Getting Started with LuaJ

James Roseborough, Ian Farmer, Version 3.0-beta2

Copyright © 2009-2014 Freely available under the terms of the Luaj license.

introduction · examples · concepts · libraries · luaj api · parser · building · downloads · release notes

1 - Introduction

Goals of Luaj

Luaj is a lua interpreter based on the 5.2.x version of lua with the following goals in mind:

Luaj version and Lua Versions

Luaj 3.0.x

Support for lua 5.2.x features: It also includes miscellaneous improvements over luaj 2.0.x:

Luaj 2.0.x

Support for lua 5.1.x features, plus:

Luaj 1.0.x

Support for most lua 5.1.x features.


Good performance is a major goal of luaj. The following table provides measured execution times on a subset of benchmarks from the computer language benchmarks game in comparison with the standard C distribution.
Project Version Mode    Benchmark execution time (sec)    Language Sample command
binarytrees 15 fannkuch 10 nbody 1e6 nsieve 9
luaj 3.0 -b (luajc) 2.980 5.073 16.794 11.274 Java java -cp luaj-jse-3.0-beta2.jar;bcel-5.2.jar lua -b fannkuch.lua 10
-n (interpreted) 12.838 23.290 36.894 15.163 java -cp luaj-jse-3.0-beta2.jar lua -n fannkuch.lua 10
lua 5.1.4 17.637 16.044 15.201 5.477 C lua fannkuch.lua 10
jill 1.0.1 44.512 54.630 72.172 20.779 Java
kahlua 1.0 jse 22.963 63.277 68.223 21.529 Java
mochalua 1.0 50.457 70.368 82.868 41.262 Java
Luaj in interpreted mode performs well for the benchmarks, and even better when the lua-to-java-bytecode (luajc) compiler is used, and actually executes faster than C-based lua in some cases. It is also faster than Java-lua implementations Jill, Kahlua, and Mochalua for all benchmarks tested.

2 - Simple Examples

Run a lua script in Java SE

From the main distribution directory line type:

	java -cp lib/luaj-jse-3.0-beta2.jar lua examples/lua/hello.lua

You should see the following output:

	hello, world
To see how luaj can be used to acccess most Java API's including swing, try:
	java -cp lib/luaj-jse-3.0-beta2.jar lua examples/lua/swingapp.lua

Compile lua source to lua bytecode

From the main distribution directory line type:

	java -cp lib/luaj-jse-3.0-beta2.jar luac examples/lua/hello.lua
	java -cp lib/luaj-jse-3.0-beta2.jar lua luac.out

The compiled output "luac.out" is lua bytecode and should run and produce the same result.

Compile lua source or bytecode to java bytecode

Luaj can compile lua sources or binaries directly to java bytecode if the bcel library is on the class path. From the main distribution directory line type:

	ant bcel-lib
	java -cp "lib/luaj-jse-3.0-beta2.jar;lib/bcel-5.2.jar" luajc -s examples/lua -d . hello.lua
	java -cp "lib/luaj-jse-3.0-beta2.jar;." lua -l hello

The output hello.class is Java bytecode, should run and produce the same result. There is no runtime dependency on the bcel library, but the compiled classes must be in the class path at runtime, unless runtime jit-compiling via luajc and bcel are desired (see later sections).

Lua scripts can also be run directly in this mode without precompiling using the lua command with the -b option and providing the bcel library in the class path:

	java -cp "lib/luaj-jse-3.0-beta2.jar;lib/bcel-5.2.jar" lua -b examples/lua/hello.lua

Run a script in a Java Application

A simple hello, world example in luaj is:

	import org.luaj.vm2.*;
	import org.luaj.vm2.lib.jse.*;

	Globals globals = JsePlatform.standardGlobals();
	LuaValue chunk = globals.load("print 'hello, world'");;
Loading from a file is done via Globals.loadFile():
	LuaValue chunk = globals.loadfile("examples/lua/hello.lua");
Chunks can also be loaded from a Reader as text source
	chunk = globals.load(new StringReader("print 'hello, world'"), "main.lua");
or an InputStream to be loaded as text source "t", or binary lua file "b":
	chunk = globals.load(new FileInputSStream("examples/lua/hello.lua"), "main.lua", "bt"));

A simple example may be found in


You must include the library lib/luaj-jse-3.0-beta2.jar in your class path.

