kevinhinterlong / jsonit

A guide to setting up antlr4 grammar projects

jsonit

jsonit is a simple example of how to use Antlr to define a grammar and use it in a Java (and gradle) based project

The project uses the JSON.g4 grammar from the antlr/grammars-v4 repo

NOTE: This project is not meant to be a useful json parser.

Antlr setup with gradle

The following lines from build.gradle define how to set up the antlr plugin which will generate the Java sources for you Parser/Lexer

// First we must pull in the antlr plugin
apply plugin: 'antlr'

// then decide where to store the generated java code
def generatedSources = [
    rootDir: 'generated-src/',
    javaDir: 'generated-src/java'
]

// then add the generated sources to our project
sourceSets {
    main {
        java {
            srcDir generatedSources.javaDir
        }
    }
}

dependencies {
    // pull in 'org.antlr:antlr4:4.7' for the `antlr` configuration
    // (this is just for building the sources files)
    antlr 'org.antlr:antlr4:4.7'
    // add the antlr runtime so we can use the generated sources
    implementation 'org.antlr:antlr4-runtime:4.7'
}

// configure the grammar generation
// more info: https://docs.gradle.org/current/userguide/antlr_plugin.html
generateGrammarSource {
    arguments += ['-visitor', '-long-messages']
    outputDirectory = file(generatedSources.javaDir)
}
// ensures the grammar source files are generated before building the project
compileJava.dependsOn generateGrammarSource

// delete the generated sources as part of the 'clean' step
clean {
    delete generatedSources.rootDir
}

Working with IntelliJ

IntelliJ by default will use it's own configuration for running Gradle project which is great for most cases; however it does not work with complex configurations.

If you try to run the project with IntelliJ it will fail because it does not run this step

compileJava.dependsOn generateGrammarSource

To solve this you can set

Build, Execution, Deployment → Build Tools → Gradle → Runner → Delegate IDE build/run actions to gradle to true

NOTE: You could also manually generate the grammar once (or whenever you change it)

$ ./gradlew generateGrammarSource 

Build

To build the project, clone it then run

$ ./gradlew build