Annotation CheckReturnValue for subscribe() and other builder-like methods

[jreznot]

Popular reactive libraries such as RxJava has @CheckReturnValue annotation on methods returning objects without side-effects, so developers may rely on IDE to check that there are no calls with ignored return value.

It would be great if Mutiny APIs would be annotated with this kind of annotation.

Example:

package io.reactivex.rxjava3.annotations;

import java.lang.annotation.*;

/**
 * Marks methods whose return values should be checked.
 */
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Target(ElementType.METHOD)
public @interface CheckReturnValue {
}

Problem example:

    @Test
    public void testLog() {
        Uni.createFrom().item(1)
                .log()
                .subscribe();  // nothing happens, because I didn't finish subscribe configuration
    }

Once methods are annotated IDE can help here:

[jreznot]

The corresponding issue in IntelliJ IDEA: https://youtrack.jetbrains.com/issue/IDEA-277014

I believe it would be better to maintain such rules right in the framework to help all editors and IDEs with these usages.

[jponge]

Do we have any standard for such annotations?

[jreznot]

Do we have any standard for such annotations?

IntelliJ IDEA recognizes any annotation with short name CheckReturnValue. Google Checker Framework uses the same name too.

[jponge]

Ok so we can create our own right?

[jreznot]

Yes, you may simply create your own, no dependencies required

[gavinking]

This sounds like a really worthwhile thing to do.

[jponge]

Planned for 1.2.0 🚀

[cescoffier]

+1, sounds like a great idea and will improve the user experience. Once defined in mutiny I would pull the annotation in the Vert.x bindings and annotate all methods returning a Uni or a Multi (how many times did we call a method returning a Uni and nothing happens?).

@Sanne This could be interesting for Hibernate Reactive too.

[jponge]

• Annotate in Mutiny
• Bring the annotation to the Mutiny Vert.x bindings code generator

[jreznot]

Please be careful with methods that receive lambdas with effect, e.g. log or something like forEach. As far as I understand, there might be hot streams that do work even without a subscription, such most-likely-have-effect methods must not be annotated as CheckReturnValue.

[Sanne]

Yes look nice. But how does it work, is it an annotation processor? I would prefer an annotation processor (or similar) over trying to rely on IDEA specific conventions.

[jreznot]

Yes look nice. But how does it work, is it an annotation processor? I would prefer an annotation processor (or similar) over trying to rely on IDEA specific conventions.

This is simply inspection in IntelliJ IDEA ) I believe it provides faster feedback loop than compiling everything to see this kind of errors.

[jreznot]

is it an annotation processor

AFAIK this kind of checks cannot be implemented as annotation processor, only if you use something like ByteBuddy enhancer that visits all the code.

[Sanne]

Ok I would have preferred something that works in all IDEs (and no IDE) but I suppose it's still very nice to have this in IDEA.

Funny, reminds me of an old tool we had explored to identify (and take advantage of) code which would do a Map#put but ignored the return value: https://github.com/infinispan/jokre (it's pretty much dead though, just somewhat related)

[jponge]

See #729