The Ultimate Guide to Java Virtual Threads Rock the JVM Blog

The framework will invoke that callback on some thread, but not necessarily the same thread that initiated the operation. This means developers must break their logic down into alternating IO and computational steps which are stitched together into a sequential workflow. Java thread pool was designed to avoid the overhead of creating new OS threads because creating them was a costly operation. But creating virtual threads is not expensive, so, there is never a need to pool them. It is advised to create a new virtual thread everytime we need one.

java loom virtual threads

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. Now that we know how to create virtual threads let’s see how they work. This allows us to process a large number of requests in parallel with a small pool of carrier threads. Structured concurrency aims to simplify multi-threaded and parallel programming.

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In order to avoid this wasting of resource without modifying Netty upstream, we wrote an extension that modifies the bytecode of the class responsible for creating the thread locals at build time. Using this extension, performance of virtual threads in Quarkus for the Json Serialization test of the Techempower suite increased by nearly 80%, making it almost as good as reactive endpoints. The main challenge, to implement decomposition of your task into sub-task is on you, anyway. What these classes provide you, are features specifically for dealing with the Fork/Join pool and balancing the workload with the available platform threads. When you want to perform each sub-task on its own virtual thread, you don’t need this. So you can easily implement a recursive task with virtual threads without the built-in classes, e.g.

java loom virtual threads

So we may get scalability from this model, but we have to give up on using parts of the language and ecosystem to get it. The following example illustrates using virtual threads to concurrently fetch two URLs and aggregate their results as part of handling a request. It creates an ExecutorService that runs each task in a new virtual thread, submits two tasks to it, and waits for the results. ExecutorService has been retrofitted to implement AutoCloseable, so it can be used with try-with-resources, and the close method shuts down the executor and waits for tasks to complete. However, there are some cases where a blocking operation doesn’t unmount the virtual thread from the carrier thread, blocking the underlying carrier thread.

1. Classic Threads or Platform Threads

But external killing of a linked actor/vthread -one of erlang usually ignored secret sauces- is fundamental, if not, you need adhoc mechanisms like Go cancellation contexts who IMHO adds a lot of error prone accidental complexity. Think usages beyond supervisors/fault-tolerance like killing obsolete requests/computation or speculative execution. BTW, even with OS threads, when you run transaction-handling code, as opposed to long-running computation, time-sharing preemption is the exception rather than the rule. For JSON serialization, Netty uses their custom implementation of thread locals, FastThreadLocal to store buffers. When using virtual threads in quarkus, the number of virtual threads simultaneously living in the service is directly related to the incoming traffic.

Platform threads have always been easy to model, program and debug because they use the platform’s unit of concurrency to represent the application’s unit of concurrency. As we can see, it takes a lot of work to follow the life cycle of a virtual thread and its continuation. We hope that the JDK team will provide better documentation of the virtual threads implementation in the future. 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.

Quarkus and virtual threads

Hence the path to stabilization of the features should be more precise. Another common use case is parallel processing or multi-threading, where you might split a task into subtasks across multiple threads. Here you have to write solutions to avoid data corruption and data races. In some cases, you must also ensure thread synchronization when executing a parallel task distributed over multiple threads.

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It would be nice to allow users to make the platform/virtual decision based on their needs, for example by setup of ManagedThreadFactory or ManagedExecutorService. I suppose it is good to have just one type of thread in one factory/service and not to mix them. And while pooling does immensely help because you are not paying a hefty price of creating a new thread every time, it does not increase the total number of available threads. While Scoped Values is a brand new thing the two following features we’ll discuss are the Second Preview of Record Patterns and the Second Incubator of Structured Concurrency. These two were marginally changed from their first appearance in Java 19 few months back, hence the content below is mostly the same as my last years Java 19 overview.

Why Do We Need Virtual Threads?

Why would we implement yet another unit of concurrency — one that is only syntax-deep — which does not align with the threads we already have? This might be more attractive in another language, where language-runtime co-evolution was not an option, but fortunately we didnt have to make that choice. Virtual threads are an alternative implementation of java.lang.Thread which store their stack frames in Javas garbage-collected heap rather than in monolithic blocks of memory allocated by the operating system. Virtual threads dramatically reduce the effort of writing, maintaining, and observing high-throughput concurrent applications. With the growing demand of scalability and high throughput in the world of microservices, virtual threads will prove a milestone feature in Java history. Notice the blazing fast performance of virtual threads that brought down the execution time from 100 seconds to 1.5 seconds with no change in the Runnable code.

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