Hawkbeans

Hawkbeans is a small (~6K LOC), MIT-licensed JVM written in C and intended for educational purposes. It does not meet the Java specification (not even close), but includes enough functionality for students to become familiar with concepts important to the JVM, viz. byte code interpretation, exceptions, class loading, resolution of symbolic references, exceptions, stack frames, and garbage collection.

Below are a list of features not supported in this JVM:

Full exception support: only the basics.
Reflection (required for assertions)
Floating point math is hardly tested
User-defined class loaders
Very limited JNI
No JIT compilation
Wide word (long/double) support is very limited
No actual protection is implemented (i.e. private/public etc.)

Hawkbeans uses a simple buddy allocator with power of 2 free lists for heap management. This code was taken from several other OS code bases.

Hawkbeans was developed referencing Oracle's Java Virtual Machine Specification.

Setup

You'll need a JDK 1.8 environment installed to build programs to run with Hawkbeans (i.e. using javac). By default, Hawkbeans will compile with Clang, but will use GCC if Clang isn't available. You'll also want to have libreadline installed. Feel free to use the provided Vagrantfile to get a dev environment up and running (no GUI necessary).

Building

To build the JVM, simply run make. To build the Java libs (required for building test programs) run make jlibs. To build test programs (in testcode/), run make testcode.

Running

Hawkbeans can be run as follows:

$> bin/hawkbeans-clang-debug SomeClass

Be careful about where you put classes. The hardcoded class search paths are in javasrc, in testcode, and in the Hawkbeans top-level directory. If you try to use a class outside these paths, it will fail.

Source Tree

The source is organized into the following directories:

jvm: The code for the platform-independent portion of Hawkbeans. Note this organization is not very sane at the moment.
- arch/x64-linux: The linux specific code, and the only one that currently works.
- arch/hawknest: A platform directory for the Hawknest emulator. Not working.
include: The header files for the code.
javasrc: The Java code for the Hawkbeans standard libraries.
scripts: Various utility scripts, mostly used when I wrote the code.
testcode: A set of simple Java programs to test functionality
bin: This is where the build toolchain puts binaries.

Questions/Concerns