jMonkeyEngine/jme3-android-native/decode.gradle

String tremorZipFile = "TremorAndroid.zip"
String stbiUrl = 'https://raw.githubusercontent.com/jMonkeyEngine/stb/0224a44a10564a214595797b4c88323f79a5f934/stb_image.h'

// Working directories for the ndk build.
String decodeBuildDir = "${buildDir}" + File.separator + 'decode'
String decodeClassesBuildDir = "${buildDir}" + File.separator + 'decode_classes'
String decodeBuildJniDir = decodeBuildDir + File.separator + 'jni'
String decodeBuildLibsDir = decodeBuildDir + File.separator + 'libs'

// Pre-compiled libs directory
def rootPath = rootProject.projectDir.absolutePath
String decodePreCompiledLibsDir = rootPath + File.separator + 'build' + File.separator + 'native' + File.separator +  'android' + File.separator + 'decode'

// jME Android Native source files path
String decodeSourceDir = 'src/native/jme_decode'
String jmeHeaders = 'src/native/headers'

task downloadStbImage(type: MyDownload) {
    sourceUrl = stbiUrl
    target = file(decodeBuildDir + File.separator + 'stb_image.h')
}

// Copy stb_image.h to the jni directory.
task copyStbiFiles(type: Copy) {
    def sourceDir = file(decodeBuildDir + File.separator + 'stb_image.h')
    def outputDir = file(decodeBuildJniDir + File.separator + "STBI")
    from sourceDir
    into outputDir
}
copyStbiFiles.dependsOn {
    def stbiFile = file(decodeBuildDir + File.separator + 'stb_image.h')
    if (!stbiFile.exists()) {
        downloadStbImage
    }
}

// Copy libtremor source to the jni directory.
task copyTremorFiles(type: Copy) {
    def zipFile = file(tremorZipFile)
    def outputDir = file(decodeBuildJniDir + File.separator + "Tremor")

    from (zipTree(zipFile)) {
        include '*.c'
        include '*.h'
    }

    into outputDir
}

task copyJmeHeadersDecode(type: Copy) {
    from file(jmeHeaders)
    into file(decodeBuildJniDir + File.separator + "headers")
}

// Copy jME Android native files to jni directory
task copySourceToBuild(type: Copy, dependsOn:[copyTremorFiles, copyStbiFiles, copyJmeHeadersDecode]) {
    def sourceDir = file(decodeSourceDir)
    def outputDir = file(decodeBuildJniDir)

    from sourceDir
    into outputDir
}

task buildNativeLib(type: Exec, dependsOn: copySourceToBuild) {
    workingDir decodeBuildDir
    executable rootProject.ndkCommandPath
    args "-j" + Runtime.runtime.availableProcessors()
}

task updatePreCompiledLibs(type: Copy, dependsOn: buildNativeLib) {
    def sourceDir = new File(decodeBuildLibsDir)
    def outputDir = new File(decodePreCompiledLibsDir)

    from sourceDir
    into outputDir
}

// Copy pre-compiled libs to build directory (when not building new libs)
task copyPreCompiledLibs(type: Copy) {
    def sourceDir = file(decodePreCompiledLibsDir)
    def outputDir = file(decodeBuildLibsDir)

    from sourceDir
    into outputDir
}

// ndkExists is a boolean from the build.gradle in the root project
// buildNativeProjects is a string set to "true"
if (ndkExists && buildNativeProjects == "true") {
    // build native libs and update stored pre-compiled libs to commit
    compileJava.dependsOn { updatePreCompiledLibs }
} else {
    // use pre-compiled native libs (not building new ones)
    compileJava.dependsOn { copyPreCompiledLibs }
}

jar.into("lib") { from decodeBuildLibsDir }

// Helper class to wrap ant download task
class MyDownload extends DefaultTask {
    @Input
    String sourceUrl

    @OutputFile
    File target

    @TaskAction
    void download() {
       ant.get(src: sourceUrl, dest: target)
    }
}