Getting Started with LuaJ

Copyright © 2009-2010 Luaj.org. 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

• Java-centric implementation of lua vm built to leverage standard Java features.
• Lightweight, high performance execution of lua.
• Multi-platform to be able to run on JME, JSE, or JEE environments.
• Complete set of libraries and tools for integration into real-world projects.
• Dependable due to sufficient unit testing of vm and library features.

Differences with 1.0

• Support for compiling lua source code into Java source code.
• Support for compiling lua bytecode directly into Java bytecode.
• Improved performance of of lua bytecode processing.
• Stackless vm design centered around dynamically typed objects.
• More alignment with C API (see names.csv for details)
• Improved class and package naming conventions.
• Improved unit tests of core classes.
• Improved quality due to major redesign and rewrite of core elements.
• More complete implementation including weak keys and values, and all metatags.

Performance

2 - Simple Examples

Run a lua script in Java SE

From the main distribution directory line type:

	java -cp lib/luaj-jse-2.0.2.jar lua examples/lua/hello.lua

You should see the following output:

	hello, world

Compile lua source to lua bytecode

From the main distribution directory line type:

	java -cp lib/luaj-jse-2.0.2.jar luac examples/lua/hello.lua
	java -cp lib/luaj-jse-2.0.2.jar lua luac.out

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

Compile lua source to java source

Luaj can compile to lua source code to Java source code:

	java -cp lib/luaj-jse-2.0.2.jar lua2java -s examples/lua -d . hello.lua
	javac -cp lib/luaj-jse-2.0.2.jar hello.java
	java -cp "lib/luaj-jse-2.0.2.jar;." lua -l hello

The output hello.java is Java source, that implements the logic in hello.lua directly. Once hello.java is compiled into hello.class it can be required and used in place of the original lua script, but with better performance. There are no additional dependencies for compiling or running source-to-source compiled lua.

Lua scripts can also be run directly in this mode without precompiling using the lua command with the -j option when run in JDK 1.5 or higher:

	java -cp lib/luaj-jse-2.0.2.jar lua -j examples/lua/hello.lua

Compile lua 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-2.0.2.jar;lib/bcel-5.2.jar" luajc -s examples/lua -d . hello.lua
	java -cp "lib/luaj-jse-2.0.2.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-2.0.2.jar;lib/bcel-5.2.jar" lua -b examples/lua/hello.lua

Run a script in a Java Application

The following pattern is used within Java SE

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

	String script = "examples/lua/hello.lua";
	LuaValue _G = JsePlatform.standardGlobals();
	_G.get("dofile").call( LuaValue.valueOf(script) );

A simple example may be found in

	examples/jse/SampleJseMain.java

You must include the library lib/luaj-jse-2.0.2.jar in your class path.

Run a script in a MIDlet

The for MIDlets the JmePlatform is used instead:

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

	String script = "examples/lua/hello.lua";
	LuaValue _G = JmePlatform.standardGlobals();
	_G.get("dofile").call( LuaValue.valueOf(script) );

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

A simple example may be found in

	examples/jme/SampleMIDlet.java

You must include the library lib/luaj-jme-2.0.2.jar in your midlet jar.

An ant script to build and run the midlet is in

	build-midlet.xml

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.getEngineByExtension(".lua");
	e.put("x", 25);
	e.eval("y = math.sqrt(x)");
	System.out.println( "y="+e.get("y") );

All standard aspects of script engines including compiled statements should be supported.

You must include the library lib/luaj-jse-2.0.2.jar in your class path.

A working example may be found in

	examples/jse/ScriptEngineSample.java

	javac examples/jse/ScriptEngineSample.java
	java -cp "lib/luaj-jse-2.0.2.jar;examples/jse" ScriptEngineSample

Excluding the lua bytecode compiler

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:

	org.luaj.vm2.compiler.LuaC.install();

Including the Lua2Java lua-source-to-Java-source compiler

To compile from lua sources to Java sources for all lua loaded at runtime, install the Lua2Java compiler after globals have been created using:

	org.luaj.vm2.jse.lua2java.Lua2Java.install();

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 after globals have been created using:

	org.luaj.vm2.jse.luajc.LuaJC.install();

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

Globals

JsePlatform

JmePlatform

4 - Libraries

Standard Libraries

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

	base
	coroutine
	io
	math
	os
	package
	string
	table

The JsePlatform.standardGlobals() globals also include:

	luajava 

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

	debug

I/O Library

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

	src/jme/org/luaj/vm2/lib/jme/JmeIoLib.java

	src/jse/org/luaj/vm2/lib/jse/JseIoLib.java

OS Library

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

	src/core/org/luaj/lib/OsLib.java

	src/jse/org/luaj/vm2/lib/jse/JseOsLib.java

Debug Library

	require 'org.luaj.vm2.lib.DebugLib'

The Luajava Library

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

	jframe = luajava.bindClass( "javax.swing.JFrame" )
	frame = luajava.newInstance( "javax.swing.JFrame", "Texts" );
	frame:setDefaultCloseOperation(jframe.EXIT_ON_CLOSE)
	frame:setSize(300,400)
	frame:setVisible(true)

See a longer sample in examples/lua/swingapp.lua for details, or try running it using:

	java -cp lib/luaj-jse-2.0.2.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 SE.

The lua connand line tool includes luajava.

5 - LuaJ API

API Javadoc

	 http://luaj.sourceforge.net/api/2.0

	 ant doc

LuaValue and Varargs

	 org.luaj.vm2.LuaValue

Common Functions

	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	 

Varargs

	 org.luaj.vm2.Varargs

Common Functions

	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

LibFunction

	 org.luaj.vm2.lib.ZeroArgFunction
	 org.luaj.vm2.lib.OneArgFunction
	 org.luaj.vm2.lib.TwoArgFunction
	 org.luaj.vm2.lib.ThreeArgFunction
	 org.luaj.vm2.lib.VarArgFunction

For example, to implement a "hello, world" function, we could supply:

	pubic class hello extends ZeroArgFunction {
		public LuaValue call() {
			env.get("print").call(valueOf("hello, world"));
		}
	}

Calling this function from lua could be done by:

 
	require( 'hello' )()

 
	new hello().call();

Closures

6 - Parser

Javacc Grammar

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

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: "+exp.name.name);
			}
		} );
	} catch ( ParseException e ) {
		System.out.println( "parse failed: "+e );
	}

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

7 - Building and Testing

Building the jars

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:

	test/junit/org/luaj/vm2/AllTests.lua

Unit test scripts can be found in these locations

	test/lua/*.lua
	test/junit/org/luaj/vm2/compiler/lua5.1-tests.zip
	test/junit/org/luaj/vm2/compiler/regressions.zip
	test/junit/org/luaj/vm2/vm1/luajvm1-tests.zip

Code coverage

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

	build-coverage.xml

8 - Downloads

Downloads and Project Pages

	SourceForge Luaj Project Page
	SourceForge Luaj Download Area

and LuaForge:

	LuaForge Luaj Project Page
	LuaForge Luaj Project Area

9 - Release Notes

Main Changes by Version