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Project Loom is to intended to explore, incubate and deliver Java VM features and APIs built on top of them for the purpose of supporting easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform. 

This OpenJDK project is sponsored by the HotSpot Group.


Info
titleSource Code

https://github.com/openjdk/loom

Info
titleEarly Access Binaries

http://jdk.java.net/loom/

Info
titleResources

State of LoomJEP 425: Virtual Threads (Preview)

On the Performance of User-Mode Threads and Coroutines

More on inside.java

Outdated:

State of Loom

Info
titleTalks

Joker 2020 - Video

AccentoDev 2020 - Video

Devoxx BE 2019 - Video

JVMLS 2019 - Video

Curry On 2019 - Video

QCon London 2019 - Video and Slides

FOSDEM 2019 - Video

Devoxx BE 2018 - Video | Slides

JVMLS 2018 – Video | Slides

JFokus 2018 – Video

Info
titleMeetings

October 2018 - Slides

Info
titleMailing List

Subscribe | Archive

Info
titleProject

Proposal | JEP | Members | Page



Warning
titleNote

Loom is under active development, which means that information and advice given here might change in the future.


Table of Contents

Supported Platforms

Mac and Linux on x86-64

Download and Build from Source


Code Block
$ git clone https://github.com/openjdk/loom
$ cd loom  
$ git checkout fibers 
$ sh configure   
$ make images

How to Contribute

The most valuable way to contribute at this time is to try out the current prototype and provide feedback and bug reports to the loom-dev mailing list.  In particular, we welcome feedback that includes a brief write-up of experiences adapting existing libraries and frameworks to work with Fibers.

If you have a login on the JDK Bug System then you can also submit bugs directly. We plan to use an Affects Version/s value of "repo-loom" to track bugs.

How to run the JDK tests

  1. Download jtreg (the JDK test harness) and place its bin subdirectory on your path.

  2. Create a debug JDK configuration (inside the top directory of the Loom repo) and build it. This step requires having jtreg on your path, or running the tests would fail:

    Code Block
    $ sh configure --with-jtreg --with-debug-level=fastdebug
    $ make images


  3. Run the tests. The following example assumes a Mac build (replace macosx with linux for a Linux build), and the java/lang/Continuation/Basic.java test, which contains some basic Continuation tests. The java/lang/Continuation directory contains Continuation test, while the java/lang/Continuation directory contains fiber tests. Supplying just the directory name runs all tests in the directory.

    Code Block
    $ make run-test TEST=open/test/jdk/java/lang/Continuation/Basic.java CONF=macosx-x86_64-server-fastdebug

Missing Features

  • Yielding while a native VM frame is on the stack in the case of a priviliged action, reflective invocation and MethodHandle invocation.

  • Cloning continuations
  • Serialization of fiber/continuation

    Virtual Threads

    Design

    See JEP 425: Virtual Threads (Preview)

    Implementation

    Virtual threads are implemented in the core libraries. A virtual thread is implemented as a continuation that is wrapped as a task and scheduled by a j.u.c.Executor. Parking (blocking) a virtual thread results in yielding its continuation, and unparking it results in the continuation being resubmitted to the scheduler. The scheduler worker thread executing a virtual thread (while its continuation is mounted) is called a carrier thread.

    The continuations used in the virtual thread implementation override onPinned so that if a virtual thread attempts to park while its continuation is pinned (see above), it will block the underlying carrier thread.

    The implementation of the networking APIs in the java.net and java.nio.channels  packages have as been updated so that virtual threads doing blocking I/O operations park, rather than block in a system call, when a socket is not ready for I/O. When a socket is not ready for I/O it is registered with a background multiplexer thread. The virtual thread is then unpacked when the socket is ready for I/O.

    Debugging

    See the Virtual Thread Debugging Support page.

    Continuations

    Design

    The primitive continuation construct is that of a scoped (AKA multiple-named-prompt), stackful, one-shot (non-reentrant) delimited continuation. The continuation can be cloned, and thus used to implement reentrant delimited continuations. The construct is exposed via the java.lang.Continuation class. Continuations are intended as a low-level API, that application authors are not intended to use directly. They will use higher-level constructs built on top of continuations, such as virtual threads or generators.

    Tail Calls

    Design

    We envision explicit tail-call elimination. It is not the intention of this project to implement automatic tail-call optimization.

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