从反射的Java类实例中获取ClassTag

Question

是否有可能ClassTag从Class通过反射获得的Java 实例中获取信息？

这是情况.我有一个case class看起来像这样的Scala :

case class Relation[M : ClassTag](id: UUID, 
                                  model: Option[M] = None)

它是这样使用的(虽然有更多的类相互关联):

case class Organization(name: String)

case class Person(firstName: String, 
                  lastName: String,
                  organization: Relation[Organization])

我想要做的是以编程方式使用如下所示的东西构建这些关系的树:

private def generateFieldMap(clazz: Class[_]): Map[String, Class[_]] = {
    clazz.getDeclaredFields.foldLeft(Map.empty[String, Class[_]])((map, field) => {
        map + (field.getName -> field.getType)
    })
}

private def getRelationModelClass[M : ClassTag](relationClass: Class[_ <: Relation[M]]): Class[_] = {
    classTag[M].runtimeClass
}

def treeOf[M: ClassTag](relations: List[String]): Map[String, Any] = {
    val normalizedRelations = ModelHelper.normalize(relations)
    val initialFieldMap = Map("" -> generateFieldMap(classTag[M].runtimeClass))
    val relationFieldMap = relations.foldLeft(initialFieldMap)((map, relation) => {
        val parts = relation.split('.')
        val parentRelation = parts.dropRight(1).mkString(".")
        val relationClass = map(parentRelation)(parts.last)
        val relationModelClass = relationClass match {
            case clazz: Class[_ <: Relation[_]] => getRelationModelClass(clazz)
            case _ => throw ProcessStreetException("cannot follow non-relation: " + relation)
        }
        val fieldMap = generateFieldMap(relationModelClass)
        map + (relation -> fieldMap)
    })
    relationFieldMap
}

val relations = List("organization")
val tree = treeOf[Person](relations)

这不会编译.我收到此错误:

[error] Foo.scala:148: not found: type _$12
[error]                 case clazz: Class[_ <: Relation[_]] => getRelationModelClass(clazz)
[error]                                   ^
[error] one error found
[error] (compile:compile) Compilation failed

基本上,我想做的就是能够ClassTag在我拥有Java时获取信息Class.这可能吗？

Answer 1

是的,绝对可能也很容易:

val clazz = classOf[String]
val ct = ClassTag(clazz)  // just use ClassTag.apply() method

在你的例子中,你想要调用这样的getRelationModelClass方法:

getRelationModelClass(clazz)(ClassTag(clazz))

这是可能的,因为[T: ClassTag]语法隐式地创建了第二个参数列表(implicit ct: ClassTag[T]).通常它由编译器填充,但没有任何东西阻止您显式使用它.

您也不需要同时将此clazz的类AND类标记传递给该方法.你甚至没有在它的体内使用显式类对象.只需传递类标签,就足够了.