Q: How does ExecutorService and Thread Pools work?

Answer:

Why Thread Pools?

Creating a new Thread for every task is expensive (OS thread creation, memory allocation). Thread pools reuse a fixed set of threads to execute many tasks.

ExecutorService

The ExecutorService interface is the standard API for managing thread pools in Java.

// Fixed-size pool: always 4 threads
ExecutorService pool = Executors.newFixedThreadPool(4);

// Cached pool: creates threads on-demand, reuses idle ones
ExecutorService pool = Executors.newCachedThreadPool();

// Single thread: tasks execute sequentially in one thread
ExecutorService pool = Executors.newSingleThreadExecutor();

// Scheduled: run tasks after a delay or periodically
ScheduledExecutorService pool = Executors.newScheduledThreadPool(2);

Submitting Tasks

ExecutorService pool = Executors.newFixedThreadPool(4);

// Fire-and-forget (Runnable)
pool.submit(() -> System.out.println("Task 1"));

// Get a result (Callable)
Future<Integer> future = pool.submit(() -> computeExpensiveThing());
Integer result = future.get(); // Blocks until done

// Shutdown
pool.shutdown();            // Finish current tasks, reject new ones
pool.shutdownNow();         // Interrupt running tasks immediately
pool.awaitTermination(5, TimeUnit.SECONDS); // Wait for completion

ThreadPoolExecutor (Full Control)

The Executors factory methods are shortcuts. For production, configure ThreadPoolExecutor directly:

ThreadPoolExecutor executor = new ThreadPoolExecutor(
    4,                // corePoolSize: minimum threads kept alive
    8,                // maxPoolSize: maximum threads allowed
    60, TimeUnit.SECONDS,  // keepAliveTime for idle threads above core
    new ArrayBlockingQueue<>(100),  // work queue capacity
    new ThreadPoolExecutor.CallerRunsPolicy()  // rejection policy
);

Rejection Policies

When both the thread pool and work queue are full:

[!WARNING] Avoid Executors.newCachedThreadPool() in production unless you're sure about your workload. It creates an unbounded number of threads. A sudden traffic spike can spawn thousands of threads, exhausting memory and crashing the JVM. Always use ThreadPoolExecutor with explicit bounds.