Java 19 brings the first preview of virtual threads to the Java platform; this is the main deliverable of OpenJDKs Project Loom. This is one of the biggest changes to come to Java in a long time — and at the same time, is an almost imperceptible change. There is almost zero new API surface, and virtual threads behave almost exactly like the threads we already know.

Call the start() method of the virtual thread to start its execution. Virtual threads are also easier to manage than traditional threads, as they are managed by the JVM and do not require explicit management by the application developer. This can help to reduce the complexity of multi-threaded applications, making them easier to develop and maintain. You can see how the example above uses a cached thread pool to submit 10,000 tasks which simulate a small IO operation that takes 10ms plus the time taken to print to the console and incrementing a counter. The only changes are in the way we define if a thread we create represents a virtual or platform thread. To achieve this, the JDK brings a Thread.Builder to be able to instantiate and configure both easily.

Java Concurrency Utilities

The asynchronous APIs do not wait for the response, rather they work through the callbacks. Whenever a thread invokes an async API, the platform thread is returned to the pool until the response comes back from the remote system or database. Later, when the response arrives, the JVM will allocate another thread from the pool that will handle the response and so on. This way, multiple threads are involved in handling a single async request. Note that virtual threads have a much smaller memory footprint than regular threads, so you can create many more of them without exhausting system resources. However, you should still be mindful of the total number of threads in your application and ensure that you are not creating too many threads, which can lead to contention and reduced performance.

Virtual threads are not just syntactic sugar for an asynchronous framework, but an overhaul to the JDK libraries to be more “blocking-aware”. Without that, an errant call to a synchronous blocking method from an async task will still tie up a platform thread for the duration of the call. Merely making it syntactically easier to manage asynchronous operations does not offer any scalability benefit unless you find every blocking operation in your system and turn it into an async method.

Java Interview Questions and Answers – The ULTIMATE List (PDF Download & video)

Since virtual threads are managed by the JVM, no underlying OS thread is blocked when they perform a blocking operation. Their state is simply stored in the heap and another Virtual thread is executed on the same Java platform thread. As cool as it may sound, you can’t run a million threads simultaneously without having a CPU with a core for each thread. A carrier thread is nothing more than a platform thread used to run virtual threads. Meaning that if you have an 8-core CPU, you will have 8 carrier threads that can run virtual threads. This means that virtual threads are mapped many-to-many to the number of carrier threads available.

• When a virtual thread is running on a carrier thread and is being blocked by, for example, a database call.
• What makes virtual threads so great is that they are good at waiting.
• Whenever a thread invokes an async API, the platform thread is returned to the pool until the response comes back from the remote system or database.
• In this post, we look at what virtual threads are, what makes them special, and how we can create them.
• However, when using the smallrye-mutiny-vertx-sqlclient it is possible to use a variant method that will await for the completion of the transaction, mimicking a blocking behaviour.
• If there is existing compiled code that extends Thread and the subclass declares a method with the same name and return type then an IncompatibleClassChangeError will be thrown at run-time if the subclass is loaded.

When using threads before Java 19 and Project Loom, creating a thread using the constructor was relatively uncommon. Instead, we preferred to use a thread pool or an executor service configured with a thread pool. In fact, those threads were what we now call platform threads, and the reason was that creating such threads was quite expensive operation. As we said, the blocking sleep operation is inside the synchronized useTheToilet method, so the virtual thread is not unmounted. So, the riccardo virtual thread is pinned to the carrier thread, and the daniel virtual thread finds no available carrier thread to execute. In fact, it is scheduled when the riccardo virtual thread is done with the bathroom.

Unleashing the Power of Lightweight Concurrency: A Comprehensive Guide to Java Virtual Threads

The platform thread will be assigned to a different virtual thread to continue doing useful work instead of waiting. This means that we will have a much better resource https://globalcloudteam.com/javas-project-loom-and-virtual-threads/ utilisation in our system! In the example shown below, we have two platform threads, which are mapped to a corresponding OS thread in our operating system.

Virtual threads are so lightweight that it is perfectly OK to create a virtual thread even for short-lived tasks, and counterproductive to try to reuse or recycle them. Indeed, virtual threads were designed with such short-lived tasks in mind, such as an HTTP fetch or a JDBC query. Operating systems typically allocate thread stacks as monolithic blocks of memory at thread creation time that cannot be resized later.

Full Quote from JEP

The implementations of these blocking operations compensate for the capture of the OS thread by temporarily expanding the parallelism of the scheduler. Consequently, the number of platform threads in the scheduler’s ForkJoinPool may temporarily exceed the number of available processors. The maximum number of platform threads available to the scheduler can be tuned with the system property jdk.virtualThreadScheduler.maxPoolSize. The JDK’s virtual thread scheduler is a work-stealing ForkJoinPool that operates in FIFO mode. The parallelism of the scheduler is the number of platform threads available for the purpose of scheduling virtual threads.

For example, you can create a virtual thread that executes a Runnable or Callable task, or one that runs indefinitely until it is explicitly stopped. The Executor framework is a higher-level abstraction for managing threads in Java. It provides a way to separate task submission from thread management, which allows for greater flexibility and scalability. However, the Executor framework still uses traditional threads, which have the limitations discussed above.

Using Java virtual threads: A demo

Unfortunately, the number of available threads is limited because the JDK implements threads as wrappers around operating system threads. OS threads are costly, so we cannot have too many of them, which makes the implementation ill-suited to the thread-per-request style. If each request consumes a thread, and thus an OS thread, for its duration, then the number of threads often becomes the limiting factor long before other resources, such as CPU or network connections, are exhausted. The JDK’s current implementation of threads caps the application’s throughput to a level well below what the hardware can support.

Typically, while experimenting with virtual-threads, we realized that using the postgresql-JDBC driverresults in frequent pinning. The previous example is trivial and doesn’t capture how imperative style can simplify complex reactive operations. The endpoints must now fetch all the fortunes in the database, then append a quote to each fortune before finally returning the result to the client. Replace with the specific version you’d like to install, such as the early access build of JDK 21 that includes virtual thread support.