Get Scala annotation from Java Method

I have an instance of Java Method that represents a Scala function with a Scala annotation (that extends StaticAnnotation). I know I can use Method.getAnnotation(classOf[SomeJavaAnnotation]) to get a Java annotation, but this returns null for Scala annotations.

How can I get the Scala annotation from this? It seems I need to convert it to a Scala MethodSymbol first, but I don't see an obvious way to do that and I've only seen resources showing how to go the other way (from Scala MethodSymbol to Java Method).

Scala annotations are invisible for Java reflection.

Either

• use Java annotations (with runtime retention policy), then they will be visible for Java reflection, or

• use Scala reflection, it can see Scala annotations.

This is how java.lang.reflect.Method can be converted into scala.reflect.runtime.universe.MethodSymbol

import java.lang.reflect.Method
import scala.reflect.runtime
import scala.reflect.runtime.universe._

def methodToMethodSymbol(method: Method): MethodSymbol = {
  val runtimeMirror = runtime.currentMirror
  val classSymbol = runtimeMirror.classSymbol(method.getDeclaringClass)
  classSymbol.typeSignature.decl(TermName(method.getName)).asMethod // (*)
}

If there are overloaded versions of the method, you'll have to replace
the line (*)
with

classSymbol.typeSignature.decl(TermName(method.getName)).alternatives.find(
  _.asMethod.paramLists.flatten.map(_.typeSignature.erasure.typeSymbol.asClass) == 
    method.getParameterTypes.map(runtimeMirror.classSymbol).toList
).get.asMethod

Another implementation:

def methodToMethodSymbol(method: Method): MethodSymbol = {
  val runtimeMirror = runtime.currentMirror
  val castedRuntimeMirror = runtimeMirror.asInstanceOf[{
    def methodToScala(meth: Method): scala.reflect.internal.Symbols#MethodSymbol
  }]

  castedRuntimeMirror.methodToScala(method).asInstanceOf[MethodSymbol]
}

Testing:

class myAnnotation extends StaticAnnotation

class MyClass {
  @myAnnotation
  def myMethod(i: Int): Unit = ()
}

val clazz = classOf[MyClass]
val method = clazz.getMethod("myMethod", classOf[Int])
  
val methodSymbol = methodToMethodSymbol(method) // method myMethod
methodSymbol.annotations // List(myAnnotation)

Just in case, here is reverse conversion of scala.reflect.runtime.universe.MethodSymbol into java.lang.reflect.Method

def methodSymbolToMethod(methodSymbol: MethodSymbol): Method = {
  val runtimeMirror = runtime.currentMirror
  val classSymbol = methodSymbol.owner.asClass
  val clazz = runtimeMirror.runtimeClass(classSymbol)
  val paramClasses = methodSymbol.paramLists.flatten.map(paramSymbol =>
    runtimeMirror.runtimeClass(paramSymbol.typeSignature.erasure)
  )
  clazz.getMethod(methodSymbol.name.toString, paramClasses: _*)
}

Another implementation:

def methodSymbolToMethod(methodSymbol: MethodSymbol): Method = {
  type InternalMethodSymbol = scala.reflect.internal.Symbols#MethodSymbol
  val runtimeMirror = runtime.currentMirror
  val castedRuntimeMirror = runtimeMirror.asInstanceOf[{
    def methodToJava(sym: InternalMethodSymbol): Method
  }]

  castedRuntimeMirror.methodToJava(
    methodSymbol.asInstanceOf[InternalMethodSymbol]
  )
}

https://stackoverflow.com/questions/16787163/get-a-java-lang-reflect-method-from-a-reflect-runtime-universe-methodsymbol