Java has long been a preferred language for building high-performance applications, thanks in part to its powerful concurrency capabilities. As demands for responsiveness and scalability grow, understanding Java’s concurrency model especially with the introduction of virtual threads in Java 21 has become essential for backend developers. In this post, we’ll explore traditional multithreading, the Executor Framework, and virtual threads, diving into how these tools can help you build efficient, scalable applications.

1. What is Concurrency?

Concurrency is the ability to handle multiple tasks simultaneously, which improves an application’s responsiveness. For example, a web server processing thousands of simultaneous user requests can perform better when tasks run in parallel rather than waiting in line. In Java, concurrency is often achieved using threads.

2. Threads in Java

A thread is an independent path of execution within a program. Java has built-in support for threads through the java.lang.Thread class and the Runnable interface. Creating threads allows you to split tasks and execute them concurrently, enhancing performance. Here’s a simple example:

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}

public class Main {
    public static void main(String[] args) {
        MyThread thread = new MyThread();
        thread.start();
    }
}

With this, Java’s traditional model of multithreading gives developers the ability to create multiple threads to perform different tasks concurrently.

3. The Executor Framework

Starting with Java 5, the Executor Framework improved thread management. It allows developers to create and control pools of threads, making it easier to run a large number of concurrent tasks without exhausting system resources. Here’s an example using Executors:

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(5);

        for (int i = 0; i < 10; i++) {
            executor.submit(() -> {
                System.out.println("Executing Task: " + Thread.currentThread().getName());
            });
        }
        executor.shutdown();
    }
}

This approach enables better control over system resources by limiting the number of threads running concurrently.

4. Managing Synchronization

Concurrency introduces challenges like data inconsistency, which occurs when multiple threads access and modify shared resources simultaneously. Java provides synchronized blocks to prevent multiple threads from executing a critical section at the same time, maintaining data integrity:

public class Counter {
    private int count = 0;

    public synchronized void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

By synchronizing the increment method, we ensure that only one thread at a time can execute it, preventing conflicts.

5. Advanced Concurrency Utilities

Java’s java.util.concurrent package includes utilities like:

CountDownLatch: Allows threads to wait for other threads to complete.
CyclicBarrier: Useful for coordinating multiple threads that must meet at a certain point before continuing.
Semaphore: Controls the number of threads that can access a resource.

For example, using CountDownLatch:

import java.util.concurrent.CountDownLatch;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(3);

        for (int i = 0; i < 3; i++) {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName() + " completed its task");
                latch.countDown();
            }).start();
        }

        latch.await();
        System.out.println("All tasks completed");
    }
}

6. CompletableFuture and Asynchronous Programming

Java 8 introduced CompletableFuture for asynchronous programming. Unlike synchronous methods, it allows you to run tasks in the background without blocking other processes:

import java.util.concurrent.CompletableFuture;

public class Main {
    public static void main(String[] args) {
        CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
            System.out.println("Running asynchronously in " + Thread.currentThread().getName());
        });

        future.join();
    }
}

7. Virtual Threads: A New Paradigm for Concurrency in Java

With the release of Java 21, virtual threads bring a game-changing approach to concurrency. Traditional threads (or “platform threads”) are managed by the OS and are relatively resource-intensive. In contrast, virtual threads are lightweight threads managed by the JVM itself, allowing applications to handle thousands or even millions of threads efficiently.

How Virtual Threads Work

Virtual threads are decoupled from OS threads, enabling the JVM to manage and schedule them with minimal resource overhead. They are ideal for I/O-bound applications and significantly improve scalability.

Here’s an example of using virtual threads:

public class VirtualThreadExample {
    public static void main(String[] args) {
        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 0; i < 10; i++) {
                executor.submit(() -> {
                    System.out.println("Running in virtual thread: " + Thread.currentThread());
                });
            }
        }
    }
}

Using newVirtualThreadPerTaskExecutor(), each task runs in a lightweight virtual thread, allowing the application to handle massive concurrency with ease.

Advantages of Virtual Threads

Massive Concurrency: Virtual threads make it feasible to handle thousands or even millions of tasks concurrently.
Seamless Integration: They work with existing Java APIs, making it easy to migrate from traditional threads.
Resource Efficiency: Virtual threads reduce the need for complex thread pools, simplifying code while maintaining high performance.

Limitations and Best Practices

Virtual threads work best in applications with I/O-bound tasks. For CPU-bound operations, traditional platform threads might still be more efficient. Additionally, blocking calls that don’t yield control to the JVM may reduce the efficiency of virtual threads, so understanding which operations are blocking remains crucial.

Final Thoughts

Java’s concurrency capabilities have evolved immensely, offering developers flexible and powerful tools for building efficient, scalable applications. From traditional multithreading and the Executor Framework to the revolutionary virtual threads, Java now makes it easier than ever to handle massive concurrency with minimal complexity.

Whether you’re building a high-performance web server or managing complex background tasks, virtual threads are a valuable addition to your toolkit. By mastering these tools, you’ll be well-equipped to create applications that can handle today’s demanding workloads and prepare for the future of Java development.

Mastering Concurrency in Java: From Traditional Multithreading to Virtual Threads

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