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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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