How to Start Multiple Threads Simultaneously in Java?

How can I start multiple threads simultaneously in Java?

Thread thread1 = new Thread(new MyRunnable()); 
Thread thread2 = new Thread(new MyRunnable()); 
thread1.start(); 
thread2.start();

In Java, threading allows a program to perform multiple tasks concurrently. To start multiple threads at the same time, developers often utilize the `Thread` class or implement the `Runnable` interface. This article will delve into the methods to initiate several threads simultaneously, ensuring optimal performance and efficiency.

import java.util.concurrent.CountDownLatch;

public class MultiThreadStart {
    public static void main(String[] args) throws InterruptedException {
        final int threadCount = 5;
        CountDownLatch latch = new CountDownLatch(1);
        for (int i = 0; i < threadCount; i++) {
            new Thread(() -> {
                try {
                    latch.await(); // Wait until latch is counted down
                    System.out.println(Thread.currentThread().getName() + " started.");
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }).start();
        }
        latch.countDown(); // Release all waiting threads
    }
}

Causes

• Threads may not start simultaneously if there is a delay in the initiation of thread executions due to JVM thread scheduler.
• Using `Thread.sleep()` or long-running initializations in threads can cause staggered starts.

Solutions

• Create a thread pool using `ExecutorService` which manages thread execution and improves simultaneous execution.
• Utilize `CountDownLatch` from `java.util.concurrent` to synchronize the start of multiple threads.
• Ensure any initialization processes are completed before starting threads to prevent delays.

Mistake: Starting threads without proper synchronization which can lead to race conditions.

Solution: Use synchronization tools like `CountDownLatch` or `CyclicBarrier` to manage thread start.

Mistake: Using Thread.sleep to delay starts, which can lead to unpredictable behavior of threads.

Solution: Avoid using sleeps for synchronization; instead, rely on concurrency utilities.