ExecutorService and thread pools

Key points

• A pool reuses a fixed set of threads. You submit tasks and the pool decides who runs them and when.
• newFixedThreadPool(n) for steady load, newCachedThreadPool() for short bursts, newSingleThreadExecutor() to force work into order.
• Size CPU-bound pools near the core count. I/O-bound pools need to be much larger, or use virtual threads instead.
• shutdown() drains the queue, shutdownNow() interrupts running tasks. Neither one blocks, so follow with awaitTermination.
• Since Java 19 ExecutorService is AutoCloseable, so try-with-resources shuts down and waits for you.
• An exception thrown inside execute() disappears quietly. Catch and log in the task, or use submit() and check the Future.

Example

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        ExecutorService pool = Executors.newFixedThreadPool(3);   // 3 threads, 6 jobs
        try {
            for (int i = 1; i <= 6; i++) {
                int job = i;
                pool.execute(() -> {
                    System.out.println("job " + job + " ran on " + Thread.currentThread().getName());
                    sleep(100);
                });
            }
        } finally {
            pool.shutdown();                                      // stop accepting, drain the queue
            boolean clean = pool.awaitTermination(5, TimeUnit.SECONDS);
            System.out.println("all jobs finished: " + clean);
        }
    }

    static void sleep(long ms) {
        try { Thread.sleep(ms); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
    }
}

Create pools, not threads, and always shut them down.