Thread Pools & Executors

Creating threads manually is expensive. Thread pools reuse a fixed number of threads for many tasks, improving performance. The Executor framework (java.util.concurrent) provides high-level thread management. This is a foundational concept in enterprise application development that professional developers rely on daily. The explanations below are written to be beginner-friendly while covering the depth and nuance that comes from real-world Java experience. Take your time with each section and practice the examples

40 min•By Priygop Team•Last updated: Feb 2026

Thread Pool Example

Example

import java.util.concurrent.*;
import java.util.*;

public class ThreadPoolDemo {
    public static void main(String[] args) throws Exception {
        // Fixed thread pool — 3 worker threads
        ExecutorService pool = Executors.newFixedThreadPool(3);
        
        System.out.println("=== Thread Pool Demo ===");
        
        // Submit 6 tasks (only 3 run at a time)
        for (int i = 1; i <= 6; i++) {
            final int taskId = i;
            pool.submit(() -> {
                String thread = Thread.currentThread().getName();
                System.out.println("Task " + taskId + " starting on " + thread);
                try { Thread.sleep(1000); } catch (InterruptedException e) {}
                System.out.println("Task " + taskId + " completed on " + thread);
            });
        }
        
        // Shut down the pool
        pool.shutdown();
        pool.awaitTermination(10, TimeUnit.SECONDS);
        System.out.println("\nAll tasks completed!");
        
        // Callable + Future: get results from threads
        ExecutorService executor = Executors.newFixedThreadPool(3);
        
        List<Future<Long>> futures = new ArrayList<>();
        long[] ranges = {100000, 200000, 300000};
        
        for (long range : ranges) {
            final long r = range;
            Future<Long> future = executor.submit(() -> {
                long sum = 0;
                for (long i = 1; i <= r; i++) sum += i;
                return sum;
            });
            futures.add(future);
        }
        
        System.out.println("\n=== Computation Results ===");
        for (int i = 0; i < futures.size(); i++) {
            System.out.println("Sum to " + ranges[i] + ": " + futures.get(i).get());
        }
        
        executor.shutdown();
    }
}

Try It Yourself: Parallel Task Processor

Try It Yourself: Parallel Task ProcessorJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.concurrent.*;
import java.util.*;

public class ParallelProcessor {
    public static void main(String[] args) throws Exception {
        ExecutorService pool = Executors.newFixedThreadPool(4);
        
        // Simulate processing multiple data files
        String[] tasks = {
            "Process user data",
            "Generate reports",
            "Send emails",
            "Update database",
            "Clean cache",
            "Sync backups"
        };
        
        System.out.println("=== Parallel Task Processor ===");
        System.out.println("Using 4 worker threads for " + tasks.length + " tasks\n");
        
        long start = System.currentTimeMillis();
        
        List<Future<String>> futures = new ArrayList<>();
        for (String task : tasks) {
            Future<String> future = pool.submit(() -> {
                String thread = Thread.currentThread().getName();
                System.out.println("[START] " + task + " (" + thread + ")");
                Thread.sleep((long)(Math.random() * 2000 + 500));
                System.out.println("[DONE]  " + task + " (" + thread + ")");
                return task + " -> OK";
            });
            futures.add(future);
        }
        
        // Collect results
        System.out.println("\n=== Results ===");
        for (Future<String> f : futures) {
            System.out.println(f.get());
        }
        
        long elapsed = System.currentTimeMillis() - start;
        System.out.println("\nTotal time: " + elapsed + "ms");
        
        pool.shutdown();
    }
}

import java.util.concurrent.*;
import java.util.*;

public class ParallelProcessor {
    public static void main(String[] args) throws Exception {
        ExecutorService pool = Executors.newFixedThreadPool(4);
        
        // Simulate processing multiple data files
        String[] tasks = {
            "Process user data",
            "Generate reports",
            "Send emails",
            "Update database",
            "Clean cache",
            "Sync backups"
        };
        
        System.out.println("=== Parallel Task Processor ===");
        System.out.println("Using 4 worker threads for " + tasks.length + " tasks\n");
        
        long start = System.currentTimeMillis();
        