Run a script in a MIDlet

For MIDlets the JmePlatform is used instead:

	import org.luaj.vm2.*;
	import org.luaj.vm2.lib.jme.*;

	Globals globals = JmePlatform.standardGlobals();
	LuaValue chunk = globals.loadfile("examples/lua/hello.lua");;

The file must be a resource within within the midlet jar for the loader to find it. Any files included via require() must also be part of the midlet resources.

A simple example may be found in


You must include the library lib/luaj-jme-3.0-beta2.jar in your midlet jar.

An ant script to build and run the midlet is in


You must install the wireless toolkit and define WTK_HOME for this script to work.

Run a script using JSR-223 Dynamic Scripting

The standard use of JSR-223 scripting engines may be used:

	ScriptEngineManager mgr = new ScriptEngineManager();
	ScriptEngine e = mgr.getEngineByName("luaj");
	e.put("x", 25);
	e.eval("y = math.sqrt(x)");
	System.out.println( "y="+e.get("y") );
You can also look up the engine by language "lua" or mimetypes "text/lua" or "application/lua".

All standard aspects of script engines including compiled statements are supported.

You must include the library lib/luaj-jse-3.0-beta2.jar in your class path.

A working example may be found in

To compile and run it using Java 1.6 or higher:
	javac -cp lib/luaj-jse-3.0-beta2.jar examples/jse/
	java -cp "lib/luaj-jse-3.0-beta2.jar;examples/jse" ScriptEngineSample

Excluding the lua bytecode compiler

By default, the compiler is included whenever standardGlobals() or debugGlobals() are called. Without a compiler, files can still be executed, but they must be compiled elsewhere beforehand. The "luac" utility is provided in the jse jar for this purpose, or a standard lua compiler can be used.

To exclude the lua-to-lua-bytecode compiler, do not call standardGlobals() or debugGlobals() but instead initialize globals with including only those libraries that are needed and omitting the line:


Including the LuaJC lua-bytecode-to-Java-bytecode compiler

To compile from lua to Java bytecode for all lua loaded at runtime, install the LuaJC compiler into a globals object use:


This will compile all lua bytecode into Java bytecode, regardless of if they are loaded as lua source or lua binary files.

The requires bcel to be on the class path, and the ClassLoader of JSE or CDC.

3 - Concepts


The old notion of platform has been replaced with creation of globals. Two classes are provided to encapsulate common combinations of libraries.


This class can be used as a factory for globals in a typical Java SE application. All standard libraries are included, as well as the luajava library. The default search path is the current directory, and the math operations include all those supported by Java SE.


This class can be used to set up the basic environment for a Java ME application. The default search path is limited to the jar resources, and the math operations are limited to those supported by Java ME. All libraries are included except luajava, and the os, io, and math libraries are limited to those functions that can be supported on that platform.

Thread Safety

Luaj 3.0 can be run in multiple threads, with the following restrictions: For an example of loading allocating per-thread Globals and invoking scripts in multiple threads see examples/jse/

As an alternative, the JSR-223 scripting interface can be used, and should always provide a separate Globals instance per script engine instance by using a ThreadLocal internally.

4 - Libraries

Standard Libraries

Libraries are coded to closely match the behavior specified in See standard lua documentation for details on the library API's

The following libraries are loaded by both JsePlatform.standardGlobals() and JmePlatform.standardGlobals():


The JsePlatform.standardGlobals() globals also include:


The JsePlatform.debugGlobals() and JsePlatform.debugGlobals() functions produce globals that include:


I/O Library

The implementation of the io library differs by platform owing to platform limitations.

The JmePlatform.standardGlobals() instantiated the io library io in

The JsePlatform.standardGlobals() includes support for random access and is in

OS Library

The implementation of the os library also differs per platform.

The basic os library implementation us used by JmePlatform and is in:

A richer version for use by JsePlatform is :
Time is a represented as number of seconds since the epoch, and locales are not implemented.

Coroutine Library

The coroutine library is implemented using one JavaThread per coroutine. This allows coroutine.yield() can be called from anywhere, as with the yield-from-anywhere patch in C-based lua.

