如何获得选项[_]类的None与反射

Question

我有这样的设置，我想从XML NodeSeq填充对象字段。下面显示了我的设置的简化版本。它工作得很好，但是当我把None放在testObj中时，我得到了一个java.util.NoSuchElementException: None.get。

我认为的问题是，我找不到一种方法来获得Option[_]的类，当它是None。我已经被困在这个问题上很久了，我找不到出路。我觉得必须有一种方法来分支和填充对象字段时，原始值是None，但如何?

import xml.NodeSeq
object test {
  case class TestObject(name: Option[String] = None, value: Option[Double] = None)
  def main(args: Array[String]): Unit = {
  val testObj = TestObject(Some("xxx"), Some(12.0))
//    val testObj = TestObject(None, Some(12.0))
    val nodeSeq = <test> <name> yyy </name> <value> 34.0 </value> </test>
    val result = getObject[TestObject](nodeSeq, Option(testObj))
    println("result = " + result) // result = Some(TestObject(Some(yyy),Some(34.0)))
  }
  def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
    if ((nodeSeq.isEmpty) || (!obj.isDefined)) None else {
      for (field <- obj.get.getClass.getDeclaredFields) {
    field.setAccessible(true)
    val fieldValue = field.get(obj.get)
    if (fieldValue == null || !fieldValue.isInstanceOf[Option[_]]) None
    var newValue: Option[_] = None
    fieldValue.asInstanceOf[Option[_]].get match {    // <---- None.get is the error here
      case x: Double => newValue = Some((nodeSeq  field.getName).text.trim.toDouble)
      case x: String => newValue = Some((nodeSeq  field.getName).text.trim)
    }
    val decField = obj.get.getClass.getDeclaredField(field.getName)
    decField.setAccessible(true)
    decField.set(obj.get, newValue)
   }
    obj
    }
  }
}

Answer 1

我忽略了这段代码中我不喜欢的其他部分，但是，您不能在None上调用.get。你可以调用map，它会返回你想要的。下面是修改后的代码:

  def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
    if (nodeSeq.isEmpty || obj.isEmpty) {
      None
    }
    else {
      for (field <- obj.get.getClass.getDeclaredFields) {
        field.setAccessible(true)
        val fieldValue = field.get(obj.get)
        if (fieldValue == null || !fieldValue.isInstanceOf[Option[_]]) None
        val newValue = fieldValue.asInstanceOf[Option[_]].map(a => {
          a match {
            case x: Double => Some((nodeSeq  field.getName).text.trim.toDouble)
            case x: String => Some((nodeSeq  field.getName).text.trim)
          }
        })
        val decField = obj.get.getClass.getDeclaredField(field.getName)
        decField.setAccessible(true)
        decField.set(obj.get, newValue)
      }
      obj
    }
  }

在实践中，我通常避免在选项上调用.get，因为它经常会导致错误。

更新:

这个版本功能更强一些，尽管它仍然改变了您发送进来的对象:

  def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
    obj.map(o => {
      for {
        field <- o.getClass.getDeclaredFields
        _ = field.setAccessible(true)
        fieldValue <- Option(field.get(o))
      } yield {
        val newValue = fieldValue match {
          case Some(a) => {
            a match {
              case x: Double => Some((nodeSeq  field.getName).text.trim.toDouble)
              case x: String => Some((nodeSeq  field.getName).text.trim)
            }
          }
          case _ => None
        }
        val decField = o.getClass.getDeclaredField(field.getName)
        decField.setAccessible(true)
        decField.set(o, newValue)
      }
      o
    })
  }

Answer 2

实际上，这可能更接近您想要的:检查字段的类型。

import xml.NodeSeq
object test {
  case class TestObject(name: Option[String] = None, value: Option[Double] = None)
  def main(args: Array[String]): Unit = {
  val testObj = TestObject(Some("xxx"), Some(12.0))
//    val testObj = TestObject(None, Some(12.0))
    val nodeSeq = <test> <name> yyy </name> <value> 34.0 </value> </test>
    val result = getObject[TestObject](nodeSeq, Option(testObj))
    println("result = " + result) // result = Some(TestObject(Some(yyy),Some(34.0)))
  }
  def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
    if ((nodeSeq.isEmpty) || (!obj.isDefined)) None else {
      for (field <- obj.get.getClass.getDeclaredFields) {
    field.setAccessible(true)
    val newValue = field.getGenericType match {
      case t: ParametrizedType if (t.getActualType == classOf[Option[_]]) => 
        val paramType = t.getActualTypeArguments()(0)
        if (paramType == classOf[java.lang.Double]) // since Double can't be a generic type parameter on JVM
          // from your original code, but it probably needs to be modified
          // to handle cases where field isn't in nodeSeq or not a number
          Some((nodeSeq  field.getName).text.trim.toDouble) 
        else if (paramType == classOf[String])
          Some((nodeSeq  field.getName).text.trim)
        else None
      case _ => None
    }
    if (newValue.isDefined)
      field.set(obj.get, newValue)
   }
    obj
    }
  }
}

创建新对象的版本(可能需要根据您的需求进行修改):

  def getObject[A](nodeSeq: NodeSeq, objClass: Class[A]): Option[A] = {
    if (nodeSeq.isEmpty) None else {
      try {
        val fieldValues = objClass.getDeclaredFields.flatMap { field =>
          field.setAccessible(true)
          field.getGenericType match {
            case t: ParametrizedType if (t.getActualType == classOf[Option[_]]) => 
              val paramType = t.getActualTypeArguments()(0)
              if (paramType == classOf[java.lang.Double])
                Some(Some((nodeSeq  field.getName).text.trim.toDouble))
              else if (paramType == classOf[String])
                Some(Some((nodeSeq  field.getName).text.trim))
              else None
            case _ => None
          }
        }
        // assumes only one constructor, otherwise get the desired one
        val ctor = objClass.getConstructors()(0)
        Some(ctor.newInstance(fieldValues))
      } catch {
        case _ => None
      }
    }
  }