Kaffe is a cleanroom, open source implementation of a Java virtual machine and class libraries.

This is the oldest and by far the most complete clean room open source Java virtual machine implementation. However, the performance leaves a lot to be desired.

LaTTe is a virtual machine which is able to execute Java bytecode. It includes a novel JIT compiler targeted to RISC machines (specifically the UltraSPARC). The JIT compiler generates RISC code of good quality through a clever mapping of Java stack operands to registers with a negligible overhead. Additionally, the runtime components of LaTTe, including thread synchronization, exception handling, and garbage collection, have been optimized. As a result, the performance of LaTTe is competitive with that of Sun's JDK 1.2 production release and JDK 1.3 release candidate client/server VMs (HotSpot).

Originally based on Kaffe, this has come a long way.

ORP, Open Runtime Platform, implements advanced JIT and GC interfaces. These interfaces allow the JIT, GC and remaining runtime system to be developed in complete isolation and at independent rates. A major benefit is that JIT and GC researchers are relieved of the intractable task of developing a full system before they can innovate in their area of expertise.

The basic system incorporates a fast code generating JIT as well as an optimizing JIT. It also includes several GC algorithms, ranging from a simple mark-sweep algorithm to an advanced train algorithm.

Blackdown was the first group to port Java 1.0, 1.1, and then 1.2 to Linux.

Basically an uninspiring reference implementation, but the only one for Linux until fairly recently.

kissme started its life as a Honours project by Stephen Tjasink while he was studying at the University of Cape Town. Initially the JVM was developed for a set-top box that came with a TV Satellite Decoder. To fulfill this goal the JVM was designed to run on a slow processor and with limited memory. One of unconventional aspects of the project was the need to store class files in ROM. The JVM was modified to be able to store the runtime class information on the ROM and then load it again when booting.

When Stephen did his Masters degree the next year, he decided to extend the VM to support orthogonal persistence. Persistent programming languages allow the programmer to indicate which objects in memory should be saved on secondary storage. The runtime (or a compiled in library) will ensure that the data is saved automatically and transparently on disk. When the program restarts it can retrieve this information transparently.

It uses Classpath libraries. JIT work started in February 2000, still not completed.

SableVM is an open-source virtual machine for Java, intended as a research framework for efficient execution of Java bytecode. The framework is essentially composed of an extensible bytecode interpreter using state-of-the-art and innovative techniques.

Interpreter-only, just barely runs "Hello, world!". However, they have some interesting techniques for run time services (thin locks, etc.)

Japhar is the Hungry Programmers' Java VM. It has been built from the ground up without consulting Sun's sources.

Development seems to be slow. Moving from version 0.08 to 0.09 took 18 months (?).

CACAO is 64 bit just-in-time (JIT) compiler for Java. It translates Java byte code on demand into native code for the ALPHA processor.

This doesn't seem to be in development anymore.