Java Concurrency: An Introduction to Project Loom

The JVM does delimited continuation on IO operations, no IO for virtual threads. To utilize the CPU effectively, the number of context switches should be minimized. From the CPU’s point of view, it would be perfect if exactly one thread ran permanently on each core and was never replaced. We won’t usually be able to achieve this state, since there are other processes running on the server besides the JVM. But “the more, the merrier” doesn’t apply for native threads – you can definitely overdo it. With virtual threads on the other hand it’s no problem to start a whole million threads.

• Many of these projects are aware of the need to improve their synchronized behavior to unleash the full potential of Project Loom.
• If you ever wrote tests which involve Thread.sleep, you probably know that they are fragile and prone to flakiness.
• While the main motivation for this goal is to make concurrency easier/more scalable, a thread implemented by the Java runtime and over which the runtime has more control, has other benefits.
• The sole purpose of this addition is to acquire constructive feedback from Java developers so that JDK developers can adapt and improve the implementation in future versions.
• It’s just a matter of a single bit when choosing between them.
• This post looked at the benefits of structured concurrency to the Java language and how it is implemented in Loom.

Typically, ExecutorService has a pool of threads that can be reused in case of new VirtualThreadExecutor, it creates a new virtual thread every time you submit a task. You can also create a ThreadFactory if you need it in some API, but this ThreadFactory just creates virtual threads. You just create treads as if it was a very native, very low footprint abstraction, which is not the case right now. The first takeaway is that this may revolutionize the way you work with concurrent code.

Hence the path to stabilization of the features should be more precise. Structured concurrency aims to simplify multi-threaded and parallel programming. It treats multiple tasks running in different threads as a single unit of work, streamlining error handling and cancellation while improving reliability and observability. This helps to avoid issues like thread leaking and cancellation delays.

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JDK 8 brought asynchronous programming support and more concurrency improvements. While things have continued to improve over multiple versions, there has been nothing groundbreaking in Java for the last three decades, apart from support for concurrency and multi-threading using OS threads. With Project Loom, we will have at least one more such option to choose from.

The problem with real applications is them doing silly things, like calling databases, working with the file system, executing REST calls or talking to some sort of queue/stream. When you open up the JavaDoc of inputStream.readAllBytes() , it gets hammered into you that the call is blocking, i.e. won’t return until all the bytes are read – your current thread is blocked until then. Unlike continuations, the contents of the unwound stack frames is not preserved, and there is no need in any object reifying this construct. The capitalized words Thread and Fiber would refer to particular Java classes, and will be used mostly when discussing the design of the API rather than of the implementation. The word thread will refer to the abstraction only and never to a particular implementation, so thread may refer either to any implementation of the abstraction, whether done by the OS or by the runtime. Fibers will be mostly implemented in Java in the JDK libraries, but may require some support in the JVM.

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And of course, there would have to be some actual I/O or other thread parking for Loom to bring benefits. Project Loom has revisited all areas in the Java runtime libraries that can block and updated the code to yield if the code encounters blocking. Java’s concurrency utils (e.g. ReentrantLock, CountDownLatch, CompletableFuture) can be used on Virtual Threads without blocking underlying Platform Threads.

This is quite similar to coroutines, like goroutines, made famous by the Go programming language . Java has had good multi-threading and concurrency capabilities from early on in its evolution and can effectively utilize multi-threaded and multi-core CPUs. Java Development Kit 1.1 had basic support for platform threads (or Operating System threads), and JDK 1.5 had more utilities and updates to improve concurrency and multi-threading.

Understanding Java’s Project Loom

We are doing everything we can to make the preview experience as seamless as possible for the time being, and we expect to provide first-class configuration options once Loom goes out of preview in a new OpenJDK release. The backend web server receives connections and requests from the frontend web server. It responds to each request after a configured delay of 1/3 seconds. The target latency from browser to frontend web service is therefore 1 second. This arrangement is ideal for experimenting with concurrency. The server latency is overwhelmingly due to wait time.

But in the example, we created a dependency between the executorServices; ExecutorService X can’t finish before Y. This example works because the resources in the try are closed in reversed order. First, we wait for ExecutorService Y to close, and then the close method on X will is called. Structured concurrency ties your threads to a scope, but all your threads will be canceled in parallel when you exit that scope.

Why Use Project Loom?

