Java.Multithreding.What happens when an exception is thrown in a thread?

✅ 1. If it’s a checked or unchecked exception thrown from run():

• The thread terminates immediately.
• The exception is not rethrown to the main thread.
• If uncaught, it’s passed to the thread’s UncaughtExceptionHandler (if any).

---

📦 Example:

Thread t = new Thread(() -> {
    System.out.println("Running...");
    throw new RuntimeException("Something went wrong!");
});
t.start();

🧨 Output:

Running...
Exception in thread "Thread-0" java.lang.RuntimeException: Something went wrong!
    at ...

The thread crashes.

Other threads (like main) continue running.

🤔 What If I Try to Catch the Exception in main()?

try {
    Thread t = new Thread(() -> {
        throw new RuntimeException("Boom!");
    });
    t.start();
} catch (Exception e) {
    System.out.println("Caught in main!"); // ❌ won't happen
}

❌ Does not work!
Exceptions thrown in one thread don’t propagate to another thread (like main).

✅ How to Handle It Properly

🛡 1. Catch exceptions inside the thread

Thread t = new Thread(() -> {
    try {
        System.out.println("Thread working...");
        throw new RuntimeException("Oops!");
    } catch (Exception e) {
        System.out.println("Caught in thread: " + e.getMessage());
    }
});
t.start();

🧯 2. Use Thread.setUncaughtExceptionHandler

This is like a global “oops catcher” for a thread.

Thread t = new Thread(() -> {
    throw new RuntimeException("Boom!");
});

t.setUncaughtExceptionHandler((thread, ex) -> {
    System.out.println("Thread " + thread.getName() + " crashed: " + ex.getMessage());
});
t.start();

🧠 You can also set a default for all threads:

Thread.setDefaultUncaughtExceptionHandler((t, e) -> {
    System.out.println("Unhandled in thread " + t.getName() + ": " + e.getMessage());
});

🚀 What About Callable and Future?

If you’re using Callable with ExecutorService, the exception is wrapped inside a Future.

ExecutorService executor = Executors.newSingleThreadExecutor();
Future<String> future = executor.submit(() -> {
    throw new IllegalStateException("Error inside task");
});

try {
    future.get(); // throws ExecutionException
} catch (ExecutionException e) {
    System.out.println("Caught: " + e.getCause()); // prints original exception
}

✅ Summary

Situation	What Happens
Exception in Thread.run()	Thread terminates
Exception not caught	Sent to UncaughtExceptionHandler
Try-catch in main thread?	❌ Doesn’t catch child thread exceptions
Callable submitted to executor	Exception is wrapped in ExecutionException
Best practice	Catch inside thread or set handler