Run task before compilation using Android Gradle plugin

Question

I have a very simple build.gradle file with the following content:

buildscript {
    repositories {
        mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:0.4.1'
    }
}

apply plugin: 'android'

android {
    buildToolsVersion "17.0.0"
    compileSdkVersion 17

    sourceSets {
        main {
            manifest.srcFile 'AndroidManifest.xml'
            res.srcDirs = ['res']
            assets.srcDirs = ['assets']
        }
    }
}

task generateSources {
    doFirst {
        def script = "python GenerateSources.py".execute()
        script.in.eachLine {line -> println line}
        script.err.eachLine {line -> println "ERROR: " + line}
        script.waitFor()
    }
}

What I want is to run generateSources task before java compilation is started. I found several solutions how to do that, like compileJava.dependsOn("generateSources"), but unfortunately they give an error:

A problem occurred evaluating root project 'Android'.
> Could not find property 'compileJava' on root project 'Android'.

I don't know Gradle and can't understand what's wrong with this code. So I would like to know how I can fix this error.

Answer 1

Apparently, the android plugin doesn't add a compileJava task (like the java plugin would). You can check which tasks are available with gradle tasks --all, and pick the right one for your (otherwise correct) dependency declaration.

EDIT:

Apparently, the android plugin defers creation of tasks in such a way that they can't be accessed eagerly as usual. One way to overcome this problem is to defer access until the end of the configuration phase:

gradle.projectsEvaluated {
    compileJava.dependsOn(generateSources)
}

Chances are that there is a more idiomatic way to solve your use case, but quickly browsing the Android plugin docs I couldn't find one.

Answer 2

The proper way to run a task before Java compilation on Android is to make a compilation task for each variant depend on your task.

afterEvaluate {
  android.applicationVariants.all { variant ->
    variant.javaCompiler.dependsOn(generateSources)
  }
}

Answer 3

You can see task execution in terminal running task for example gradle assemble. Try this one, it is started practically before anything.

android {
    ...
    gradle.projectsEvaluated {
         preBuild.dependsOn(generateSources)
    }
    ...
}

Edit, this may not work in Android Studio, as the Android Gradle DSL does not have a projectsEvaluated method.