ExtendJ Extension Base

This project is a minimal working example of an extension to the extensible Java compiler ExtendJ. This should provide a useful base for creating your own extensions.


Obviously this stuff is just simple template code that is meant to be copied, and I give anyone permission to use it without attribution. If you copy this code to create your own project, you can delete the LICENSE file. The license is there just to make it possible to use this project if your employer is strict about Open Source licensing.

Cloning this Project

To clone this project you will need Git installed.

Use this command to clone the project using Git:

git clone --recursive

The --recursive flag makes Git also clone the ExtendJ submodule while cloning the extension-base repository.

If you forgot the --recursive flag you can manually clone the ExtendJ submodule using these commands:

cd extension-base
git submodule init
git submodule update

This should download the ExtendJ Git repository into a local directory named extendj.

Build and Run

This project is built with Gradle. Don't worry, you do not need to install Gradle. You just need a Java installation. If you have Java installed, run the following commands to build the project:

./gradlew jar
java -jar checker.jar testfiles/

If you are on Windows, replace ./gradlew by just gradlew.

If everything went well, you should see this output:

testfiles/ contained no errors

Backend Extensions

This project is set up to build a frontend extension, i.e., static analysis tools.

To build a backend extension, i.e., a full compiler, you should apply the following changes in the file build.gradle:

  • In the java block in sourceSets.main: add a line srcDir' 'extendj/src/backend-main'
  • Change the Main-Class Jar attribute from org.extendj.ExtensionMain to org.extendj.JavaCompiler
  • Change the line imports "java8 frontend" to imports "java8 backend"

File Overview

Here is a short description of some notable files in this project:

  • build.gradle - the main Gradle build script. More about this below.
  • gradlew.bat - script for building on Windows.
  • gradlew - script for building on Unix-likes.
  • src/java/org/extendj/ - main class for the base extension. Parses Java files supplied on the command-line and runs the process() method on each parsed AST.
  • src/jastadd/ExtensionBase.jrag - simple aspect containing a single inter-type declaration: the CompilationUnit.process() method.
  • testfiles/ - simple Java file to test the generated compiler.
  • settings.gradle - configures the Gradle project name.
  • checker.jar - the generated compiler Jar file (based on project name).

How this Extension Works

This extension builds a compiler that prints out the filenames of Java files you supply to the compiler. This is obviously super simple to do, but the compiler does error-check each file for semantic errors first before printing the file name, so it can be used as a simple Java error checker.

The src/java/org/extendj/ class is the entry-point for the compiler. This is where command-line arguments are processed. The ExtensionMain class is very small because it extends the org.jastadd.extendj.JavaChecker class from ExtendJ.

The processNoErrors method in ExtensionMain is called for each CompilationUnit, i.e. each Java source file, that contained no semantic errors.

protected void processNoErrors(CompilationUnit unit) {
  // Called when there were no errors in the compilation unit.

The process method is defined in src/jastadd/ExtensionBase.jrag:

aspect ExtensionBase {
  /** Called by ExtensionMain.processNoErrors() after error-checking a compilation unit. */
  public void CompilationUnit.process() {

Extension Architecture

This section explains how the module system in the JastAdd Gradle plugin works.

A module definition usually starts like this:


This includes the core ExtendJ modules by loading the file with the path extendj/jastadd_modules. That file is a module specification which in turn includes modules from subdirectories in the extendj directory.

Each jastadd_modules file can define multiple JastAdd modules. In the build script, there is just one module named extension-base:

module "extension-base", {

    imports "java8 frontend"

    java {
        basedir "src/java/"
        include "**/*.java"

    jastadd {
        basedir "src/jastadd/"
        include "**/*.ast"
        include "**/*.jadd"
        include "**/*.jrag"

The module has some comments to show how to add parser or scanner files, but we don't use that and it is likely that you wont need to either if you just want to parse Java code.

The module uses an imports statement to import all of the JastAdd files from the core ExtendJ module java8 frontend. Each supported Java version in ExtendJ has a frontend and backend module. The frontend module is used if you don't need to generate bytecode.

JastAdd only uses .ast, .jadd, and .jrag files to generate Java code, but a JastAdd compiler needs some supporting code to run the compiler, so our module has a Java class src/java/org/extendj/ to run the compiler, and ExtendJ includes some Java code and scanner/parser code that is not generated by JastAdd.

The Java code generated by JastAdd is output to the src/gen directory.

Gradle Build Walkthrough

The build script build.gradle may need an introduction even if you are already familiar with Gradle. The first part of the build script declares which plugins will be used. We use the JastAdd Gradle plugin to generate code with JastAdd:

plugins {
  id 'java'
  id 'org.jastadd' version '1.12.0'

Next comes the jastadd {...} configuration. This part provides information about the JastAdd modules that you want to build:

jastadd {

  modules {
    include("extendj/jastadd_modules") // Include the core ExtendJ modules.

    module "extension-base", {
      imports "java8 frontend" // This module depends on "java8 frontend" from ExtendJ.

      java {
        basedir "src/java/"
        include "**/*.java"

      jastadd {
        basedir "src/jastadd/"
        include "**/*.ast"
        include "**/*.jadd"
        include "**/*.jrag"

      //scanner {
      // TODO List your scanner specification additions here.

      //parser {
      // TODO List your parser specification additions here.

  // Target module to build:
  module = 'extension-base'

  astPackage = 'org.extendj.ast'
  buildInfoDir = 'src/gen-res' = 'JavaParser' = 'OriginalScanner'

ExtendJ is organized into a modular structure where each Java version of the compiler has its own module. Each Java version also has a frontend and backend module. The backend modules add bytecode generation to the corresponding frontend.

The modules {...} block above declares the modules that will be visible to the JastAdd Gradle plugin. Then, the module = ... line tells the plugin which module you want to build. The JastAdd Gradle plugin will include all source files in the module, plus the source files from modules that your module depends on (recursively).

The next part of the build script specifies source and resource directories for the build. We need to do this here to include a few Java files from ExtendJ that will be used by `src/java/org/extendj/':

sourceSets.main {
  java {
    srcDir 'extendj/src/frontend-main'
  resources {
    srcDir 'extendj/src/res'
    srcDir jastadd.buildInfoDir

The remaining parts of the build script are not very interesting.


Although the Gradle plugin can handle some automatic rebuilding when a source file changes, it does not handle all possible cases. In some situations you will need to force Gradle to rebuild your project. This can be done passing the --rerun-tasks option to Gradle:

./gradlew --rerun-tasks

Upgrading ExtendJ

If you want to update to the latest ExtendJ version, you can use these commands:

cd extendj
git fetch origin
git reset --hard origin/master

This may be necessary if a bugfix that you need was committed to ExtendJ in a version later than the version that this repository links to.

It is recommended that you use a test suite to ensure that your extension functionality is preserved after upgrading the core ExtendJ compiler.

Additional Resources

More examples on how to build ExtendJ-like projects with the JastAdd Gradle plugin can be found here: