-- Benjamin P. Jung, Jan. 26th 2018
Copyright © 2009-2014 Luaj.org. Freely available under the terms of the Luaj license.
introduction · examples · concepts · libraries · luaj api · parser · building · downloads · release notes
1 – Introduction
- 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.
- _ENV environments model.
- yield from pcall or metatags.
- Bitwise operator library.
- Better thread safety.
- More compatible table behavior.
- Better coroutine-related garbage collection.
- Maven integration.
- Better debug reporting when using closures.
- Line numbers in parse syntax tree.
- Support for compiling lua source code into Java source code.
- Support for compiling lua bytecode directly into Java bytecode.
- Stackless vm design centered around dynamically typed objects.
- Good alignment with C API (see names.csv for details)
- Implementation of weak keys and values, and all metatags.
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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 – Examples
From the main distribution directory line type:
java -cp luaj-jse-3.0.2.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 luaj-jse-3.0.2.jar lua examples/lua/swingapp.lua
Links to sources:
examples/lua/hello.lua examples/lua/swingapp.lua
From the main distribution directory line type:
java -cp luaj-jse-3.0.2.jar luac examples/lua/hello.lua java -cp luaj-jse-3.0.2.jar lua luac.out
The compiled output "luac.out" is lua bytecode and should run and produce the same result.
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 "luaj-jse-3.0.2.jar;lib/bcel-5.2.jar" luajc -s examples/lua -d . hello.lua java -cp "luaj-jse-3.0.2.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 "luaj-jse-3.0.2.jar;lib/bcel-5.2.jar" lua -b examples/lua/hello.lua
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'");
chunk.call();
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
examples/jse/SampleJseMain.java
You must include the library luaj-jse-3.0.2.jar in your class path.
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");
chunk.call();
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
examples/jme/SampleMIDlet.java
You must include the library luaj-jme-3.0.2.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.
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 luaj-jse-3.0.2.jar in your class path.
A working example may be found in
examples/jse/ScriptEngineSample.java
To compile and run it using Java 1.6 or higher:
javac -cp luaj-jse-3.0.2.jar examples/jse/ScriptEngineSample.java java -cp "luaj-jse-3.0.2.jar;examples/jse" ScriptEngineSample
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:
org.luaj.vm2.compiler.LuaC.install(globals);
To compile from lua to Java bytecode for all lua loaded at runtime, install the LuaJC compiler into a globals object use:
org.luaj.vm2.jse.luajc.LuaJC.install(globals);
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
Android applications should use the JsePlatform, and can include the Luajava library
to simplify access to underlying Android APIs.
A specialized Globals.finder should be provided to find scripts and data for loading.
See examples/android/src/android/LuajView.java
for an example that loads from the "res" Android project directory.
The ant build script is examples/android/build.xml.
Applets in browsers should use the JsePlatform. The permissions model in applets is highly restrictive, so a specialization of the Luajava library must be used that uses default class loading. This is illustrated in the sample Applet examples/jse/SampleApplet.java, which can be built using build-applet.xml.
The JmePlatform 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.MIDlets require the JmePlatform.
The JME platform has several limitations which carry over to luaj.
In particular Globals.finder is overridden to load as resources, so scripts should be
colocated with class files in the MIDlet jar file. Luajava cannot be used.
Camples code is in
examples/jme/SampleMIDlet.java,
which can be built using build-midlet.xml.
Luaj 3.0 can be run in multiple threads, with the following restrictions:
- Each thread created by client code must be given its own, distinct Globals instance
- Each thread must not be allowed to access Globals from other threads
- Metatables for Number, String, Thread, Function, Boolean, and and Nil are shared and therefore should not be mutated once lua code is running in any thread.
For an example of loading allocating per-thread Globals and invoking scripts in multiple threads see examples/jse/SampleMultiThreaded.java
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.
Lua and luaj are allow for easy sandboxing of scripts in a server environment.Considerations include
- The debug and luajava library give unfettered access to the luaj vm and java vm so can be abused
- Portions of the os, io, and coroutine libraries are prone to abuse
- Rogue scripts may need to be throttled or killed
- Shared metatables (string, booleans, etc.) need to be made read-only or isolated via class loaders such as LuajClassLoader
Luaj provides sample code covering various approaches:
- examples/jse/SampleSandboxed.java A java sandbox that limits libraries, limits bytecodes per script, and makes shared tables read-only
- examples/jse/samplesandboxed.lua A lua sandbox that limits librares,limits bytecodes per script, and makes shared tables read-only
- examples/jse/SampleUsingClassLoader.java A heavier but strong sandbox where each script gets its own class loader and a full private luaj implementation
4 – 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():
base bit32 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:
debugThe implementation of the io library differs by platform owing to platform limitations.
