AndroidGradlePreprocessing

if you want add some preprocessing into Android project (under Gradle build system) then you should add some prefix into your build script

buildscript {
    repositories {
        mavenLocal()
        mavenCentral()
    }
    dependencies {
        classpath 'com.igormaznitsa:jcp:7.0.1'
    }
}
apply plugin: 'com.igormaznitsa.jcp'

and postfix

preprocessSettings {
    def targetFolder = 'build/preprocessed/java'
    target = file(targetFolder)
    sources = ['src/main/java']
    android.sourceSets.main.java.srcDirs = [targetFolder]
}
preBuild.dependsOn preprocess

I made experiments with Android project created with Android Plug-in under Intellij IDEA and played with build.gradle on App module level which is created by the plug-in. So total build.gradle in my case looked like

// section for preprocessor
buildscript {
    repositories {
        mavenLocal()
        mavenCentral()
    }
    dependencies {
        classpath 'com.igormaznitsa:jcp:7.0.1'
    }
}
apply plugin: 'com.igormaznitsa.jcp'
// end section for preprocessor

apply plugin: 'com.android.application'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "android.it.igormaznitsa.com.jcpandroid"
        minSdkVersion 15
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.android.support:design:28.0.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

// preprocessor settings
preprocessSettings {
    def targetFolder = 'build/preprocessed/java'
    target = file(targetFolder)
    sources = ['src/main/java']
    android.sourceSets.main.java.srcDirs = [targetFolder]
}
preBuild.dependsOn preprocess