Fixed thread pool

Definition

An executor runs tasks for you on a pool of reusable threads. A fixed thread pool has a set number of threads: extra tasks wait in a queue until a thread is free. You submit work and the pool handles the threads.

Example code

Ten tasks on three threads

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

class Demo {
    public static void main(String[] args) throws InterruptedException {
        ExecutorService pool = Executors.newFixedThreadPool(3);
        Set<String> threadsUsed = ConcurrentHashMap.newKeySet();

        for (int i = 0; i < 10; i++) {
            pool.submit(() -> threadsUsed.add(Thread.currentThread().getName()));
        }
        pool.shutdown();
        pool.awaitTermination(5, TimeUnit.SECONDS);

        System.out.println("10 tasks ran on at most 3 threads: " + (threadsUsed.size() <= 3));
    }
}

// Output:
// 10 tasks ran on at most 3 threads: true