Class, method or constructor annotation which indicates that each parameter should be checked to ensure it isn't null. If placed at the class level, all explicit methods and constructors will be checked. Explicit means those declared within the class and not inherited or added by transforms (but see includeGenerated()).

Example usage:

import groovy.transform.NullCheck
import static groovy.test.GroovyAssert.shouldFail

@NullCheck
class Greeter {
    private String audience

    Greeter(String audience) {
        this.audience = audience.toLowerCase()
    }

    String greeting(String salutation) {
        salutation.toUpperCase() + ' ' + audience
    }
}

assert new Greeter('World').greeting('hello') == 'HELLO world'

def ex = shouldFail(IllegalArgumentException) { new Greeter(null) }
assert ex.message == 'audience cannot be null'

ex = shouldFail(IllegalArgumentException) { new Greeter('Universe').greeting(null) }
assert ex.message == 'salutation cannot be null'

The produced code for the above example looks like this:

class Greeter {
    private String audience

    Foo(String audience) {
        if (audience == null) throw new IllegalArgumentException('audience cannot be null')
        this.audience = audience.toLowerCase()
    }

    String greeting(String salutation) {
        if (salutation == null) throw new IllegalArgumentException('salutation cannot be null')
        salutation.toUpperCase() + ' ' + audience
    }
}