For incoming requests, use Netty or Spring Reactive or Vertx. All the other options you listed seem a lot more work, so even if preview Loom wouldn’t work, it would only take a little time to find out. So, am working on a Java 11 web service that makes thousands of remote network calls, per each request. Pub/Sub can help decoupling components, reducing latency and transparently adding/removing logging and monitoring, even at runtime. Applications using WebSockets or Queues might also benefit from Pub/Sub, as their domain is event based.

This particular experiment is about simple, “legacy” Java 1.0 code enjoying terrific scalability. The new Fiber class in Project Loom provides support for structured concurrency. Structured concurrency is a programming paradigm that focuses on the structure and organization of concurrent code, intending to make it easier to write and reason about concurrent programs. It emphasizes the use of explicit control structures and coordination mechanisms to manage concurrent execution, as opposed to the traditional approach of using low-level thread synchronization primitives.

What Are Virtual Threads in Java?

Nothing drastic, but you might find a new parameter in some methods. Lightweight means that you can have many of them, and in fact Fibry will be happy to keep running several million of fibers at the same time, if that’s what you need. With threads, depending on your configuration, you can maybe have some tens of thousands. It’s available since Java 19 in September 2022 as a preview feature.

Reasons for Using Java is Project Loom

For example, such a thread could be paused and serialized on one machine and then deserialized and resumed on another. A fiber would then have methods like parkAndSerialize, and deserializeAndUnpark. A real implementation challenge, however, may be how to reconcile fibers with internal JVM code that blocks kernel threads. Examples include hidden code, like loading classes from disk to user-facing java enhancement proposals pursue virtual threads functionality, such as synchronized and Object.wait. As the fiber scheduler multiplexes many fibers onto a small set of worker kernel threads, blocking a kernel thread may take out of commission a significant portion of the scheduler’s available resources, and should therefore be avoided. Virtual threads are lightweight threads that are not tied to OS threads but are managed by the JVM.

Data Engineering

Also, the kotlin support of vert.x has already encapsulated all JDK blocking methods. JEP 353 in JDK 13, and JEP 373 in JDK 15, replaced the implementations of, ServerSocket, and DatagramSocket with new implementations designed for use with virtual threads. New tests will exercise all new and revised APIs, and all areas changed to support virtual threads. Introduce a new public class to represent user-mode threads, unrelated to java.lang.Thread. This would be an opportunity to jettison the unwanted baggage that the Thread class has accumulated over 25 years. We explored and prototyped several variants of this approach, but in every case grappled with the issue of how to run existing code.

Because when your virtual thread runs, it’s a normal Java thread. It’s a normal platform thread because it uses carrier thread underneath. However, you just have to remember on the back of your head, that there is something special happening there, that there is a whole variety of threads that you don’t see, because they are suspended.


Although JavaRX is a powerful and potentially high-performance approach to concurrency, it is not without drawbacks. In particular, it is quite different from the existing mental constructs that Java developers have traditionally used. Also, JavaRX can’t match the theoretical performance achievable by managing virtual threads at the virtual machine layer. In this example, suspendCoroutine is introduced to obtain a reference to the current Continuation, run a segment of code, and ultimately suspend the current coroutine. Continuation represents a continuation of the current calculation. Use Continuation.resume() to resume the execution context.

Reasons for Using Java is Project Loom

Project Loom’s Fibers are a new form of lightweight concurrency that can coexist with traditional threads in the JVM. They are a more efficient and scalable alternative to traditional threads for certain types of workloads, and provide a more intuitive programming model. Other Java technologies, such as thread pools and the Executor framework, can be used to improve the performance and scalability of Java applications, but they do not provide the same level of concurrency and efficiency as fibers. 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. By default it is equal to the number of available processors, but it can be tuned with the system property jdk.virtualThreadScheduler.parallelism.

For example, there are many potential failure modes for RPCs that must be considered; network failures, retries, timeouts, slowdowns etc; we can encode logic that accounts for a realistic model of this. To demonstrate the value of an approach like this when scaled up, I challenged myself to write a toy implementation of Raft, according to the simplified protocol in the paper’s figure 2 . I chose Raft because it’s new to me , and is supposed to be hard to get right and so a good target for experimenting with bug-finding code.

