Rembulan

(Rembulan is Javanese/Indonesian for Moon.)

About

Rembulan is an implementation of Lua 5.3 for the Java Virtual Machine (JVM), written in pure Java with minimal dependencies. The goal of the Rembulan project is to develop a correct, complete and scalable Lua implementation for running sandboxed Lua programs on the JVM.

Rembulan implements Lua 5.3 as specified by the Lua Reference Manual, explicitly attempting to mimic the behaviour of PUC-Lua whenever possible. This includes language-level features (such as metamethods and coroutines) and the standard library.

Status

The majority of language-level features is implemented, and may be expected to work. If you find behaviour that does not conform to Lua 5.3 as defined by the Lua Reference Manual, please open a new issue.

See also the completeness table that maps out the current completeness status of Rembulan with regard to PUC-Lua, in particular the standard library.

Frequently asked questions (FAQ)

What is Rembulan good for?

Lua is a small, beautifully-designed and simple-yet-powerful programming language. Lua has been traditionally used as an embedded scripting language. Rembulan aims to serve a similar purpose on the JVM, with an explicit focus on sandboxing the client Lua programs.

There are two main use-cases for Rembulan: running untrusted Lua scripts on the JVM, and enhancing Java applications by adding the ability to script them with Lua.

Does Rembulan implement the Lua C API?

No, at this point Rembulan requires libraries to be written against its Java interface.

Does Rembulan work with Lua bytecode?

No. The Lua bytecode (i.e., the bytecode generated by PUC-Lua's luac) is considered an implementation detail by both Rembulan and the Lua Reference Manual. Rembulan implements its own compiler and compiles to Java bytecode directly. It uses its own intermediate representation (IR) annotated with statically-inferred type information, but does not expose it to the user, and the IR has no serialisable form.

For more information about the Rembulan compiler, see the compiler overview.

How are coroutines implemented?

See How are coroutines implemented?

Using Rembulan

Rembulan requires a Java Runtime Environment (JRE) version 7 or higher.

Documentation

Generated JavaDocs are available online:

• Runtime module
• Compiler
• Standard Library

There are also a few short texts in the doc folder:

• How are coroutines implemented?
• Overview of the Rembulan compiler
• Rembulan completeness table

Building from source

To build Rembulan, you will need the following:

• Java Development Kit (JDK) version 7 or higher
• Maven version 3 or higher

Maven will pull in the remaining dependencies as part of the build process.

To fetch the latest code on the master branch and build it, run

git clone https://github.com/mjanicek/rembulan.git
cd rembulan    
mvn install

This will build all modules, run tests and finally install all artifacts into your local Maven repository.

Standalone REPL

Much like PUC-Lua, Rembulan contains a standalone REPL. This is provided in the module rembulan-standalone. To build the REPL, run

mvn package -DskipTests -Dmaven.javadoc.skip=true -DstandaloneFinalName=rembulan

The standalone REPL is packaged as a self-contained, executable Capsule and is placed in the directory rembulan-standalone/target.

To run the REPL:

cd rembulan-standalone/target
./rembulan-capsule.x

The standalone REPL mimics the behaviour or the standalone PUC-Lua interpreter and may be used as its drop-in replacement.

$ ./rembulan-capsule.x
Rembulan 0.1-SNAPSHOT (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_60)
> print("hello world!")
hello world!

Using Rembulan from Maven

There are no releases yet, but snapshot artifacts are published to the Sonatype OSSRH Snapshot Repository. To use the snapshot artifacts, add the following configuration to your pom.xml:

<repositories>
  <repository>
    <id>sonatype-ossrh-snapshots</id>
    <name>Sonatype OSSRH (Snapshots)</name>
    <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
    <snapshots />
  </repository>
</repositories>

To include the runtime as a dependency:

<dependency>
  <groupId>net.sandius.rembulan</groupId>
  <artifactId>rembulan-runtime</artifactId>
  <version>0.1-SNAPSHOT</version>
</dependency>

To include the compiler as a dependency:

<dependency>
  <groupId>net.sandius.rembulan</groupId>
  <artifactId>rembulan-compiler</artifactId>
  <version>0.1-SNAPSHOT</version>
</dependency>

To include the standard library as a dependency:

<dependency>
  <groupId>net.sandius.rembulan</groupId>
  <artifactId>rembulan-stdlib</artifactId>
  <version>0.1-SNAPSHOT</version>
</dependency>

Note that rembulan-compiler and rembulan-stdlib both pull in rembulan-runtime as a dependency, but are otherwise independent. (I.e., to use the compiler and the standard library, you need to declare both -compiler and -stdlib as dependencies, but do not need to include -runtime).

Getting started

Rembulan compiles Lua functions into Java classes and loads them into the JVM; the compiler performs a type analysis of the Lua programs in order to generate a more tightly-typed code whenever feasible.

Since the JVM does not directly support coroutines, Rembulan treats Lua functions as state machines and controls their execution (i.e., yields, resumes and pauses) using exceptions. Since the Rembulan runtime retains control of the control state, this technique is also used to implement CPU accounting and scheduling of asynchronous operations.

Example: Hello world

The following snippet loads the Lua program print('hello world!'), compiles it, loads it into a (non-sandboxed) state, and runs it:

(From rembulan-examples/.../HelloWorld.java)

String program = "print('hello world!')";

// initialise state
StateContext state = StateContexts.newDefaultInstance();
Table env = StandardLibrary.in(RuntimeEnvironments.system()).installInto(state);

// compile
ChunkLoader loader = CompilerChunkLoader.of("hello_world");
LuaFunction main = loader.loadTextChunk(new Variable(env), "hello", program);

// execute
DirectCallExecutor.newExecutor().call(state, main);

The output (printed to System.out) is:

hello world!

Example: CPU accounting

Lua functions can be called in a mode that automatically pauses their execution once the given number of operations has been performed:

(From rembulan-examples/.../InfiniteLoop.java)

String program = "n = 0; while true do n = n + 1 end";

// initialise state
StateContext state = StateContexts.newDefaultInstance();
Table env = StandardLibrary.in(RuntimeEnvironments.system()).installInto(state);

// compile
ChunkLoader loader = CompilerChunkLoader.of("infinite_loop");
LuaFunction main = loader.loadTextChunk(new Variable(env), "loop", program);

// execute at most one million ops
DirectCallExecutor executor = DirectCallExecutor.newExecutorWithTickLimit(1000000);

try {
    executor.call(state, main);
    throw new AssertionError();  // never reaches this point!
}
catch (CallPausedException ex) {
    System.out.println("n = " + env.rawget("n"));
}

Prints:

n = 199999

The CallPausedException contains a continuation of the call. The call can be resumed: the pause is transparent to the Lua code, and the loop does not end with an error (it is merely paused).

Further examples

For further examples, see the classes in rembulan-examples/src/main/java/net/sandius/rembulan/examples.

Project structure

Rembulan is a multi-module Maven build, consisting of the following modules that are deployed to Sonatype OSSRH:

• rembulan-runtime ... the core classes and runtime;
• rembulan-compiler ... a compiler of Lua sources to Java bytecode;
• rembulan-stdlib ... the Lua standard library;
• rembulan-standalone ... standalone REPL, a (mostly) drop-in replacement for the lua command from PUC-Lua.

There are also auxiliary modules that are not deployed:

• rembulan-tests ... project test suite, including benchmarks from the Benchmarks Game;
• rembulan-examples ... examples of the Rembulan API.

Contributing

Contributions of all kinds are welcome!

License

Rembulan is licensed under the Apache License Version 2.0. See the file LICENSE.txt for details.