How to Wait for Multiple Threads to Finish Execution Before Proceeding?

How to Synchronize Multithreaded Execution

In multithreaded applications, it's often necessary to coordinate the execution of parallel tasks. This article explores a method to wait for a group of threads to complete before proceeding.

The Problem:

Consider the following code:

<code class="java">public class DoSomethingInAThread implements Runnable {...}

public static void main(String[] args) {
    for (int n=0; n<1000; n++) {
        Thread t = new Thread(new DoSomethingInAThread());
        t.start();
    }
    // wait for all threads' run() methods to complete before continuing
}</code>

In this example, multiple new threads are created and started. However, the main() method continues execution immediately, without waiting for these threads to finish their tasks.

The Solution:

To synchronize the execution, we can create an array of thread references and then use the join() method on each thread object. This blocks the current thread until the target thread has completed its execution:

<code class="java">import java.lang.Thread;

public class DoSomethingInAThread implements Runnable {...}

public static void main(String[] args) {
    Thread[] threads = new Thread[1000];
    for (int n=0; n<threads.length; n++) {
        threads[n] = new Thread(new DoSomethingInAThread());
        threads[n].start();
    }
    for (Thread t: threads) {
        t.join();
    }
}</code>

In this revised code, the main() method creates an array of thread references and starts each thread. It then enters a loop that calls join() on every thread in the array. This ensures that all threads complete their tasks before the main() method continues.

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