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Java LESS CSS compiler

Active development of this project has stopped

There are two alternatives:

  • less4j : native java port of lesscss
  • use the original javascript lesscss via npm, grunt, etc.

Lesscss compiles LESS code into CSS stylesheets (see

Lesscss is compatible with LESS version 1.7.0. In fact, Lesscss executes the official JavaScript LESS compiler in a JVM.

In daemon mode, Lesscss monitors your LESS files (and all their imports), and automatically compiles them when necessary. So you only need to save and refresh your browser.

Lesscss can be used at the commandline, but it also provides a simple API for embedded usage in build tools.

Why yet another Java LESS compiler?

  • Lesscss can start a daemon that automatically starts a compilation when the source is modified.
  • Lesscss caches information about imported source files so that an initial compilation to gather imports is rarely necessary.
  • Lesscss supports all options, which is not the case for the other Java implementations (at the time of writing).
  • Lesscss is up to 5 times faster then existing Java implementations.

Download and installation

Lesscss requires Java 1.6 or higher.

You can download Lesscss from the maven central repository and un-zip it.

Or you can declare it as dependency in your project:

  • Gradle, grab, grails, etc.: build "com.github.houbie:lesscss:1.3.2-less-1.7.0"

  • Maven:


Commandline usage

The zip distribution contains OS specific shell scripts that are compatible with the official lessc command.

Just type lessc source.less destination.css to compile a LESS file.

The complete syntax is lessc [option option=parameter ...] <source> [destination]

Type lessc -h to see the full list of options.

Tip for compiling Twitter Bootstrap

When using Twitter Bootstrap in multiple projects, it is not necessary to copy all the LESS files to all your projects. Only copy the ones that you want to customize (typically variables.less), and then compile with:

lessc --include-path your/project/less:path/to/bootstrap-3.0.0/less bootstrap.less css/bootstrap.css

This will first look for less files in your/project/less, and when not found it will fall back to path/to/bootstrap-3.0.0/less

When you use the lesscss-gradle-plugin, you can even avoid downloading Twitter Bootstrap manually and nor will you have to copy/modify any files. Read this blog to see how.

Lesscss API

You can create a CompilationTask instance to compile one or more LESS files (Groovy example):

CompilationUnit bootstrap= newCompilationUnit('less/bootstrap.less', 'css/bootstrap.css', new Options(), new FileSystemResourceReader(new File('less')))
CompilationUnit myLess= newCompilationUnit('src/myLess.less', 'css/myLess.css')
CompilationTask compilationTask = new CompilationTask(cacheDir: new File('/tmp'), compilationUnits: [bootstrap, myLess])
compilationTask.execute() //only once
compilationTask.startDaemon(100) //check every 100 milliseconds for changes

You can also use the LessCompiler directly (again Groovy):

//simple usage
new LessCompilerImpl().compile(new File('source'), new File('destination'))

//full option API
LessCompiler.CompilationDetails details= new LessCompilerImpl(new File('customJavaScriptFunctions.js').text).compileWithDetails(
    new File('source.less').text,
    new FileSystemResourceReader('importsDir'),
    new Options(minify: true),

println details.result //generated CSS
println details.imports //list of imports encountered during compilation

There are 3 ResourceReader implementations available for resolving source and imported LESS files:

  • FileSystemResourceReader: search resources in one or more directories, ex. new FileSystemResourceReader(new File('webapp/less'), new File('/bootstrap/less'))
  • ClasspathResourceReader : search resources in the classpath relative to a base path, ex. new ClasspathResourceReader('bootstrap/less')
  • CombiningResourceReader : delegates to the ResourceReader's in an array until the resource is resolved, ex. new CombiningResourceReader(srcResourceReader, jarResourceReader)


Except for the cleancss option (see further), all standard lessc options ar supported:

Options options = new Options()
//standard LESS options
options.compress = true //Compress output by removing some whitespaces. Default: false
options.strictImports = true //Force evaluation of imports. Default: false
options.rootpath = /root/path //Set rootpath for url rewriting in relative imports and urls. Works with or without the relative-urls option. Default: null
options.relativeUrls = true //Re-write relative urls to the base less file. Default: false
options.minify = true //Compress output using YUI minifier (standard LESS uses clean-css, which is only available in node.js). Default: false
options.strictMath = true //In strict mode, math requires brackets. Default: false
options.strictUnits = true //Disallow mixed units, e.g. 1px+1em or 1px*1px which have units that cannot be represented. Default: false
options.ieCompat = false //Enable IE compatibility checks. Default: true
options.javascriptEnabled = false //Enable JavaScript in less files. Default: true
options.sourceMap = true //Outputs a v3 sourcemap.  Default: false
options.sourceMapRootpath = "/path/to/sourcemaps" //adds this path onto the sourcemap filename and less file paths. Default: null
options.sourceMapBasepath = file("$buildDir/css").absolutePath //Sets sourcemap base path (will be subtracted from generated paths). Default: null
options.sourceMapLessInline = true //Puts the less files into the map instead of referencing them. Default: false
options.sourceMapMapInline = true //Puts the map (and any less files) into the output css file. Default: false
options.sourceMapURL = "http://localhost:8080/myApp/css/" //The complete url and filename put in the less file. Default: null (calculated)
options.globalVars = [fancyColor: "#123456"] //Defines a variable that can be referenced in the less. Default: empty Map
options.modifyVars = ["btn-warning-bg": "red"] //Modifies a variable already declared in the less. Default: empty Map

//deprecated options
options.optimizationLevel = 1 //Set the parser's optimization level. The lower the number, the less nodes it will create in the tree
options.dumpLineNumbers= NONE //Outputs filename and line numbers in comments (COMMENTS) or in a fake media query (MEDIA_QUERY). Use source maps instead.


Lesscss passes all the tests of the official JavaScript LESS 1.7.0 compiler.

There is however one minor incompatibility: lesscss still uses the YUI minifier, while less.js 1.5+ switched to cleancss. Unfortunately cleancss depends on node.js and is not usable on the JVM.

Fast compilation in dev mode

Native less compilation (typically lessc via node.js) is still a lot faster than all the java less compilers. When compiling large less files like Twitter Bootstrap f.i., the differences are significant.

Lesscss can now achieve the same speed by using the new CommandLineLesscCompilationEngine (which replaced the unstable jav8 engine), when lessc is available on your system.

This makes sense when using lesscss in your java based build process: not all the developers and build machines require the native lessc, but if available it can be used transparently.

Be aware that there are a few semantic differences when generating source maps: file names and locations can differ. This can be mitigated by explicitly specifying the source map rootpath, basepath and url and/or the source-map-less-inline option.

Building from source

If you use the included gradle wrapper, you don't have to install anything (except a JDK)

git clone
cd lesscss
./gradlew installApp

Or use gradle 1.8 or higher

Useful gradle tasks:

  • install : installs the lesscss jar into the local maven repo
  • installApp : installs lesscss in build/install/lesscss
  • distZip : creates the zip distribution
  • clean, test, jar, etc.

Note that some tests related to the CommandLineLesscCompilationEngine fail on Windows.

What's up next?