使用 Scala 对象中在 Java 代码中具有隐式参数的函数

Question

Is there a way that I can use the fromJson and toJson methods in my java code? I would want to call it to transform a bson document that I get from mongoldb to a model class that I have in Scala

My Scala object is below,

import com.fasterxml.jackson.databind.{DeserializationFeature, ObjectMapper}
import com.fasterxml.jackson.module.scala.{DefaultScalaModule, ScalaObjectMapper}

object JsonUtils {

  val mapper = new ObjectMapper() with ScalaObjectMapper

  mapper.registerModule(DefaultScalaModule)
  mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)

  def toJson(value: Map[Symbol, Any]): String = {
    toJson(value map { case (k,v) => k.name -> v})
  }

  def toJson(value: Any): String = {
    mapper.writeValueAsString(value)
  }

  def toMap[V](json:String)(implicit m: Manifest[V]) = fromJson[Map[String,V]](json)

  def fromJson[T](json: String)(implicit m : Manifest[T]): T = {
    mapper.readValue[T](json)
  }

  implicit class Marshallable[T](marshallMe: T) {
    def toJson: String = JsonUtils.toJson(marshallMe)
  }

  implicit class Unmarshallable(unMarshallMe: String) {
    def toMap: Map[String,Any] = JsonUtils.toMap(unMarshallMe)
    def fromJson[T]()(implicit m: Manifest[T]): T =  JsonUtils.fromJson[T](unMarshallMe)
  }
}

Answer 1

In Java code you have to resolve Scala implicits manually.

Try to call fromJson like

JsonUtils.fromJson("a", scala.reflect.Manifest.classType(LocalUser.class));

or

JsonUtils$.MODULE$.fromJson("a", scala.reflect.Manifest$.MODULE$.classType(LocalUser.class));

Based on Calling a Scala from Java Play Framework which takes a ClassTag

Regarding toJson I can't see problems. Although if you call overloaded version of toJson with Map argument, maybe you'll have to convert between scala.Predef.Map aka scala.collection.immutable.Map and java.util.Map

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By the way, Manifest is deprecated in favor of ClassTag / TypeTag

https://docs.scala-lang.org/overviews/reflection/typetags-manifests.html