Listing 2 will run on the Project Loom JVM without any problems. We first want to close the threads that generate a value before we close the DB thread. This problem is solved by providing an extra ExecutorService in the try-with-resources. In the example below, we start one thread for each ExecutorService.

This article dives into the throughput and quality of the async code review process, which are very important dimensions to optimize for in product development teams. It also explains why co-creation patterns – Pair and Mob programming – as an alternative way of working are able to optimize for both of those dimensions, instead of needing to trade off between them. Loom and Java in general are prominently devoted to building web applications. Obviously, Java is used in many other areas, and the ideas introduced by Loom may well be useful in these applications.

All of this can be achieved using a so-called continuation. Continuation is a programming construct that was put into the JVM, at the very heart of the JVM. There are actually similar concepts in different languages. Continuation, the software construct is the thing that allows multiple virtual threads to seamlessly run on very few carrier threads, the ones that are actually operated by your Linux system. Project Loom’s mission is to make it easier to write, debug, profile and maintain concurrent applications meeting today’s requirements.

Is it possible to combine some desirable characteristics of the two worlds? Be as effective as asynchronous or reactive programming, but in a way that one can program in the familiar, sequential command sequence? Oracle’s Project Loom aims to explore exactly this option with a modified JDK. It brings a new lightweight construct for concurrency, named virtual threads.

The main goal of this project is to add a lightweight thread construct, which we call fibers, managed by the Java runtime, which would be optionally used alongside the existing heavyweight, OS-provided, implementation of threads. Fibers are much more lightweight than kernel threads in terms of memory footprint, and the overhead of task-switching among them is close to zero. Millions of fibers can be spawned in a single JVM instance, and programmers need not hesitate to issue synchronous, blocking calls, as blocking will be virtually free. When these features are production ready, it will be a big deal for libraries and frameworks that use threads or parallelism.

Loom – Fibers, Continuations and Tail-Calls for the JVM

By the way, you can find out if code is running in a virtual thread with Thread.currentThread().isVirtual(). Virtual threads are one of the most important project loom java innovations in Java for a long time. They were developed in Project Loom and have been included in the JDK since Java 19 as a preview feature .

With Project Loom, you no longer consume the so-called stack space. The virtual threads that are not running at the moment, which is technically called pinned, so they are not pinned to a carrier thread, but they are suspended. These virtual threads actually reside on heap, which means they are subject to garbage collection. In that case, it’s actually fairly easy to get into a situation where your garbage collector will have to do a lot of work, because you have a ton of virtual threads. You don’t pay the price of platform threads running and consuming memory, but you do get the extra price when it comes to garbage collection. The garbage collection may take significantly more time.

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Because after all, you do have to store the stack trace somewhere. Most of the time it’s going to be less expensive, you will use less memory, but it doesn’t mean that you can create millions of very complex threads that are doing a lot of work. The API may change, but the thing I wanted to show you is that every time you create a virtual thread, you’re actually https://globalcloudteam.com/ allowed to define a carrierExecutor. In our case, I just create an executor with just one thread. Even with just a single thread, single carriers, or single kernel thread, you can run millions of threads as long as they don’t consume the CPU all the time. Because, after all, Project Loom will not magically scale your CPU so that it can perform more work.

What Are Virtual Threads in Java?

Here on HappyCoders.eu, I want to help you become a better Java programmer.Read more about me here. Let’s start with the challenge that led to the development of virtual threads. 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. We’ll still use the Scala programming language so that we vary only one component of the implementation, which should make the comparison easier. However, instead of representing side effects as immutable, lazily-evaluated descriptions, we’ll use direct, virtual-thread-blocking calls.

For early adopters, is already included in the latest early access builds of JDK 19. So, if you’re so inclined, go try it out, and provide feedback on your experience to the OpenJDK developers, so they can adapt and improve the implementation for future versions. Like any other preview feature, to take advantage of it, you need to add the –enable-preview JVM argument while compiling and running. This code is not only easier to write and read but also – like any sequential code – to debug by conventional means.

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Moreover, not every blocking call is interruptible—but this is a technical, not a fundamental limitation, which at some point might be lifted. Instead, we don’t have to think about executors or pass them around. If something needs to happen in the background, we simply create a new virtual thread, and use ordinary virtual-thread-blocking calls. A few use cases that are actually insane these days, but they will be maybe useful to some people when Project Loom arrives. For example, let’s say you want to run something after eight hours, so you need a very simple scheduling mechanism. Doing it this way without Project Loom is actually just crazy.