jacobin

A more-than-minimal JVM written in Go.

Status

Intended feature set:

Java 17 functionality, but...
No JNI (Oracle intends to replace it; see JEP 389)
No security manager (Oracle intends to remove it; see JEP 411)
No JIT
Somewhat less stringent bytecode verification
Does not enforce Java 17's sealed classes

What we've done so far and what we need to do:

Command-line parsing

Gets options from the three environment variables. Details here
Parses the command line; identify JVM options and application options
Responds to most options listed in the java -help output

To do:

Handling @files (which contain command-line options)
Parsing complex classpaths

Class loading

Correctly reads and parses most classes
Extracts bytecode and parameters needed for execution
Automated pre-loading of core Java classes (Object, etc.)
java.*, javax.*, jdk.*, sun.* classes are loaded from the JAVA_HOME directory (i.e., from JDK binaries)
Handles JAR files

To do:

Handle more-complex classes
Handle interfaces
Handle inner classes

Verification, Linking, Preparation, Initialization

Performs format check of class file.
Linking, preparation, and initialization -- minimally and only as needed at execution time

To do:

Verification
Robust preparation and initialization

Execution

Execution of bytecode ✏️ The primary focus of current coding work
190 bytecodes fully operational, including one- and multi-dimensional arrays

To do:

invokedynamic
Calls to superclasses
Inner and nested classes
Exception-tree walking
Annotations

Instrumentation

Instruction-level tracing (use -trace:inst to enable this feature)
Extensive logging data (use -verbose:finest to enable. Caveat: this produces a lot of data)

To do:

Emit instrumented data to a port, for reading/display by a separate program.

Garbage Collection

GC is handled by the golang runtime, which has its own GC

Understanding the Code

A detailed roadmap to the code base can be found in the wiki.

Thanks

The project's home page carries a lengthy note at the bottom that expresses our thanks to vendors and programmers who have made the Jacobin project possible. They are many and we are deeply grateful to them.