Virtual Threads in Java Project Loom

From the output, we understand that the same virtual thread jumps from one platform thread it was running in at the beginning to another when it comes back from sleeping. Virtual threads were first introduced as a preview API in JDK 19, delivered to the second preview round in JDK 20, and are expected to become standard with JDK 21. Until now, you may have read many articles and listened to many presentations about virtual threads. The thread-per-request model ties up threads in the system, and there is a maximum number of concurrent threads allowed.

It will then be possible to perform blocking operations without blocking the platform thread upon which the virtual thread is mounted. The process includes creating a java stack, a system stack, system calls, and callbacks. It is more expensive compared to creating ‘standard’ objects, especially when a given server wants to handle multiple short-lived requests where for each request a new thread would be created.

Will your application benefit from Virtual Threads?

The above code also shows how the jdk.tracePinnedThreads flag works. The VTHREAD_SCOPE is a ContinuationScope object, a class used to group continuations. In other words, it’s a way to group continuations related to each other. In our case, we have only one ContinuationScope object, the VTHREAD_SCOPE object.

• This virtual thread does not occupy OS resources and CPU until it needs to do so.
• This way Quarkus takes care of data source configuration, both blocking and nonblocking.
• This cost is pretty cheap compared to the cost of blocking platform threads.
• The maximum number of platform threads available to the scheduler can be tuned with the system property jdk.virtualThreadScheduler.maxPoolSize.

Conveniently you can store some data in the entry point of the request handler and use that data all across the workload of the request being executed without having to explicitly pass that data as a method argument across your codebase. «The principle for structured concurrency is quite straightforward — when there is sequential code that splits into concurrent flows, they must join back in the same code unit,» Garcia-Ribeyro said. «If you write code in this way, then the error handling and cancellation can be streamlined and it makes it much easier to read and debug.» I mentioned above that the virtual threads are mounted and unmounted by the JVM, now let’s make this behavior observable. Pinned threads metrics (JEP-444, section Executing virtual threads).

Bringing virtual threads to reactive REST services

Cheap to blockWhen performing blocking I/O, the underlying OS thread wrapped by the Java platform thread is put in a wait queue and a context switch occurs to load a new thread context onto the CPU core. Since virtual threads are managed by the JVM, no underlying OS thread https://globalcloudteam.com/javas-project-loom-and-virtual-threads/ 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. This test is about observing how virtual threads handle long-running tasks compared to platform threads.

This will eventually affect the performance of the application and inhibit its scalability. In other words, the mutiny environment is a safe environment for virtual threads. This guide explains how to benefit from Java 19 virtual threads when writing REST services in Quarkus. Quarkus comes with Dev Services that ease the development process. One of the options is to provision and configure the Postgresql database automatically. This way Quarkus takes care of data source configuration, both blocking and nonblocking.

Data repository

So it’s about time we all learn about virtual threads and why we might care about them as Spring developers. JEP 353 in JDK 13, and JEP 373 in JDK 15, replaced the implementations of java.net.Socket, ServerSocket, and DatagramSocket with new implementations designed for use with virtual threads. The Thread.setPriority method has no effect on virtual threads, which always have a priority of Thread.NORM_PRIORITY. Jdk.VirtualThreadPinned indicates that a virtual thread was parked while pinned, i.e., without releasing its platform thread .

The need to share mutable states among threads, the well-known “hard part” of concurrent programming, drastically decreases. However, using such an approach, we can easily reach the limit of the number of threads we can create. And of course, there would have to be some actual I/O or other thread parking for Loom to bring benefits. Assumptions leading to the asynchronous Servlet API are subject to be invalidated with the introduction of Virtual Threads. The async Servlet API was introduced to release server threads so the server could continue serving requests while a worker thread continues working on the request. This makes lightweight Virtual Threads an exciting approach for application developers and the Spring Framework.

Demo: Using Virtual Threads in Spring Applications

In fact, the more straightforward way to write concurrent programs in Java is to create a new thread for every concurrent task. In the second variant, Thread.ofVirtual() returns a VirtualThreadBuilder whose start() method starts a virtual thread. The alternative method Thread.ofPlatform() returns a PlatformThreadBuilder via which we can start a platform thread. The “Detailed changes” section describes in detail all of the changes that have been made to the JDK to support virtual threads, including the aforementioned APIs for using virtual threads. The “Virtual threads are a preview API, disabled by default” section explains how to enable the virtual threads preview API in JDK 19 to support trialling their use.

Instead, use semaphores to make sure only a specified number of threads are accessing that resource. Note that the following syntax is part of structured concurrency, another new feature proposed in Project Loom. We can use the Thread.Builder reference to create and start multiple threads. When invoked on a virtual thread, Thread.getThreadGroup() returns a placeholder thread group with the name “VirtualThreads“.

Java 20 Project Loom updates set stage for Java LTS

«Apps might see a big performance boost without having to change the way their code is written,» he said. «That’s very appreciated by our customers who are building software for not just a year or two, but for five to 10 years — not having to rewrite their apps all the time is important to them.» After all, the entire purpose of Reactive programming is not to block threads. If you want to block threads, there’s no point in using reactive frameworks . Blocking a virtual thread results in yielding its continuation and unparking it results in the continuation being resubmitted to the scheduler. Because the Java runtime can explicitly control when a virtual thread is suspended and resumed, that structure allows a more fine-grained concurrency model for Java.

Virtual threads are not faster threads — they do not run code any faster than platform threads. There can be many more of them than platform threads, so they enable the higher concurrency needed for higher throughput according to Little’s Law. To enable applications to scale while remaining harmonious with the platform, we should strive to preserve the thread-per-request style by implementing threads more efficiently, so they can be more plentiful. Operating systems cannot implement OS threads more efficiently because different languages and runtimes use the thread stack in different ways.