Core Java

The Future of Async in Java: CompletableFuture vs Virtual Threads

Asynchronous programming is a cornerstone of modern Java applications, allowing them to handle tasks without blocking the main thread. But with Java 21 comes a new challenger: virtual threads. These lightweight alternatives to traditional OS threads promise significant performance improvements. However, the familiar CompletableFuture remains a powerful tool for asynchronous operations. This article dives into the strengths and weaknesses of both approaches, helping you choose the right weapon for your asynchronous warfare in Java!

java logo

1.The Rise of Async

In today’s fast-paced world, applications need to be responsive and efficient. Traditional, synchronous programming, where the main thread waits for each task to finish before moving on, can lead to sluggish performance. This is where asynchronous programming comes to the rescue!

Asynchronous programming allows Java applications to handle multiple tasks concurrently without blocking the main thread. Imagine a server waiting for a database query to complete. Instead of freezing the entire application, asynchronous programming lets the server continue processing other requests while the database query runs in the background. This keeps the application responsive and provides a smoother user experience.

With the introduction of virtual threads in Java 21, the landscape of asynchronous programming is poised for a significant change. These lightweight alternatives to traditional operating system (OS) threads promise improved performance and resource utilization.

However, CompletableFuture, a well-established tool in the asynchronous programming toolbox, remains a powerful option. It offers a structured way to handle asynchronous tasks and has been widely adopted in Java development.

This article delves into the world of CompletableFuture and virtual threads, exploring their strengths and weaknesses. We’ll compare and contrast these approaches to help you choose the right weapon for tackling asynchronous operations in your Java applications.

2. CompletableFuture: A Familiar Friend

CompletableFuture is a class introduced in Java 8 that represents a future result of an asynchronous computation. It enables you to write asynchronous, non-blocking code, enhancing application performance and responsiveness.

  • Key Features:
    • Asynchronous Task Chaining:
      • thenApply: Transforms the result of a completed CompletableFuture.
      • thenAccept: Performs an action on the result.
      • thenCombine: Combines results of two CompletableFutures.
      • More methods for complex workflows.
    • Result Handling:
      • join: Waits for completion and retrieves result (potentially blocking).
      • get: Similar to join, but with timeout options and exception handling.
    • Error Handling:
      • exceptionally: Provides a fallback action in case of errors.
      • handle: Handles both successful completion and exceptions.
public static CompletableFuture<String> downloadFile(String url) {
  return CompletableFuture.supplyAsync(() -> {
    // Simulate downloading a file
    try {
      Thread.sleep(2000); // Simulate download time
      return "File downloaded successfully!";
    } catch (InterruptedException e) {
      throw new RuntimeException("Download interrupted!");

public static void main(String[] args) {
      .thenAccept(result -> System.out.println(result))
      .exceptionally(error -> {
        System.err.println("Error downloading file: " + error.getMessage());
        return null;

  System.out.println("Doing other tasks while downloading...");

In this example:

  • downloadFile returns a CompletableFuture that represents the eventual download completion.
  • thenAccept is used to handle the successful download result (printing a message).
  • exceptionally defines what to do if an error occurs during download (printing an error message).
  • The main program can continue executing tasks (System.out.println("Doing other tasks while downloading...")) because it’s not blocked waiting for the download to finish.

Limitations to Consider:

  • Thread Pool Overhead: CompletableFuture relies on thread pools to execute asynchronous tasks. While lightweight compared to OS threads, there can still be some overhead associated with managing the thread pool.
  • Context Inheritance: By default, CompletableFuture doesn’t inherently inherit context (like security credentials) from the calling thread. You might need to explicitly pass this context along if required by your asynchronous operation.

Despite these limitations, CompletableFuture remains a valuable tool for asynchronous programming in Java. However, virtual threads, introduced in Java 21, offer a new contender in the asynchronous arena. Let’s explore them next!

3. Virtual Threads: A Lightweight Contender

While CompletableFuture provides a structured approach to asynchronous programming, Java 21 introduces a game-changer: virtual threads. These innovative threads offer a significant departure from traditional OS threads, bringing several advantages to the table.

Traditional OS Threads vs. Virtual Threads: A Tale of Two Worlds

  • OS Threads: These are heavyweight entities managed directly by the operating system. Creating and managing them can be resource-intensive, especially when dealing with a large number of concurrent tasks.
  • Virtual Threads: These are lightweight alternatives that operate within the Java Virtual Machine (JVM) itself. They don’t directly map to OS threads, making them significantly less resource-intensive to create and manage.

The Power of Lightweight Virtual Threads

Virtual threads boast several advantages over traditional OS threads:

  • Lightweight Creation: Creating and managing virtual threads is much faster and less resource-consuming compared to OS threads. This allows you to have a much larger pool of concurrent tasks without significant overhead.
  • Efficient Resource Utilization: Since virtual threads are lightweight, they require fewer system resources. This translates to improved overall application performance and scalability, especially on resource-constrained systems.
  • Improved Scalability: With their lightweight nature, virtual threads allow applications to handle a much larger number of concurrent tasks efficiently. This is particularly beneficial for applications that deal with a high volume of asynchronous operations.