Luaj uses WeakReferences and the OrphanedThread error to ensure that coroutines that are no longer referenced are properly garbage collected. For thread safety, OrphanedThread should not be caught by Java code. See LuaThread and OrphanedThread javadoc for details.

Debug Library

The debug library is not included by default by JmePlatform.standardGlobals() or JsePlatform.standardGlobsls() . The functions JmePlatform.debugGlobals() and JsePlatform.debugGlobsls() create globals that contain the debug library in addition to the other standard libraries. To install dynamically from lua use java-class-based require::
	require 'org.luaj.vm2.lib.DebugLib'
The lua command line utility includes the debug library by default.

The Luajava Library

The JsePlatform.standardGlobals() includes the luajava library, which simplifies binding to Java classes and methods. It is patterned after the original luajava project.

The following lua script will open a swing frame on Java SE:

	jframe = luajava.bindClass( "javax.swing.JFrame" )
	frame = luajava.newInstance( "javax.swing.JFrame", "Texts" );

See a longer sample in examples/lua/swingapp.lua for details, including a simple animation loop, rendering graphics, mouse and key handling, and image loading. Or try running it using:

	java -cp lib/luaj-jse-3.0-beta2.jar lua examples/lua/swingapp.lua

The Java ME platform does not include this library, and it cannot be made to work because of the lack of a reflection API in Java ME.

The lua connand line tool includes luajava.

5 - LuaJ API

API Javadoc

The javadoc for the main classes in the LuaJ API are on line at
You can also build a local version from sources using
	 ant doc

LuaValue and Varargs

All lua value manipulation is now organized around LuaValue which exposes the majority of interfaces used for lua computation.

Common Functions

LuaValue exposes functions for each of the operations in LuaJ. Some commonly used functions and constants include:
	call();               // invoke the function with no arguments
	call(LuaValue arg1);  // call the function with 1 argument
	invoke(Varargs arg);  // call the function with variable arguments, variable return values
	get(int index);       // get a table entry using an integer key
	get(LuaValue key);    // get a table entry using an arbitrary key, may be a LuaInteger
	rawget(int index);    // raw get without metatable calls
	valueOf(int i);       // return LuaValue corresponding to an integer
	valueOf(String s);    // return LuaValue corresponding to a String
	toint();              // return value as a Java int
	tojstring();          // return value as a Java String
	isnil();              // is the value nil
	NIL;                  // the value nil
	NONE;                 // a Varargs instance with no values	 


The interface Varargs provides an abstraction for both a variable argument list and multiple return values. For convenience, LuaValue implements Varargs so a single value can be supplied anywhere variable arguments are expected.

Common Functions

Varargs exposes functions for accessing elements, and coercing them to specific types:
	narg();                 // return number of arguments
	arg1();                 // return the first argument
	arg(int n);             // return the nth argument
	isnil(int n);           // true if the nth argument is nil
	checktable(int n);      // return table or throw error
	optlong(int n,long d);  // return n if a long, d if no argument, or error if not a long
See the Varargs API for a complete list.


The simplest way to implement a function is to choose a base class based on the number of arguments to the function. LuaJ provides 5 base classes for this purpose, depending if the function has 0, 1, 2, 3 or variable arguments, and if it provide multiple return values.
Each of these functions has an abstract method that must be implemented, and argument fixup is done automatically by the classes as each Java function is invoked.

An example of a function with no arguments but a useful return value might be:

	pubic class hostname extends ZeroArgFunction {
		public LuaValue call() {
			return valueOf(;
The value env is the environment of the function, and is normally supplied by the instantiating object whenever default loading is used.

Calling this function from lua could be done by:

	local hostname = require( 'hostname' )
while calling this function from Java would look like:
	new hostname().call();
Note that in both the lua and Java case, extra arguments will be ignored, and the function will be called. Also, no virtual machine instance is necessary to call the function. To allow for arguments, or return multiple values, extend one of the other base classes.

Libraries of Java Functions

When require() is called, it will first attempt to load the module as a Java class that implements LuaFunction. To succeed, the following requirements must be met:

If luaj can find a class that meets these critera, it will instantiate it, cast it to LuaFunction then call() the instance with two arguments: the modname used in the call to require(), and the environment for that function. The Java may use these values however it wishes. A typical case is to create named functions in the environment that can be called from lua.

