Android: Loading API Keys and other secrets from properties file using gradle

build.gradle

buildTypes {
    applicationVariants.all { variant ->
        variant.buildConfigField "String", "API_KEY", "\""+getApiKey()+"\""
    }
}

def getApiKey(){
    def Properties props = new Properties()
    props.load(new FileInputStream(new File('secrets.properties')))
    return props['API_KEY']
}

secrets.properties

API_KEY=my_awesome_api_key

UseApiKey.java

String apiKey = BuildConfig.API_KEY