April 16, 2024


My Anti-Drug Is Computer


Intro to virtual threads: A new approach to Java concurrency

One particular of the most considerably-reaching Java 19 updates is the introduction of digital threads. Virtual threads are section of Project Loom, and are obtainable in Java 19 as a preview.

How virtual threads perform

Virtual threads introduce an abstraction layer between functioning-technique processes and application-stage concurrency. Said otherwise, digital threads can be made use of to schedule jobs that the Java virtual machine orchestrates, so the JVM mediates in between the functioning system and the software. Figure 1 demonstrates the architecture of virtual threads.

Architecture of virtual threads in Java. IDG

Figure 1. The architecture of virtual threads in Java.

In this architecture, the application instantiates virtual threads and the JVM assigns the compute means to deal with them. Contrast this to traditional threads, which are mapped immediately onto functioning system (OS) procedures. With common threads, the application code is liable for provisioning and dispensing OS resources. With virtual threads, the software instantiates virtual threads and thus expresses the require for concurrency. But it is the JVM that obtains and releases the resources from the running technique.

Virtual threads in Java are analogous to goroutines in the Go language. When making use of digital threads, the JVM is only equipped to assign compute sources when the application’s virtual threads are parked, that means that they are idle and awaiting new get the job done. This idling is prevalent with most servers: they assign a thread to a request and then it idles, awaiting a new function like a response from a datastore or even more input from the network.

Using standard Java threads, when a server was idling on a ask for, an running system thread was also idling, which seriously constrained the scalability of servers. As Nicolai Parlog has described, “Running systems cannot maximize the effectiveness of platform threads, but the JDK will make better use of them by severing the a single-to-a single marriage among its threads and OS threads.”

Prior initiatives to mitigate the overall performance and scalability troubles linked with regular Java threads involve asynchronous, reactive libraries like JavaRX. What’s distinctive about digital threads is they are executed at the JVM amount, and nonetheless they in shape into the current programming constructs in Java.

Applying Java digital threads: A demo

For this demonstration, I’ve produced a very simple Java application with the Maven archetype. I’ve also manufactured a couple of alterations to allow virtual threads in the Java 19 preview. You won’t need to make these modifications the moment digital threads are promoted out of preview. 

Listing 1 reveals the adjustments I manufactured to the Maven archetype’s POM file. Note that I also set the compiler to use Java 19 and (as shown in Listing 2) added a line to the .mvn/jvm.config.

Listing 1. The pom.xml for the demo software



The --enable-preview swap is required to make exec:java perform with preview enabled. It begins the Maven process with the essential change.

Listing 2. Including permit-preview to .mvn/jvm.config


Now, you can execute the plan with mvn compile exec:java and the digital thread features will compile and execute.

Two techniques to use digital threads

Now let us take into consideration the two major strategies you’ll really use digital threads in your code. While virtual threads current a extraordinary alter to how the JVM will work, the code is in fact really similar to regular Java threads. The similarity is by design and will make refactoring present applications and servers reasonably straightforward. This compatibility also suggests that current applications for checking and observing threads in the JVM will do the job with digital threads.

Thread.startVirtualThread(Runnable r)

The most fundamental way to use a digital thread is with Thread.startVirtualThread(Runnable r). This is a substitution for instantiating a thread and contacting thread.start out(). Look at the sample code in Listing 3.

Listing 3. Instantiating a new thread

deal com.infoworld

import java.util.Random

general public class Application 
  general public static void major( String[] args ) 
    boolean vThreads = args.length > 
    System.out.println( "Applying vThreads: " + vThreads)

    extensive get started = Technique.currentTimeMillis()

    Random random = new Random()
    Runnable runnable = () ->  double i = random.nextDouble(1000) % random.nextDouble(1000)    
    for (int i =  i < 50000 i++)
      if (vThreads) 
        Thread t = new Thread(runnable)
    long finish = System.currentTimeMillis()
    long timeElapsed = finish - start
    System.out.println("Run time: " + timeElapsed)

When run with an argument, the code in Listing 3 will use a virtual thread otherwise it will use conventional threads. The program spawns 50 thousand iterations of whichever thread type you choose. Then, it does some simple math with random numbers and tracks how long the execution takes.

To run the code with virtual threads, type: mvn compile exec:java -Dexec.args="true". To run with standard threads, type: mvn compile exec:java. I did a quick performance test and got the results below:

  • With virtual threads: Runtime: 174
  • With conventional threads: Runtime: 5450

These results are unscientific, but the difference in runtimes is substantial.

There are other ways of using Thread to spawn virtual threads, like Thread.ofVirtual().start(runnable). See the Java threads documentation for more information.

Using an executor

The other primary way to start a virtual thread is with an executor. Executors are common in dealing with threads, offering a standard way to coordinate many tasks and thread pooling.

Pooling is not required with virtual threads because they are cheap to create and dispose of, and therefore pooling is unnecessary. Instead, you can think of the JVM as managing the thread pool for you. Many programs do use executors, however, and so Java 19 includes a new preview method in executors to make refactoring to virtual threads easy. Listing 4 show you the new method alongside the old.

Listing 4. New executor methods

ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor() // New method
ExecutorService executor = Executors.newFixedThreadPool(Integer poolSize) // Old method

In addition, Java 19 introduces the Executors.newThreadPerTaskExecutor(ThreadFactory threadFactory) method, which can take a ThreadFactory that builds virtual threads. Such a factory can be obtained with Thread.ofVirtual().factory().

Best practices for virtual threads

In general, because virtual threads implement the Thread class, they can be used anywhere that a standard thread would be. However, there are differences in how virtual threads should be used for best effect.  One example is using semaphores to control the number of threads when accessing a resource like a datastore, instead of using a thread pool with a limit. See Coming to Java 19: Virtual threads and platform threads for more tips.

Another important note is that virtual threads are always daemon threads, meaning they'll keep the containing JVM process alive until they complete. Also, you cannot change their priority. The methods for changing priority and daemon status are no-ops. See the Threads documentation for more about this.

Refactoring with virtual threads

Virtual threads are a big change under the hood, but they are intentionally easy to apply to an existing codebase. Virtual threads will have the biggest and most immediate impact on servers like Tomcat and GlassFish. Such servers should be able to adopt virtual threading with minimal effort. Applications running on these server will net scalability gains without any changes to the code, which could have enormous implications for large-scale applications. Consider a Java application running on many servers and cores suddenly, it will be able to handle an order-of-magnitude more concurrent requests (although, of course, it all depends on the request-handling profile).

It may be just a matter of time before servers like Tomcat allow for virtual threads with a configuration parameter. In the meantime, if you are curious about migrating a server to virtual threads, consider this blog post by Cay Horstmann, where he shows the process of configuring Tomcat for virtual threads. He enables the virtual threads preview features and replaces the Executor with a custom implementation that differs by only a single line (you guessed it, Executors.newThreadPerTaskExecutor). The scalability benefit is significant, as he says: “With that change, 200 requests took 3 seconds, and Tomcat can easily take 10,000 requests.”


Virtual threads are a major change to the JVM. For application programmers, they represent an alternative to asynchronous-style coding such as using callbacks or futures. All told, we could see virtual threads as a pendulum swing back towards a synchronous programming paradigm in Java, when dealing with concurrency. This is roughly analogous in programming style (though not at all in implementation) to JavaScript’s introduction of async/await. In short, writing correct asynchronous behavior with simple synchronous syntax becomes quite easy—at least in applications where threads spend a lot of time idling.

Check out the following resources to learn more about virtual threads:

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