A complete example of Java code for a simple toy library is in examples/jse/

import org.luaj.vm2.LuaValue;
import org.luaj.vm2.lib.*;

public class hyperbolic extends TwoArgFunction {

	public hyperbolic() {}

	public LuaValue call(LuaValue modname, LuaValue env) {
		LuaValue library = tableOf();
		library.set( "sinh", new sinh() );
		library.set( "cosh", new cosh() );
		env.set( "hyperbolic", library );
		return library;

	static class sinh extends OneArgFunction {
		public LuaValue call(LuaValue x) {
			return LuaValue.valueOf(Math.sinh(x.checkdouble()));
	static class cosh extends OneArgFunction {
		public LuaValue call(LuaValue x) {
			return LuaValue.valueOf(Math.cosh(x.checkdouble()));
In this case the call to require invokes the library itself to initialize it. The library implementation puts entries into a table, and stores this table in the environment.

The lua script used to load and test it is in examples/lua/hyperbolicapp.lua

	require 'hyperbolic'

	print('hyperbolic', hyperbolic)
	print('hyperbolic.sinh', hyperbolic.sinh)
	print('hyperbolic.cosh', hyperbolic.cosh)

	print('sinh(0.5)', hyperbolic.sinh(0.5))
	print('cosh(0.5)', hyperbolic.cosh(0.5))
For this example to work the code in must be compiled and put on the class path.


Closures still exist in this framework, but are optional, and are only used to implement lua bytecode execution, and is generally not directly manipulated by the user of luaj.

See the org.luaj.vm2.LuaClosure javadoc for details on using that class directly.

6 - Parser

Javacc Grammar

A Javacc grammarwas developed to simplify the creation of Java-based parsers for the lua language. The grammar is specified for javacc version 5.0 because that tool generates standalone parsers that do not require a separate runtime.

A plain undecorated grammer that can be used for validation is available in grammar/Lua51.jj while a grammar that generates a typed parse tree is in grammar/LuaParser.jj

Creating a Parse Tree from Lua Source

The default lu compiler does a single-pass compile of lua source to lua bytecode, so no explicit parse tree is produced.

To simplify the creation of abstract syntax trees from lua sources, the LuaParser class is generated as part of the JME build. To use it, provide an input stream, and invoke the root generator, which will return a Chunk if the file is valid, or throw a ParseException if there is a syntax error.

For example, to parse a file and print all variable names, use code like:

	try {
		String file = "main.lua";
		LuaParser parser = new LuaParser(new FileInputStream(file));
		Chunk chunk = parser.Chunk();
		chunk.accept( new Visitor() {
			public void visit(Exp.NameExp exp) {
				System.out.println("Name in use: "
					+" line "+exp.beginLine
					+" col "+exp.beginColumn);
		} );
	} catch ( ParseException e ) {
		System.out.println("parse failed: " + e.getMessage() + "\n"
			+ "Token Image: '" + e.currentToken.image + "'\n"
			+ "Location: " + e.currentToken.beginLine + ":" + e.currentToken.beginColumn 
			         + "-" + e.currentToken.endLine + "," + e.currentToken.endColumn);
An example that prints locations of all function definitions in a file may be found in

See the org.luaj.vm2.ast package javadoc for the API relating to the syntax tree that is produced.

7 - Building and Testing

Maven integration

The main jar files are now deployed in the maven central repository. To use them in your maven-based project, list them as a dependency:

For JSE projects, add this dependency for the luaj-jse jar:

while for JME projects, use the luaj-jme jar:
An example skelton maven pom file for a skeleton project is in

Building the jars

An ant file is included in the root directory which builds the libraries by default.

Other targets exist for creating distribution file an measuring code coverage of unit tests.

Unit tests

The main luaj JUnit tests are organized into a JUnit 3 suite:


Unit test scripts can be found in these locations


Code coverage

A build script for running unit tests and producing code coverage statistics is in

It relies on the cobertura code coverage library.

