How to access Gradle extra properties in src code?

Thank you for the explanation , I am a newbie in gradle and trying to set up a framework with it where I want to pass environment value from Gradle Terminal.
I am able to achieve it using system property env define in build file, giving gradle -Denv=test task command in terminal and accessing it using System.properties(env) in java file. but when I am defining ext properties in gradl3e and running command gradle -Penv=test task in that case when I am calling variable env in my java file its unrecognizable.I am looking for the way to access -Penv=value passed from terminal to java file.