[Java] Emit @Nullable for context methods

[Marcono1234]

Hello,
is it possible to have ANTLR emit @Nullable annotations for context methods (methods of RuleContext implementations generated for the parser) which can return null because the corresponding parts of the rule are optional?

For example, for the following grammar:

grammar Test;
root:   first second?;
first:  'first';
second: 'second';

ANTLR generates this Java code:

public class TestParser extends Parser {
    public static class RootContext extends ParserRuleContext {
        public TerminalNode first() { ... }
        public TerminalNode second() { ... }
        ...
    }

    ...
}

Here RootContext.second() can return null. It would therefore be useful if that method had a @Nullable annotation.

However, such generation of @Nullable should probably be opt-in, because depending on what error listener you have configured, you might also encounter null for any other methods if the input does not match the grammar, see also #4040, so @Nullable would give a false sense of security implying that other methods cannot return null.

Such a @Nullable annotation might not have to come from a specific framework; many tools recognize such annotations just from its name Nullable, ignoring the package name.

Related:

• Stack Overflow question
• Emit nullable annotations for c# 8.0 #2639