Java Thread Join Method

The join() method in Java is used to pause the execution of the current thread until the thread on which it is called has completed its execution. This is particularly useful when you want to ensure that a thread has finished its task before proceeding with the next part of your program.

Key Concepts

• Thread: A lightweight process that can run concurrently with other threads.
• Join Method: A method that allows one thread to wait for the completion of another thread.

How the Join Method Works

• When a thread calls join() on another thread, it waits for that thread to die (i.e., finish its execution).
• If the thread is already dead, the calling thread will continue immediately.

Syntax

public final void join() throws InterruptedException

Overloaded Methods

• join(long millis): Waits for the thread to die for up to the specified number of milliseconds.
• join(long millis, int nanos): Waits for the thread to die for a specified period in milliseconds and nanoseconds.

Example

Here’s a simple example to illustrate how the join() method works:

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread " + Thread.currentThread().getName() + " is running.");
    }
}

public class ThreadJoinExample {
    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        MyThread t2 = new MyThread();

        t1.start(); // Start first thread
        t2.start(); // Start second thread
        
        try {
            t1.join(); // Main thread waits for t1 to finish
            t2.join(); // Main thread waits for t2 to finish
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Both threads have finished executing.");
    }
}

Output

Thread Thread-0 is running.
Thread Thread-1 is running.
Both threads have finished executing.

When to Use Join

• Use join() when you need to ensure that a thread has completed its task before continuing with the next execution in your code.
• It's particularly useful in scenarios where thread order matters or when results from one thread are needed in another.

Conclusion

The join() method is a powerful tool for thread management in Java. By using this method, you can control the flow of your application and ensure that certain tasks are completed before proceeding, making your multithreaded programs more reliable and easier to manage.