How the current thread per task model works

The Loom project started in 2017 and has undergone many changes and proposals. Virtual threads were initially called fibers, but later on they were renamed to avoid confusion. Today with Java 19 getting closer to release, the project has delivered the two features discussed above. Hence the path to stabilization of the features should be more precise. If you are doing the actual debugging, so you want to step over your code, you want to see, what are the variables?

  • For one, it would require more work in the JVM, which makes heavy use of the Thread class, and would need to be aware of a possible fiber implementation.
  • The implications of this for Java server scalability are breathtaking, as standard request processing is married to thread count.
  • As a result, concurrent connections would also get limited due to which threads could not be appropriately scaled.
  • The new final method Thread.threadId() returns a thread’s identifier.
  • There’s a list of APIs that do not play well with Project Loom, so it’s easy to shoot yourself in the foot.
  • It will be fascinating to watch as Project Loom moves into the main branch and evolves in response to real-world use.

In the modern software world, the operating system fulfills this role of scheduling tasks to the CPU. Java makes it so easy to create new threads, and almost all the time the program ends-up creating more threads than the CPU can schedule in parallel. Let’s say that we have a two-lane road , and 10 cars want to use the road at the same time. Naturally, this is not possible, but think about how this situation is currently handled.

What does this mean to Java library developers?

Java does not make it easy to control the threads , and so influencing the interleaving of execution is very difficult except for in very isolated cases. When the FoundationDB team set out to build a distributed database, they didn’t start by building a distributed database. Instead, they built a deterministic simulation of a distributed database. They built mocks of networks, filesystems, hosts, which all worked similarly to those you’d see in a real system but with simulated time and resources allowing injection of failures. As a white box tool for bug detection, Jepsen is fantastic. If you’ve written the database in question, Jepsen leaves something to be desired.

Project Loom?

Of course, the virtual thread will not be garbage collected if it is running or if it is blocked and could ever be unblocked. Enable existing code that uses the java.lang.Thread API to adopt virtual threads with minimal change. The key to all of this isvirtual threads, which are designed to look to the programmer just like ordinary, familiar threads. However, virtual threads are managed by the Java runtime and arenotthin, one-to-one wrappers over OS threads. Instead, virtual threads are implemented in user space by the Java runtime. Our team has been experimenting with Virtual Threads since they were called Fibers.

You can use this guide to understand what Java’s Project loom is all about and how its virtual threads (also called ‘fibers’) work under the hood. Essentially, both user-mode and kernel-mode context switching are very lightweight operations and support some hardware commands. For example, PUSHA is used to store general-purpose registers. Therefore, context switching overhead is generally only caused by either storing registers or switching SPs. The call command automatically stacks PCs, and the switch is completed in dozens of commands. The probability of context switching is very low when this program is running.

In order to suspend a computation, a continuation is required to store an entire call-stack context, or simply put, store the stack. To support native languages, the memory storing the stack must be contiguous and remain at the same memory address. While virtual memory does offer some flexibility, there are still limitations on just how lightweight and flexible such kernel continuations (i.e. stacks) can be.

What the Heck Is Project Loom for Java?

It is early days for this project, and so everything — including its scope — is subject to change. At this point we do not foresee a need for a change in the Java language. Check out these additional resources to learn more about Java, multi-threading, and Project Loom. Cancellation propagation — If the thread running handleOrder() is interrupted before or during the call to join(), both forks are canceled automatically when the thread exits the scope. Error handling with short-circuiting — If either the updateInventory() or updateOrder() fails, the other is canceled unless its already completed. This is managed by the cancellation policy implemented by ShutdownOnFailure(); other policies are possible.

WISP 2 is a benchmark for Java coroutine functionality and is now an ideal product in terms of product format, performance, and stability. To date, hundreds of applications, and tens of thousands of containers have already been deployed on WISP 1 or WISP 2. The WISP coroutine is fully compatible with the code for multi-thread blocking. You only need to add JVM parameters to enable the coroutine.

How would you describe the persona and level of your target audience?

The constructors defined by the Thread class create platform threads, as before. A current limitation of virtual threads is that the G1 GC does not support humongous stack chunk objects. If a virtual thread’s stack reaches half the region size, which could be as small as 512KB, then a StackOverflowError might be thrown. In addition, from the perspective of Java code, the fact that a virtual thread and its carrier temporarily share an OS thread is invisible.

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