Code Example: Downloading a File with Virtual Threads

Here’s an example (simplified for illustrative purposes) demonstrating how to use virtual threads for a similar download task as the CompletableFuture example:

public static void downloadFileVirtualThread(String url) {
  new VirtualThread(() -> {
    // Simulate downloading a file
    try {
      Thread.sleep(2000); // Simulate download time
      System.out.println("File downloaded successfully!");
    } catch (InterruptedException e) {
      System.err.println("Download interrupted!");

public static void main(String[] args) {

  System.out.println("Doing other tasks while downloading...");

In this example:

  • downloadFileVirtualThread creates a new VirtualThread object and assigns it the download task (similar to a lambda function).
  • start() starts the execution of the virtual thread.
  • The main program can continue executing tasks (System.out.println("Doing other tasks while downloading...")) because the virtual thread handles the download asynchronously.

Limitations to Consider

  • Early Development Stage: Virtual threads are a relatively new feature in Java. While promising, they are still under development, and the API might evolve in future Java versions.
  • Maturity Compared to CompletableFuture: CompletableFuture has been around for a while and has a more mature ecosystem of libraries and tools built around it. Virtual threads are still catching up in this regard.

Although virtual threads are exciting, it’s important to acknowledge their limitations. CompletableFuture remains a well-established tool with a proven track record.

4. When to Use CompletableFuture or Virtual Threads

Here’s a table summarizing the key differences between CompletableFuture and virtual threads:

FeatureCompletableFutureVirtual Threads
Thread ModelRelies on underlying thread pool (OS threads)Lightweight threads managed by the JVM
OverheadModerate overhead for thread pool managementVery low overhead for creating and managing threads
ComplexityRelatively simple APISlightly more complex API (new concept in Java 21)
Error HandlingProvides mechanisms for handling exceptionsError handling works similarly to traditional threads
Context InheritanceDoesn’t inherently inherit context from calling threadCan inherit context from the parent thread

Scenarios for CompletableFuture:

  • Existing Codebase: If your codebase already utilizes CompletableFuture effectively, there might not be a pressing need to switch to virtual threads immediately.
  • Simpler Asynchronous Tasks: For simpler asynchronous operations that don’t require extreme resource efficiency, CompletableFuture can be a good choice due to its familiar API.

Scenarios for Virtual Threads:

  • Highly Concurrent Tasks: Applications dealing with a massive number of concurrent operations can benefit significantly from the lightweight nature of virtual threads.
  • Improved Resource Utilization: If resource efficiency is a top priority, virtual threads can help you handle more concurrent tasks with fewer system resource requirements.
  • Future-Proofing: As virtual threads mature, they have the potential to become the dominant approach for asynchronous programming in Java.

Choosing the Right Tool:

Both CompletableFuture and virtual threads have their strengths and weaknesses. The optimal choice depends on your specific project requirements:

  • For existing codebases with simpler asynchronous tasks and a focus on maintainability, CompletableFuture might be a good fit.
  • For applications requiring high concurrency, resource efficiency, and future-proofing, virtual threads offer a compelling alternative.

5. Conclusion: The Future of Async Awaits

The landscape of asynchronous programming in Java is evolving rapidly. CompletableFuture has established itself as a powerful tool for managing asynchronous tasks, offering a structured and familiar approach. However, the arrival of virtual threads in Java 21 presents an exciting new contender. Their lightweight nature and efficient resource utilization make them ideal for applications demanding high concurrency and scalability.

While virtual threads are still under development, their potential is undeniable. Choosing between CompletableFuture and virtual threads depends on your specific project needs. Existing codebases and simpler asynchronous tasks might be well-served by CompletableFuture’s established API. However, for applications prioritizing resource efficiency and high concurrency, virtual threads offer a compelling path forward.

The future of asynchronous programming in Java likely involves a coexistence of both approaches. As virtual threads mature and gain wider adoption, they might become the dominant force. Regardless of which approach you choose, understanding both CompletableFuture and virtual threads equips you to tackle asynchronous operations effectively in your Java applications. Stay tuned for further advancements in this exciting domain of software development!

Eleftheria Drosopoulou

Eleftheria is an Experienced Business Analyst with a robust background in the computer software industry. Proficient in Computer Software Training, Digital Marketing, HTML Scripting, and Microsoft Office, they bring a wealth of technical skills to the table. Additionally, she has a love for writing articles on various tech subjects, showcasing a talent for translating complex concepts into accessible content.
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

1 Comment
Newest Most Voted
Inline Feedbacks
View all comments
Nils Weinanser
Nils Weinanser
2 months ago

I would say that you may need both virtual threads and completable futures. If you want to pick up the result of a computation in a virtual thread in the main thread without blocking, you have use a completable future.

Back to top button