Experimenting with annotation processors - is ther...

# kapt

[SnowmanX95]

Experimenting with annotation processors - is there an easy in way to figure out whether an annotated property is nullable? The mirror APIs don’t have any information on this. I looked a bit into kotlinx metadata library, but I must be using it in a wrong way as all the flags I’m getting back are nullable … (the flag absolute value is 551). Here’s the code I’m using to gather the flags:

val metadataAnnotation = typeElement.getAnnotation(Metadata::class.java)
        val header = KotlinClassHeader(
            metadataAnnotation.kind,
            metadataAnnotation.metadataVersion,
            metadataAnnotation.bytecodeVersion,
            metadataAnnotation.data1,
            metadataAnnotation.data2,
            metadataAnnotation.extraString,
            metadataAnnotation.packageName,
            metadataAnnotation.extraInt
        )

        val metadata = KotlinClassMetadata.read(header)
        when (metadata) {
            is KotlinClassMetadata.Class -> {
                metadata.accept(object: KmClassVisitor()  {
                    override fun visitProperty(
                        flags: Flags,
                        name: String,
                        getterFlags: Flags,
                        setterFlags: Flags
                    ): KmPropertyVisitor? {
                        val nullable = Flag.Type.IS_NULLABLE(flags)
                        Logger.logW("* visiting property $name, flags: $flags, nullable: $nullable")
                        return super.visitProperty(flags, name, getterFlags, setterFlags)
                    }
                })
            }
            else -> {
                Logger.logW("bummer ... not a class: $metadata")
            }
        }

Anyone can see anything?

[SnowmanX95]

Also tried to read the metadata annotation at runtime using reflection, but I’m getting the same results. Looks like there is something wrong with the generated metadata in my classes …

[Shaishav Gandhi]

You can check if that property has another annotation with `Nullable`/`NonNull`/`NotNull`.

Usually Kotlin non-nulls have the NotNull annotation

[SnowmanX95]

Right - that is one possible way, but I was hoping to use the metadata as it can give me additional valuable information

[gildor]

Metdata is the only correct way, not reading nullable annotations

[Shaishav Gandhi]

That's true. Reading nullable annotations is useful when your annotation processor runs on mixed sources.

[gildor]

if it’s on mixed sources it should check is this class has kotlin metadata (it’s Kotlin type) or not (it’s Java)

[Shaishav Gandhi]

What's a nice way to do that? Any code that you can point me to?

[gildor]

not really, you can just check for Kotlin metadata annotation

[Marc Knaup]

Easy metadata extraction:
https://github.com/fluidsonic/fluid-meta

[Shaishav Gandhi]

Nice! Will check it out

[tddmonkey]

IIRC inside your visitProperty you need to return a KmPropertyVisitor and visitReturnType. The nullability will be in the Flags there

[tddmonkey]

Some code I have that does this, looks like this:

override fun visitProperty(flags: Flags, name: String, getterFlags: Flags, setterFlags: Flags): KmPropertyVisitor? {
        return MetadataPropertyVisitor(
            onBuild = { properties.put(it.name, it) },
            name = name,
            outerFlags = flags
        )

internal class MetadataPropertyVisitor(
    private val onBuild: (DeclaredProperty) -> Unit,
    private val name: String,
    private val outerFlags: Flags
): KmPropertyVisitor() {
    override fun visitReturnType(flags: Flags): KmTypeVisitor? {
        val property = DeclaredProperty(
            name = name,
            isNullable = Flag.Type.IS_NULLABLE(flags),
            isPrivate = Flag.IS_PRIVATE(outerFlags)
        )
        onBuild(property)
        return super.visitReturnType(flags)
    }
}

[tddmonkey]

(It’s a library that does what I suspect @Marc Knaup has written, but before that existed

[Marc Knaup]

The visitor pattern was way too much boilerplate for me and too complicated, so I wrote a wrapper around it which basically reutbds plenty of easily navigable data glasses.

[tddmonkey]

@Marc Knaup I did the same but never got around to open sourcing it. When I saw you did one I made a note to check it out and see if I can dump mine

[SnowmanX95]

Thanks @tddmonkey, this is exactly what I needed. How did you figure this one out? The documentation is not exactly explaining this one ….

[tddmonkey]

Failing tests, the debugger and a lot of patience 😄

[tddmonkey]

Once you get very familiar with the AST it’s not too bad

[SnowmanX95]

Nice. Thanks for taking the one for the team 🙂