ExecutorService

The ExecutorService interface represents the standard Java framework for executing asynchronous tasks. Decoupling task submission from thread management allows configuring complex pooling strategies and task queues.

ThreadPoolExecutor Parameters

When creating a custom ThreadPoolExecutor, you configure:

• corePoolSize: The number of threads to keep in the pool, even if they are idle.
• maximumPoolSize: The maximum number of threads allowed in the pool.
• workQueue: The blocking queue used to hold tasks before execution.
• handler: The Rejection Policy executed when the queue is full and threads are saturated (e.g. AbortPolicy, CallerRunsPolicy, DiscardPolicy, DiscardOldestPolicy).

Growth Mechanics (Crucial Interview Detail)

When a task is submitted: 1. If active threads < corePoolSize, a new thread is created. 2. If active threads >= corePoolSize, the task is added to the workQueue. 3. If the queue is full and active threads < maximumPoolSize, a new thread is created. 4. If the queue is full and active threads == maximumPoolSize, the rejection policy is triggered.

Code Walkthrough

This program shows how to configure a thread pool and shut it down cleanly.

import java.util.concurrent.*;
public class ExecutorServiceDemo {
    public static void main(String[] args) {
        // Create pool using Executors utility
        ExecutorService executor = Executors.newFixedThreadPool(3);
for (int i = 0; i < 5; i++) {
            int taskId = i;
            executor.submit(() -> {
                System.out.println("Executing Task " + taskId + " via " + Thread.currentThread().getName());
            });
        }
executor.shutdown(); // Refuses new tasks, completes existing ones
        try {
            if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
                executor.shutdownNow(); // Force cancel pending tasks
            }
        } catch (InterruptedException e) {
            executor.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
}

Interview-Relevant Information