[REQ] Add Support for JSpecify Annotations

[jorgerod]

Following Spring Framework's decision to use JSpecify for nullability annotations, I think it would be very interesting to add this support to Openapi Generator.

TL;TR JSpecify
In Java code, whether an expression may evaluate to null is often documented only in natural language, if at all. JSpecify's nullness annotations let programmers express the nullness of Java code in a consistent and well-defined way.

JSpecify defines annotations that describe whether a Java type contains the value null. Such annotations are useful to (for example):

• programmers reading the code,
• tools that help developers avoid NullPointerExceptions,
• tools that perform run-time checking and test generation, and
• documentation systems.

It would be possible to add an option in which it would be configurable the option to follow (jakarta or JSpecify).

What do you think?

[dafriedmann]

I recently ran into the same requirement, so I'm sharing my solution here:

You can overwrite the template default for nullability: https://openapi-generator.tech/docs/templating/#modifying-templates
With #19617 a new mustache template for nullability annotations was added.
Previously (v6) it was necessary to modify this directly in pojo.mustache.

With the mustache nullable_var_annotations.mustache the annotation can be changed quite easily:

{{#required}}{{#isNullable}}@org.jspecify.annotations.Nullable{{/isNullable}}{{^isNullable}}@org.jspecify.annotations.NonNull{{/isNullable}}{{/required}}{{^required}}@org.jspecify.annotations.Nullable{{/required}}

This allowed me to add JSpecify annotations to the generated classes.
If your project already uses JSpecify, it is sufficient to set the dependency in Maven to provided.

Hope this helps 👋🏼