        List<Future<String>> futures = new ArrayList<>();
        for (String task : tasks) {
            Future<String> future = pool.submit(() -> {
                String thread = Thread.currentThread().getName();
                System.out.println("[START] " + task + " (" + thread + ")");
                Thread.sleep((long)(Math.random() * 2000 + 500));
                System.out.println("[DONE]  " + task + " (" + thread + ")");
                return task + " -> OK";
            });
            futures.add(future);
        }
        
        // Collect results
        System.out.println("\n=== Results ===");
        for (Future<String> f : futures) {
            System.out.println(f.get());
        }
        
        long elapsed = System.currentTimeMillis() - start;
        System.out.println("\nTotal time: " + elapsed + "ms");
        
        pool.shutdown();
    }
}

Output

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Edit the code on the left, then click Run

Java|46 lines|1598 chars|✓ Valid syntax

UTF-8

Quick Quiz: Thread Pools

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Thread Pools & Executors

40 min•By Priygop Team•Last updated: Feb 2026

Thread Pool Example

Example

import java.util.concurrent.*;
import java.util.*;

public class ThreadPoolDemo {
    public static void main(String[] args) throws Exception {
        // Fixed thread pool — 3 worker threads
        ExecutorService pool = Executors.newFixedThreadPool(3);
        
        System.out.println("=== Thread Pool Demo ===");
        
        // Submit 6 tasks (only 3 run at a time)
        for (int i = 1; i <= 6; i++) {
            final int taskId = i;
            pool.submit(() -> {
                String thread = Thread.currentThread().getName();
                System.out.println("Task " + taskId + " starting on " + thread);
                try { Thread.sleep(1000); } catch (InterruptedException e) {}
                System.out.println("Task " + taskId + " completed on " + thread);
            });
        }
        
        // Shut down the pool
        pool.shutdown();
        pool.awaitTermination(10, TimeUnit.SECONDS);
        System.out.println("\nAll tasks completed!");
        
        // Callable + Future: get results from threads
        ExecutorService executor = Executors.newFixedThreadPool(3);
        
        List<Future<Long>> futures = new ArrayList<>();
        long[] ranges = {100000, 200000, 300000};
        
        for (long range : ranges) {
            final long r = range;
            Future<Long> future = executor.submit(() -> {
                long sum = 0;
                for (long i = 1; i <= r; i++) sum += i;
                return sum;
            });
            futures.add(future);
        }
        
        System.out.println("\n=== Computation Results ===");
        for (int i = 0; i < futures.size(); i++) {
            System.out.println("Sum to " + ranges[i] + ": " + futures.get(i).get());
        }
        
        executor.shutdown();
    }
}

Try It Yourself: Parallel Task Processor

Try It Yourself: Parallel Task ProcessorJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.concurrent.*;
import java.util.*;

public class ParallelProcessor {
    public static void main(String[] args) throws Exception {
        ExecutorService pool = Executors.newFixedThreadPool(4);
        
        // Simulate processing multiple data files
        String[] tasks = {
            "Process user data",
            "Generate reports",
            "Send emails",
            "Update database",
            "Clean cache",
            "Sync backups"
        };
        
        System.out.println("=== Parallel Task Processor ===");
        System.out.println("Using 4 worker threads for " + tasks.length + " tasks\n");
        
        long start = System.currentTimeMillis();
        
        List<Future<String>> futures = new ArrayList<>();
        for (String task : tasks) {
            Future<String> future = pool.submit(() -> {
                String thread = Thread.currentThread().getName();
                System.out.println("[START] " + task + " (" + thread + ")");
                Thread.sleep((long)(Math.random() * 2000 + 500));
                System.out.println("[DONE]  " + task + " (" + thread + ")");
                return task + " -> OK";
            });
            futures.add(future);
        }
        
        // Collect results
        System.out.println("\n=== Results ===");
        for (Future<String> f : futures) {
            System.out.println(f.get());
        }
        
        long elapsed = System.currentTimeMillis() - start;
        System.out.println("\nTotal time: " + elapsed + "ms");
        
        pool.shutdown();
    }
}

import java.util.concurrent.*;
import java.util.*;

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|46 lines|1598 chars|✓ Valid syntax

UTF-8

Thread Pools & Executors

Thread Pool Example

Try It Yourself: Parallel Task Processor

Quick Quiz: Thread Pools

Topics in This Module

Thread Pools & Executors

Thread Pool Example

Try It Yourself: Parallel Task Processor

Quick Quiz: Thread Pools

Topics in This Module