8 - Downloads

Downloads and Project Pages

Downloads for all version available on SourceForge or LuaForge. Sources are hosted on SourceForge and available via
	SourceForge Luaj Project Page
	SourceForge Luaj Download Area

Files are no longer hosted at LuaForge.

9 - Release Notes

Main Changes by Version

  • Initial release of 2.0 version
  • Improve correctness of singleton construction related to static initialization
  • Fix nan-related error in constant folding logic that was failing on some JVMs
  • JSR-223 fixes: add META-INF/services entry in jse jar, improve bindings implementation
  • JSR-223 bindings change: non Java-primitives will now be passed as LuaValue
  • JSR-223 enhancement: allow both ".lua" and "lua" as extensions in getScriptEngine()
  • JSR-223 fix: use system class loader to support using luaj as JRE extension
  • Improve selection logic when binding to overloaded functions using luajava
  • Enhance javadoc, put it in distribution and on line
  • Major refactor of luajava type coercion logic, improve method selection.
  • Add lib/luaj-sources-2.0.2.jar for easier integration into an IDE such as Netbeans
  • Improve coroutine state logic including let unreferenced coroutines be garbage collected
  • Fix lua command vararg values passed into main script to match what is in global arg table
  • Add arithmetic metatag processing when left hand side is a number and right hand side has metatable
  • Fix load(func) when mutiple string fragments are supplied by calls to func
  • Allow access to public members of private inner classes where possible
  • Turn on error reporting in LuaParser so line numbers ar available in ParseException
  • Improve compatibility of table.remove()
  • Disallow base library setfenv() calls on Java functions
  • Convert internal and external API's to match lua 5.2.x environment changes
  • Add bit32 library
  • Add explicit Globals object to manage global state, especially to imrpove thread safety
  • Drop support for lua source to java surce (lua2java) in favor of direct java bytecode output (luajc)
  • Remove compatibility functions like table.getn(), table.maxn(), table.foreach(), and math.log10()
  • Add ability to create runnable jar file from lua script with sample build file build-app.xml
  • Supply environment as second argument to LibFunction when loading via require()
  • Fix bug 3597515 memory leak due to string caching by simplifying caching logic.
  • Fix bug 3565008 so that short substrings are backed by short arrays.
  • Fix bug 3495802 to return correct offset of substrings from string.find().
  • Add artifacts to Maven central repository.
  • Limit pluggable scripting to use compatible bindings and contexts, implement redirection.
  • Fix bug that didn't read package.path from environment.
  • Fix pluggable scripting engine lookup, simplify implementation, and add unit tests.
  • Coerce script engine eval() return values to Java.
  • Fix Lua to Java coercion directly on Java classes.
  • Fix Globals.load() to call the library with an empty modname and the globals as the environment.
  • Fix hash codes of double.
  • Fix bug in luajava overload resolution.
  • Fix luastring bug where parsing did not check for overflow.
  • Fix luastring bug where circular dependency randomly caused NullPointerException.
  • Major refactor of table implementation.
  • Improved behavior of next() (fixes issue #7).
  • Existing tables can now be made weak (fixes issue #16).
  • More compatible allocation of table entries in array vs. hash (fixes issue #8).
  • Fix os.time() to return a number of seconds instead of milliseconds.
  • Implement formatting with, and table argument for os.time().
  • LuaValue.checkfunction() now returns LuaFunction.
  • Refactor APIs related to compiling and loading scripts to provide methods on Globals.
  • Add API to compile from Readers as well as InputStreams.
  • Add optional -c encoding flag to lua, luac, and luajc tools to control source encoding.
  • Let errors thrown in debug hooks bubble up to the running coroutine.
  • Make error message handler function in xpcall per-thread instead of per-globals.
  • Establish "org.luaj.debug" and "org.luaj.luajc" system properties to configure scripting engine.

Known Issues


File Character Encoding

Source files can be considered encoded in UTF-8 or ISO-8859-1 and results should be as expected, with literal string contianing quoted characters compiling to the same byte sequences as the input. For a non ASCII-compatible encoding such as EBSDIC, however, there are restrictions: These restrictions are mainly a side effect of how the language is defined as allowing byte literals within literal strings in source files. Code that is generated on the fly within lua and compiled with lua's load() function should work as expected, however, since these strings will never be represented with the host's native character encoding.