Scala Note – Reflection

原文：Fun with Scala’s new Reflection API (2.10)
反射是程序可以检查、甚至修改自身的能力。在面向对象编程、函数式编程和逻辑程序设计范型中都有着悠久的历史。虽然有些语言将反射作为一个指导原则，也有很多语言逐渐发展出了反射的API等能力。Scala也在2.10引入了新的反射API，这个API比老的Manifest更加接近了实际类型系统（编译器实际上使用了TypeTags，参考官方文档typetags-manifests ）。Scala在Java反射API的基础上提供了反射的原因之一是JVM擦出了泛型类型。下面让我们看下泛型类型在运行时被擦除：

case class A[T]
  println(A[String].isInstanceOf[A[String]]) // true
  println(A[String].isInstanceOf[A[Int]])    
  // true aswell - doesn't sounds right, no? 
  // that's because the type of T gets erased at runtime...

case class A[T]

println(A[String].isInstanceOf[A[String]]) // true

println(A[String].isInstanceOf[A[Int]])

// true aswell - doesn't sounds right, no?

// that's because the type of T gets erased at runtime...

Scala 2.10引入了TypeTag，从而为泛型类型T的运行时带来了编译时信息。

import scala.reflect.runtime.universe._
    case class B[T: TypeTag] {
      val tpe = typeOf[T]
  }
  println(B[String].tpe == typeOf[String]) // true
  println(B[String].tpe == typeOf[Int])    // false

  //and this also works for nested parameterised types:
  println(B[List[String]].tpe == typeOf[List[String]]) //true
  println(B[List[String]].tpe == typeOf[List[Int]])    //false

import scala.reflect.runtime.universe._

case class B[T: TypeTag] {

val tpe = typeOf[T]

}

println(B[String].tpe == typeOf[String]) // true

println(B[String].tpe == typeOf[Int]) // false

//and this also works for nested parameterised types:

println(B[List[String]].tpe == typeOf[List[String]]) //true

println(B[List[String]].tpe == typeOf[List[Int]]) //false

另一个反射API的使用场景如下面的例子所示，抓取一个给定类型的对象内的所有成员，获得获得它们的泛型类型T并检查他们是否混入了给定的特质Required，Scala的运行时mirror允许我们在运行时对于给定的实例去查找这些符号和类型(runtimeMirror(this.getClass.getClassLoader))。

 import scala.reflect.runtime.universe._
  case class Field[T: TypeTag] {//设置TypeTag
    val tpe = typeOf[T]
  }
  trait Required //可以混入到字段中来标记它是否被需要。

  object SomeClass extends BaseClass {
    val stringField = Field[String]
    val requiredIntField = new Field[Int] with Required
  }

  abstract class BaseClass {
    val typeMirror = runtimeMirror(this.getClass.getClassLoader)
    val instanceMirror = typeMirror.reflect(this)
    val members = instanceMirror.symbol.typeSignature.members
    def fieldMirror(symbol: Symbol) = instanceMirror.reflectField(symbol.asTerm)

    def fields = members.filter(_.typeSignature <:< typeOf[Field[_]])
    // filters all members that conform to type Field[_], i.e. also subclasses of Field
  }

  SomeClass.fields.foreach { symbol ⇒
    val name = symbol.name.toString.trim
    val required = symbol.typeSignature <:< typeOf[Required]
    val tpe = SomeClass.fieldMirror(symbol).get match {
      case field: Field[_] ⇒ field.tpe
    }
    println(s"field: $name; type=$tpe; required: $required")
    /** prints:
     *  field: requiredIntField; type=Int; required: true
     *  field: stringField; type=String; required: false
     */
  }

import scala.reflect.runtime.universe._

case class Field[T: TypeTag] {//设置TypeTag

val tpe = typeOf[T]

}

trait Required //可以混入到字段中来标记它是否被需要。

object SomeClass extends BaseClass {

val stringField = Field[String]

val requiredIntField = new Field[Int] with Required

}

abstract class BaseClass {

val typeMirror = runtimeMirror(this.getClass.getClassLoader)

val instanceMirror = typeMirror.reflect(this)

val members = instanceMirror.symbol.typeSignature.members

def fieldMirror(symbol: Symbol) = instanceMirror.reflectField(symbol.asTerm)

def fields = members.filter(_.typeSignature <:< typeOf[Field[_]])

// filters all members that conform to type Field[_], i.e. also subclasses of Field

}

SomeClass.fields.foreach { symbol ⇒

val name = symbol.name.toString.trim

val required = symbol.typeSignature <:< typeOf[Required]

val tpe = SomeClass.fieldMirror(symbol).get match {

case field: Field[_] ⇒ field.tpe

}

println(s"field: $name; type=$tpe; required: $required")

/** prints:

* field: requiredIntField; type=Int; required: true

* field: stringField; type=String; required: false

}

参考：
Reflection Overview

Scala Note – Reflection

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