A more than minimal JVM written in Go and capable of running Java 17 classes.
Jacobin is an implementation of the JVM specification for Java 17. It is written entirely in Go with no dependencies.
The goal is to provide a more-than-minimal implementation of the JVM that can run most class files and JARs and deliver the same results as the OpenJDK-based JVMs (that is, the majority of JVM implementations today). A paramount consideration in the design and implementation of Jacobin is the codebase: making it cohesive and containing clear code. The cohesiveness, extensive commenting, and large test suite enable professionals who want to know more about how the JVM works to find the information quickly and in an easily accessible setting. Additional information on the Jacobin wiki provides more background and insight. Because Jacobin is strictly a JVM, its code is tightly focused on Java program execution. An important factor in reducing the size of the codebase and executable is that Jacobin relies on Go’s built-in memory management to perform garbage collection, and so it contains no GC code.
Due to our desire for an utterly reliable product, Jacobin is heavily tested during development. As of February 2024, the test code is 322% the size of the production code and consists of more than 700 tests. We’re committed to increasing these numbers. When Jacobin advances some more, we intend to run the OpenJDK test suites against it.
The current status is shown here. Updates are also posted in realtime on the Jacobin Twitter account.There are currently no packaged releases of Jacobin available (although you can always compile the code). We’ll issue releases when Jacobin is mature enough to run classes as expected.
At present, all tasks and defects are logged in an instance of JetBrains’ YouTrack (kindly provided at no cost). The task numbers appear at the start of the comment for every commit and push. The GitHub ‘issues’ facility is used strictly for issues posted by users. This design allows users to find solutions without needing to dig through numerous unrelated matters.
As we progress, we post short explanations of project decisions and explanations of how the JVM works. Current material can be found below:
Jacobin status at the 1-year mark
Why was Go chosen for this project?
Inside Java class files: the constant pool
Inside the JVM: How fields are handled
Jacobin is presently being developed by Andrew Binstock (platypusguy), Spencer Uresk, and Richard Elkins. Contributors are more than welcome. If you’d like to show your support the project but can’t contribute code, we’d love a GitHub star or for you to follow the project.
This project could not have been possible without Github (for the excellent platform), JetBrains (for superb tools), Oracle’s Java team (for the great technology and best-in-class documentation), and these JVM experts: Ben Evans, Aleksey Shipilev, Chris Newland, and Bill Venners who have written helpful, in-depth articles on the machinery of the JVM. A big thanks to all!