How Can I Effectively Determine the Completion Status of Child Threads in Multithreaded Programming?

Determining Thread Completion Status

In multithreaded programming, it's crucial to know when child threads have finished execution. This question explores various approaches to monitoring thread completion status.

Thread.join():
This method in the main thread blocks execution until all child threads complete. It ensures synchronized completion but is a blocking approach.

Thread.isAlive():
Polling Thread.isAlive() allows you to check if a thread is still executing. However, it's recommended to avoid polling as it's inefficient and can impact performance.

Exception Handling:
In an unconventional approach, you can configure uncaught exception handling for each thread. When the thread completes, it throws an exception that can be handled in your main object.

Concurrent Utilities:
Java's concurrent utilities like locks and synchronizers can be used to coordinate thread execution and establish dependencies.

Thread Completion Listener:

A more orthodox approach is to create a listener interface:

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

Then, define an abstract base thread class that extends Thread and implements the following methods:

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

    public final void addListener(final ThreadCompleteListener listener) {
        listeners.add(listener);
    }

    public final void removeListener(final ThreadCompleteListener listener) {
        listeners.remove(listener);
    }

    private final void notifyListeners() {
        for (ThreadCompleteListener listener : listeners) {
            listener.notifyOfThreadComplete(this);
        }
    }

    @Override
    public final void run() {
        try {
            doRun();
        } finally {
            notifyListeners();
        }
    }

    public abstract void doRun();
}

Each child thread should extend NotifyingThread and implement doRun() instead of run(). Upon completion, the thread automatically invokes notifyListeners().

In your main class, implement ThreadCompleteListener and add yourself as a listener when creating each thread:

NotifyingThread thread1 = new OneOfYourThreads();
thread1.addListener(this); // add ourselves as a listener
thread1.start(); // Start the Thread

When each thread completes, the notifyOfThreadComplete method is invoked, providing access to the completed thread instance.

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