@ParametersAreNonnullByDefault for all subpackages

nonnull.gradle

/**
  *  File: nonnull.gradle
  *
  *  Generates package-info.java for appropriate packages
  *  inside src/main/java folder.
  *
  *  This is a workaround to define @ParametersAreNonnullByDefault for all Java classes in a package 
  *  i.e. including all subpackages (note: edit line no. 19).
 */
task generateNonNullJavaFiles(dependsOn: "assembleDebug", type: Copy) {
    group = "Copying"
    description = "Generate package-info.java classes"

    def infoFileContentHeader = getFileContentHeader();
    def infoFileContentFooter = getFileContentFooter();

    def sourceDir = file( "${projectDir}" + File.separatorChar + "src" + File.separatorChar +
            "main" + File.separatorChar + "java" + File.separatorChar +
            "com" + File.separatorChar + "company" + File.separatorChar + "name" )
    sourceDir.eachDirRecurse { dir ->
        def infoFilePath = dir.getAbsolutePath() + File.separatorChar + "package-info.java"
        
        if (!file(infoFilePath).exists()) {
            def infoFileContentPackage = getFileContentPackage(dir.getAbsolutePath());
            new File(infoFilePath).write(infoFileContentHeader +
                    infoFileContentPackage + infoFileContentFooter)
            println "[dir] " + infoFilePath + "  created";
        }
    }
    println "[SUCCESS] NonNull generator: package-info.java files checked"
}

def getFileContentPackage(path) {
    def mainSrcPhrase = "src" + File.separatorChar + "main" + File.separatorChar +
            "java" + File.separatorChar
    def mainSrcPhraseIndex = path.indexOf(mainSrcPhrase)
    def output = path.substring(mainSrcPhraseIndex)
    
    // Win hotfix
    if (System.properties['os.name'].toLowerCase().contains('windows')) {
        output = output.replace("\\", "/")
        mainSrcPhrase = mainSrcPhrase.replace("\\", "/")
    }

    return "package " + output.replaceAll(mainSrcPhrase, "").replaceAll(
                "/", ".") + ";\n"
}

def getFileContentHeader() {
    return  "/**\n" +
            " *\n" +
            " * Make all method parameters @NonNull by default.\n" +
            " *\n" +
            " * We assume that all method parameters and return types are NON-NULL by default.\n" +
            " *\n" +
            " * e.g.\n" +
            " *\n" +
            " * String trimExampleMethod(String value) {\n" +
            " *     return value.trim();\n" +
            " * }\n" +
            " *\n" +
            " * is equal to:\n" +
            " *\n" +
            " * @NonNull\n" +
            " * String trimExampleMethod(@NonNull String value) {\n" +
            " *     return value.trim();\n" +
            " * }\n" +
            " *\n" +
            " * reverse this behaviour with: @Nullable annotation.\n" +
            " *\n" +
            " */\n" +
            "@ParametersAreNonnullByDefault\n" +
            "@ReturnValuesAreNonnullByDefault\n"
}

def getFileContentFooter() {
    return  "\n" +
            "import javax.annotation.ParametersAreNonnullByDefault;\n" +
            "\n" +
            "import edu.umd.cs.findbugs.annotations.ReturnValuesAreNonnullByDefault;"
}

Replace line: 19 with your package name

"com" + File.separatorChar + "company" + File.separatorChar + "name"

Usage:

apply plugin: 'com.android.application'
apply from: 'nonnull.gradle'

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.2"

    [...]
}