ExecutorService & Thread Pools

Syntax

ExecutorService executor = Executors.newFixedThreadPool(n);

Examples

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

ExecutorService executor = Executors.newFixedThreadPool(3);

for (int i = 1; i <= 5; i++) {
    int taskId = i;
    executor.submit(() -> {
        System.out.println("Running task " + taskId + " on " + Thread.currentThread().getName());
    });
}

executor.shutdown();  // stop accepting new tasks, let submitted ones finish

import java.util.concurrent.*;

ExecutorService executor = Executors.newFixedThreadPool(2);

Future<Integer> future = executor.submit(() -> {
    Thread.sleep(100);
    return 42;
});

try {
    Integer result = future.get();  // blocks until the task finishes
    System.out.println("Result: " + result);
} catch (InterruptedException | ExecutionException e) {
    System.out.println("Task failed: " + e.getMessage());
}

executor.shutdown();

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

ExecutorService fixedPool = Executors.newFixedThreadPool(4);   // exactly 4 threads
ExecutorService cachedPool = Executors.newCachedThreadPool();   // grows/shrinks as needed
ExecutorService singleThread = Executors.newSingleThreadExecutor(); // one thread, tasks run in order

singleThread.submit(() -> System.out.println("Task A"));
singleThread.submit(() -> System.out.println("Task B"));  // guaranteed to run after A

fixedPool.shutdown();
cachedPool.shutdown();
singleThread.shutdown();

Best practices

• Always call shutdown() (or shutdownNow() for immediate termination) when done with an ExecutorService - forgetting to do so leaves threads running indefinitely
• Use newFixedThreadPool() for a predictable, bounded number of concurrent tasks, protecting your system from unbounded thread creation
• Handle both InterruptedException and ExecutionException when calling future.get(), since either can occur if the task fails or is interrupted
• Prefer ExecutorService over manually creating raw Thread objects for any real application - it is more efficient and easier to manage correctly

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation