Answer

What is ThreadPoolExecutor?

ThreadPoolExecutor is a powerful class that manages a pool of worker threads, efficiently executing multiple tasks without the overhead of creating new threads for each task.

Why Use Thread Pools?

Key Parameters

val executor = ThreadPoolExecutor(
    2,                      // corePoolSize: Minimum threads
    4,                      // maximumPoolSize: Max threads
    60L,                    // keepAliveTime: Idle thread timeout
    TimeUnit.SECONDS,
    LinkedBlockingQueue<Runnable>(),  // Work queue
    ThreadFactory { r -> Thread(r, "MyThread") },
    ThreadPoolExecutor.AbortPolicy()  // Rejection policy
)

How It Works

1. Task submitted
2. If threads < corePoolSize → Create new thread
3. If threads >= corePoolSize → Add to queue
4. If queue full AND threads < maxPoolSize → Create new thread
5. If queue full AND threads >= maxPoolSize → Reject task

Common Executors (Factory Methods)

// Fixed thread pool
val fixed = Executors.newFixedThreadPool(4)

// Cached thread pool (creates as needed)
val cached = Executors.newCachedThreadPool()

// Single thread executor
val single = Executors.newSingleThreadExecutor()

// Scheduled executor
val scheduled = Executors.newScheduledThreadPool(2)

Usage Example

val executor = Executors.newFixedThreadPool(4)

// Submit tasks
executor.execute { 
    // Background work
}

val future = executor.submit<String> {
    // Return result
    "Result"
}
val result = future.get()  // Blocks until complete

// Shutdown properly
executor.shutdown()
executor.awaitTermination(5, TimeUnit.SECONDS)

Rejection Policies

Best Practices

• Always shutdown executors when done
• Choose pool size based on task type (CPU vs I/O bound)
• Consider using Coroutines for simpler async code
• Use meaningful thread names for debugging