How Can I Efficiently Determine the Completion Status of Multiple Threads?

How to Ascertain Thread Completion in Multiple Threads

When executing multiple threads simultaneously, it becomes crucial to determine their completion status. This article explores various methods to achieve this.

1. Thread.join(): Blocking Wait

The Thread.join() method allows the main thread to block until a specific thread completes its execution. This approach, while simple, can hinder the overall responsiveness of your application.

2. Thread.isAlive(): Polling Check

Thread.isAlive() periodically verifies if a thread is still running. However, this approach is generally discouraged due to its potential inefficiency and resource consumption.

3. UncaughtExceptionHandler:

An unorthodox approach involves setting up uncaught exception handlers. When a thread completes, it can be programmed to throw an exception, triggering the handler in your object.

4. Locks and Synchronizers:

Leveraging locks or synchronizers can provide a more synchronized mechanism to manage thread execution and completion.

5. Listener-Based Notification:

A more elegant solution involves creating a listener interface and implementing classes that extend NotifyingThread. These threads automatically notify listeners upon completion, allowing the main thread to receive notifications and respond accordingly.

Implementation of Listener-Based Notification:

To implement this approach, begin by defining a listener interface:

public interface ThreadCompleteListener {
    void notifyOfThreadComplete(final Thread thread);
}

Next, create an abstract class, NotifyingThread, which extends Thread:

public abstract class NotifyingThread extends Thread {
    private final Set<ThreadCompleteListener> listeners
                   = new CopyOnWriteArraySet<>();
}

The doRun() method should be implemented instead of run() in your specific threads. When they complete, they will notify any registered listeners.

Finally, modify the main class that starts the threads to implement ThreadCompleteListener. After creating each thread, add it to the listener list:

NotifyingThread thread1 = new OneOfYourThreads();
thread1.addListener(this); // Add main class as a listener
thread1.start();           // Start the thread

Your notifyOfThreadComplete method will be invoked whenever a thread completes or crashes, providing the completed thread instance.

Note: Implementing Runnable rather than extending Thread is generally preferred for creating new threads.

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