Run on release only

How to setup run on release only on the gradle file? 
The following setting is for all building.
android {
    ...

    defaultConfig {
       ...
    }
}

BlackObfuscator {
    enabled true
    depth 2
    obfClass = ["top.niunaijun", "com.abc"]
    blackClass = ["top.niunaijun.black"]
}

dependencies {
    ...
}