Thread Pool

Creating a thread is expensive because it requires allocating OS memory, native resources, and runtime stacks. A thread pool optimizes execution by maintaining a collection of pre-instantiated worker threads that consume tasks from a shared queue.

Mechanics of a Thread Pool

A standard thread pool consists of:

• Worker Threads: A set of threads that run in loops, continuously polling a task queue for work.
• Task Queue: A blocking queue that holds tasks submitted to the pool when all worker threads are busy.
• Task Submission: Callers submit tasks (implementing Runnable or Callable) to the pool.

Code Walkthrough

This is a basic implementation of a custom thread pool using standard queue synchronization.

import java.util.LinkedList;
import java.util.Queue;
public class CustomThreadPool {
    private final Queue<Runnable> taskQueue = new LinkedList<>();
    private final Worker[] threads;
public CustomThreadPool(int poolSize) {
        threads = new Worker[poolSize];
        for (int i = 0; i < poolSize; i++) {
            threads[i] = new Worker();
            threads[i].start();
        }
    }
public void execute(Runnable task) {
        synchronized (taskQueue) {
            taskQueue.add(task);
            taskQueue.notify(); // Wake up idle worker thread
        }
    }
private class Worker extends Thread {
        public void run() {
            Runnable task;
            while (true) {
                synchronized (taskQueue) {
                    while (taskQueue.isEmpty()) {
                        try {
                            taskQueue.wait(); // Wait for task submission
                        } catch (InterruptedException e) {
                            return;
                        }
                    }
                    task = taskQueue.poll();
                }
                try {
                    task.run(); // Execute task
                } catch (RuntimeException e) {
                    // Prevent thread death on user exceptions
                }
            }
        }
    }
}

Interview-Relevant Information