Java 21 Virtual Threads

Introduction to Virtual Threads

Java 21 introduces one of the most significant enhancements to the Java concurrency model since the introduction of the java.util.concurrent package in Java 5: Virtual Threads. This feature, developed under Project Loom, addresses the long-standing limitations of platform threads in Java and brings a new level of scalability to concurrent applications.

Virtual threads are lightweight threads that are managed by the Java Virtual Machine (JVM) rather than the operating system. This fundamental difference allows applications to create millions of virtual threads, making it possible to adopt a "one thread per task" programming model without the overhead traditionally associated with thread creation and management.

The Problem with Platform Threads

Before diving into virtual threads, it's important to understand the limitations of platform threads (the traditional Java threads):

• Resource Intensive: Each platform thread consumes approximately 1MB of stack memory and requires kernel resources.
• Limited Scalability: Most systems can only efficiently manage thousands of platform threads, not millions.
• Context Switching Overhead: Switching between platform threads involves expensive kernel operations.
• Thread Pool Management: Developers must carefully tune thread pools to avoid resource exhaustion.

These limitations have led to the widespread adoption of asynchronous programming models using callbacks, CompletableFuture, and reactive programming libraries. While these approaches improve scalability, they often come at the cost of code complexity and maintainability.

How Virtual Threads Work

Virtual threads are implemented using a technique called "continuation," which allows a thread's execution to be suspended and resumed. Here's how they differ from platform threads:

When a virtual thread performs a blocking operation (like I/O or waiting for a lock), it doesn't block the underlying OS thread (called the "carrier thread"). Instead, the JVM unmounts the virtual thread from the carrier thread, allowing the carrier thread to execute other virtual threads. When the blocking operation completes, the virtual thread is scheduled to continue its execution on an available carrier thread.

Creating and Using Virtual Threads

Java 21 provides several ways to create and work with virtual threads. Here are the most common approaches:

// Creating a single virtual thread
Thread vThread = Thread.startVirtualThread(() -> {
    System.out.println("Running in a virtual thread");
});

// Using a virtual thread builder
Thread.Builder.OfVirtual builder = Thread.ofVirtual().name("worker-", 0);
Thread vThread = builder.startt(() => {
    System.out.println("Virtual thread with custom name");
});t(() => {
    System.out.println("Virtual thread with custom name");
});

// Creating a virtual thread executor
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i -> {
        executor.submit(() => {
            // Task logic here
            return result;
        });
    });
}

The most significant advantage is that you can now create thousands or even millions of virtual threads without worrying about resource constraints. This makes it possible to handle many concurrent connections in server applications without complex asynchronous programming models.

Performance Benefits

• Higher Throughput: Handle more concurrent requests.
• Reduced Latency: Less time waiting for thread pools.

The performance improvements from virtual threads can be substantial, especially for I/O-bound applications:

• Higher Throughput: Applications can handle more concurrent requests with the same hardware.
• Reduced Latency: Less time spent waiting for available threads in a pool.
• Better Resource Utilization: CPU cores can be kept busy even when many threads are blocked on I/O.
• Simplified Code: Straightforward imperative code can achieve the scalability of asynchronous approaches.

Limitations and Considerations

While virtual threads offer significant advantages, there are some important considerations:

• Thread-Local Variables: Excessive use of thread-locals can negate memory savings of virtual threads.
• Pinning: Some operations "pin" a virtual thread to its carrier thread, reducing efficiency.
• Synchronization: Heavy use of synchronized blocks can reduce the benefits of virtual threads.
• CPU-Bound Tasks: Virtual threads don't provide significant benefits for CPU-bound workloads.

Best Practices for Virtual Threads

To get the most out of virtual threads, consider these best practices:

1. Prefer java.util.concurrent Locks Over synchronized: The ReentrantLock class and other java.util.concurrent locks are virtual thread-friendly.
2. Minimize Thread-Local Usage: If you must use thread-locals, consider using ScopedValue instead.
3. Use Virtual Thread Per Task: Don't pool virtual threads; create a new one for each task.
4. Update Libraries: Ensure your libraries and frameworks are updated to be virtual thread-friendly.
5. Monitor and Profile: Use JDK Flight Recorder to identify pinning and other virtual thread issues.

Integration with AWS Services

Virtual threads can significantly improve the efficiency of Java applications running on AWS, particularly for:

• API Gateways and Microservices: Handle more concurrent requests with fewer resources.
• AWS Lambda Functions: Improve throughput for Java functions that make multiple async calls.
• Database Access: Make multiple parallel queries to Amazon RDS or DynamoDB without thread pool limitations.
• S3 Operations: Perform many concurrent S3 operations efficiently.

Conclusion

Java 21's virtual threads represent a paradigm shift in Java concurrency. By making threads cheap and plentiful, they allow developers to write simple, straightforward code that scales to handle thousands or millions of concurrent operations. This brings Java back to the forefront of high-performance server-side development, combining the readability of synchronous code with the scalability of asynchronous approaches.

As you consider adopting Java 21 for your projects, virtual threads should be one of the most compelling reasons to upgrade. They not only improve performance but also simplify your codebase by reducing the need for complex asynchronous programming patterns.