The JmePlatform.standardGlobals() instantiated the io library io in
src/jme/org/luaj/vm2/lib/jme/JmeIoLib.java
The JsePlatform.standardGlobals() includes support for random access and is in
src/jse/org/luaj/vm2/lib/jse/JseIoLib.javaThe implementation of the os library also differs per platform.
The basic os library implementation us used by JmePlatform and is in:
src/core/org/luaj/lib/OsLib.java
A richer version for use by JsePlatform is :
src/jse/org/luaj/vm2/lib/jse/JseOsLib.java
Time is a represented as number of seconds since the epoch, and locales are not implemented.
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. The sample code in examples/jse/CollectingOrphanedCoroutines.java provides working examples.
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 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" ); frame:setDefaultCloseOperation(jframe.EXIT_ON_CLOSE) frame:setSize(300,400) frame:setVisible(true)
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 luaj-jse-3.0.2.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 ME.
The lua connand line tool includes luajava.
5 – LuaJ API
http://luaj.org/luaj/3.0/api
You can also build a local version from sources using
ant docAll lua value manipulation is now organized around LuaValue which exposes the majority of interfaces used for lua computation.
org.luaj.vm2.LuaValueLuaValue 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 valuesThe 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.
org.luaj.vm2.VarargsVarargs 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.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
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(java.net.InetAddress.getLocalHost().getHostName());
}
}
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.
- The class must be on the class path with name, modname.
- The class must have a public default constructor.
- The class must inherit from LuaFunction.
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/hyperbolic.java
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 hyperbolic.java 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
A plain undecorated grammer that can be used for validation is available in grammar/Lua52.jj while a grammar that generates a typed parse tree is in grammar/LuaParser.jj
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: "+exp.name.name
+" 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
examples/jse/SampleParser.java
See the org.luaj.vm2.ast package javadoc for the API relating to the syntax tree that is produced.
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:
<dependency>
<groupId>org.luaj</groupId>
<artifactId>luaj-jse</artifactId>
<version>3.0.2</version>
</dependency>
while for JME projects, use the luaj-jme jar:
<dependency>
<groupId>org.luaj</groupId>
<artifactId>luaj-jme</artifactId>
<version>3.0.2</version>
</dependency>
An example skelton maven pom file for a skeleton project is in
examples/maven/pom.xmlBuild the jars with maven.
mvn clean verify
All unit tests are executed during the build.
Test scripts can be found in these locations
luaj-test/src/test/resourcesExecuton is included in the build of luaj-test.
The maven build creates the coverage report in the luaj-test/target/site folder during the verify phase.
8 – Downloads
SourceForge Luaj Project Page SourceForge Luaj Download Area
The jar files may also be downloaded from the maven central repository, see Maven Integration.
Files are no longer hosted at LuaForge.
9 – Release Notes
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- debug code may not be completely removed by some obfuscators
- tail calls are not tracked in debug information
- mixing different versions of luaj in the same java vm is not supported
- values associated with weak keys may linger longer than expected
- behavior of luaj when a SecurityManager is used has not been fully characterized
- negative zero is treated as identical to integer value zero throughout luaj
- lua compiled into java bytecode using luajc cannot use string.dump() or xpcall()
- number formatting with string.format() is not supported
- shared metatables for string, bool, etc are shared across Globals instances in the same class loader
- orphaned threads will not be collected unless garbage collection is run and sufficient time elapses
For a non ASCII-compatible encoding such as EBSDIC, however, there are restrictions:
- supplying a Reader to Globals.load() is preferred over InputStream variants
- using FileReader or InputStreamReader to get the default OS encoding should work in most cases
- string literals with quoted characters may not produce the expected values in generated code
- command-line tools lua, luac, and luajc will require -c Cp037 to specify the encoding
Code that is generated on the fly within lua and compiled with lua's load() function should work as expected, since these strings will never be represented with the host's native character encoding.