What is @NonNull used for?

Placing @NonNull on a field, constructor parameter, or method parameter tells Lombok to generate a null check at the start of the relevant generated (or annotated) method, throwing a NullPointerException immediately with a clear message if the value is null, rather than letting a null slip through and fail confusingly somewhere else later.

public class Person {
    @NonNull
    private String name;

    public Person(@NonNull String name) {
        this.name = name;
    }
}
//  generates: if (name == null) {
//      throw new NullPointerException("name is marked non-null but is null");
//  }

It's a lightweight, compile-time-generated form of defensive null checking — not a substitute for a real nullability analysis tool, but a convenient way to fail fast with a clear error at the point a null was actually passed in, rather than at some later, harder-to-trace point in the code.

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