In JSpecify mode, missing meta-annotations trigger warning i.c.w. JDK 23+

[Stephan202]

See this discussion for context. I narrowed down the reproduction case to:

import reactor.core.publisher.Mono;

final class Test {
  void m() {
    Mono.justOrEmpty(null);
  }
}

In the absence of a dependency on com.google.code.findbugs:jsr305 (or similar org.jspecify:jspecify alternative), this triggers the following warning:

[WARNING] Cannot find annotation method 'when()' in type 'javax.annotation.Nonnull': class file for javax.annotation.Nonnull not found
[WARNING] /path/to/Test.java: unknown enum constant javax.annotation.meta.When.MAYBE
  reason: class file for javax.annotation.meta.When not found

The trigger seems to be this usage of reactor.util.annotation.Nullable, which is defined with reference to a few javax.annotation.* meta annotations.

The issue is not observed without -XepOpt:NullAway:JSpecifyMode=true. I could also not reproduce it with JDK 21.0.6-tem and 22.0.2-oracle, but do observe it with 23.0.2-tem and 24-open. This indicates that it's specific to JSpecify mode on JDK 23+.

[msridhar]

I can reproduce this in a standalone build, but it's hard to reproduce in a unit test, so hard to track down exactly what is going on. That said, what I think is going on is:

1. From JDK 23 on, more annotations get read from bytecodes via the Compiler Tree APIs, due to fixes in javac;
2. In JSpecify mode, we are calling one of these APIs on this method parameter that we didn't call before (due to some generics checking); and
3. reading the annotation via the Compiler Tree API leads to the annotation type being properly loaded by javac, which leads to its meta-annotations being loaded, which leads to the warning you are seeing.

The only part I'm not too sure about is why 2 is happening (which is hard to track down outside a unit test). But I don't think anything wrong is going on here. javac tries to be lazy about loading types / annotations from the classpath, but in principle it can load whatever is present there and whatever those types depend on. By using javax.annotations.* meta annotations, I think those annotations become part of reactor.util.annotation.Nullable's "API" and should be on the classpath to avoid such warnings.

While I haven't fully tracked this down, I'm pretty sure it's not a NullAway bug, so I'm going to close it out. Unfortunately I think you'll need the JSR305 annotations on the classpath to avoid these warnings for now.