Main

Supported Platforms

Mac and Linux on x86-64

Download and Build from Source

$ 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:
   

   $ 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.

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

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.