virtual threads.adoc

Vert.x Virtual Threads

Use virtual threads to write Vert.x code that looks like it is synchronous.

You still write the traditional Vert.x code processing events, but you have the opportunity to write synchronous code for complex workflows and use thread locals in such workflows.

Introduction

The non-blocking nature of Vert.x leads to asynchronous APIs. Asynchronous APIs can take various forms including callback style, promises or Rx-style. Vert.x uses futures in most places (although, it also supports Rx).

In some cases, programming using asynchronous APIs can be more challenging than using a direct synchronous style, in particular if you have several operations that you want to do in sequence. Also, error propagation is often more complex when using asynchronous APIs.

Virtual thread support allows you to work with asynchronous APIs, but using a direct synchronous style that you’re already familiar with.

It does this by using Java 21 virtual threads. Virtual threads are very lightweight threads that do not correspond to underlying kernel threads. When they are blocked they do not block a kernel thread.

Using virtual threads

You can deploy virtual thread verticles.

A virtual thread verticle is capable of awaiting Vert.x futures and gets the result synchronously.

When the verticle awaits a result, the verticle can still process events like an event-loop verticle.

AbstractVerticle verticle = new AbstractVerticle() {
  @Override
  public void start() {
    HttpClient client = vertx.createHttpClient();
    HttpClientRequest req = Future.await(client.request(
      HttpMethod.GET,
      8080,
      "localhost",
      "/"));
    HttpClientResponse resp = Future.await(req.send());
    int status = resp.statusCode();
    Buffer body = Future.await(resp.body());
  }
};

// Run the verticle a on virtual thread
vertx.deployVerticle(verticle, new DeploymentOptions().setThreadMode(ThreadMode.VIRTUAL_THREAD));

Note	Using virtual threads require to use Java 21 or above.

Blocking within a virtual thread verticle

You can use Future.await to suspend the current virtual thread until the awaited result is available.

The virtual thread is effectively blocked, but the application can still process events.

When a virtual thread awaits for a result and the verticle needs to process a task, a new virtual thread is created to handle this task.

When the result is available, the virtual thread execution is resumed and scheduled after the current task is suspended or finished.

Important

Like any verticle at most one task is executed at the same time.

You can await on a Vert.x Future

Buffer body = Future.await(response.body());

or on a JDK CompletionStage

Buffer body = Future.await(Future.fromCompletionStage(completionStage));

Field visibility

A virtual thread verticle can interact safely with fields before an await call. However, if you are reading a field before an await call and reusing the value after the call, you should keep in mind that this value might have changed.

int value = counter;
value += Future.await(getRemoteValue());
// the counter value might have changed
counter = value;

You should read/write fields before calling await to avoid this.

counter += Future.await(getRemoteValue());

Note	this is the same behavior with an event-loop verticle

Blocking without await

When your application blocks without using await, e.g. using ReentrantLock#lock, the Vert.x scheduler is not aware of it and cannot schedule events on the verticle: it behaves like a worker verticle, yet using virtual threads.

This use case is not encouraged yet not forbidden, however the verticle should be deployed with several instances to deliver the desired concurrency, like a worker verticle.

Thread local support

Thread locals are only reliable within the execution of a context task.

ThreadLocal<String> local = new ThreadLocal();
local.set(userId);
HttpClientRequest req = Future.await(client.request(HttpMethod.GET, 8080, "localhost", "/"));
HttpClientResponse resp = Future.await(req.send());