Thread.UncaughtExceptionHandler

To catch exceptions from a thread, implement the Thread.UncaughtExceptionHandler interface. Override the uncaughtException() method, which takes two parameters: the thread that threw the exception and the exception itself. Within this method, you can handle the exception as needed.

Setting the Thread Handler

Once you have implemented a handler, assign it to the thread using the setUncaughtExceptionHandler() method. This ensures that the handler is called whenever an unhandled exception occurs in the thread.

Example

Here's an example that demonstrates how to use Thread.UncaughtExceptionHandler:

Thread.UncaughtExceptionHandler handler = new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable ex) {
        System.out.println("Uncaught exception in thread: " + t.getName());
        System.out.println("Exception message: " + ex.getMessage());
    }
};

Thread workerThread = new Thread() {
    @Override
    public void run() {
        // Throw an exception to demonstrate uncaught exception handling
        throw new RuntimeException("Exception in worker thread");
    }
};

workerThread.setUncaughtExceptionHandler(handler);
workerThread.start();

This code creates a thread, assigns an uncaught exception handler to it, and starts the thread. When the thread throws a RuntimeException, the uncaught exception handler catches it and prints the thread name and exception message.

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