What is a Thread Pool and How to Create It in Java
💡 Concept: Thread Pool
A thread pool is a managed set of reusable threads used to execute tasks. It improves performance by reusing threads instead of creating new ones for every task.
📘 Quick Intro
Thread pools are managed via the Executor framework in Java. They help efficiently manage system resources when running many tasks concurrently.
🧠 Analogy
Imagine a restaurant with a fixed number of waiters. Instead of hiring a new waiter for each customer, the restaurant reuses its existing waiters — that’s a thread pool.
🔧 Technical Explanation
- Java provides
ExecutorslikenewFixedThreadPool()to create a thread pool. - Tasks are submitted using
execute()orsubmit()methods. - The pool manages a queue of submitted tasks and runs them using its threads.
- Thread pool prevents overhead of frequent thread creation and destruction.
🎯 Use Cases
- ✅ Running multiple tasks in parallel with limited system threads.
- ✅ Scenarios like web servers, data processing pipelines, and batch jobs.
- ✅ Helps manage CPU resources and avoid out-of-memory errors due to thread overload.
💻 Example: Creating a Thread Pool
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ThreadPoolExample {
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(3);
for (int i = 1; i <= 5; i++) {
final int task = i;
executor.submit(() -> {
System.out.println("Executing Task " + task + " on " + Thread.currentThread().getName());
});
}
executor.shutdown();
}
}
❓ Interview Q&A
Q1: What is a thread pool in Java?
A: A collection of worker threads used to execute tasks efficiently.
Q2: Why use a thread pool?
A: To avoid overhead of thread creation and improve system performance.
Q3: Which Java package provides thread pool support?
A: java.util.concurrent.
Q4: What does ExecutorService do?
A: Manages and controls the execution of asynchronous tasks.
Q5: Difference between execute() and submit()?
A: submit() returns a Future; execute() does not.
Q6: How to shutdown a thread pool?
A: Use shutdown() or shutdownNow() methods.
Q7: Can thread pools cause memory leaks?
A: Yes, if tasks hold references and pool isn't shut down properly.
Q8: What is newFixedThreadPool()?
A: Creates a thread pool with a fixed number of threads.
Q9: Can a thread pool have unbounded threads?
A: Yes, using newCachedThreadPool().
Q10: How does a thread pool improve scalability?
A: By reusing threads, reducing context-switching and memory usage.
📝 MCQs
Q1. What is the benefit of using a thread pool?
- Creates more threads
- Reuses threads to execute tasks efficiently
- Prevents CPU usage
- Blocks IO
Q2. Which class is used to manage a thread pool?
- Thread
- Runnable
- ExecutorService
- ThreadManager
Q3. Which method shuts down a thread pool?
- terminate()
- kill()
- shutdown()
- stop()
Q4. What is the key difference between execute() and submit()?
- submit() runs faster
- submit() returns a Future
- execute() runs twice
- submit() is deprecated
Q5. Which Executor creates a fixed number of threads?
- newThreadPool()
- newExecutor()
- newFixedThreadPool()
- newWorkerPool()
Q6. What happens when more tasks are submitted than threads in pool?
- Rejected
- Lost
- Wait in queue
- Run simultaneously
Q7. Can thread pools cause memory leaks?
- No
- Yes, if not shut down properly
- Only in Java 6
- Only if synchronized
Q8. What does shutdownNow() do?
- Waits for tasks
- Restarts pool
- Attempts to stop all tasks
- Creates new pool
Q9. What interface does ExecutorService extend?
- Callable
- Executor
- Thread
- Runnable
Q10. Can we submit Runnable and Callable to thread pool?
- Only Runnable
- Only Callable
- Yes
- No
💡 Bonus Insight
Use ThreadPoolExecutor directly for advanced control like setting queue capacity, core/max pool size, and rejection policies.
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