Scala反射 – 根据特征加载或查找类

我认为反射API确实可以更容易地理清你需要的东西(即,用于过滤而不是用于查询类加载器).

如果,通过您的短语“搜索已加载的类”,您实际上是指已经加载的类,请参阅this question以获取它们.

你可以想象一个带有初始化程序的小部件库,它只是确保加载它所知道的所有小部件类.然后客户端只需要知道初始化器.

型式试验是一样的.

val need = typeOf[Whatsit[Cog]]
for (x <- (ServiceLoader load classOf[Whatsit[_]]).asScala) { 
  val im = currentMirror reflect x 
  if (im.symbol.toType weak_<:< need)
    Console println s"$x is what I need"
  else
    Console println s"$x is not what I need,I'm looking for a $need"
}

你在哪里寻找带有类型参数的东西：

trait Whatsit[+A <: Widget] {
  def widget: A
}

class Engine extends Whatsit[Cog] {
  def widget = new Cog
}

class FlyWheel extends Whatsit[Sprocket] {
  def widget = new Sprocket
}

样品：

widgets.Engine@f9da0cd is what I need
widgets.FlyWheel@4cfdbb9f is not what I need,I'm looking for a widgets.Whatsit[widgets.Cog]

如果您使用ServiceLoader已经十年了,并且不需要复习：

apm@mara:~/tmp$ls -R META-INF
META-INF:
MANIFEST.MF  services

META-INF/services:
widgets.Whatsit  widgets.Widget
apm@mara:~/tmp$cat META-INF/services/widgets.Widget
widgets.Cog
widgets.Sprocket
apm@mara:~/tmp$cat META-INF/services/widgets.Whatsit 
widgets.Engine
widgets.FlyWheel

东西：

package widgets

trait Widget {
  def turn(): Int
  override def toString = s"Widget ${getClass.getSimpleName}"
}

class Cog extends Widget {
  def turn() = 5
}

class Sprocket extends Widget {
  def turn() = 10
}

trait Whatsit[+A <: Widget] {
  def widget: A
  override def toString = s"Whatsit ${getClass.getSimpleName} of $widget"
}

class Engine extends Whatsit[Cog] {
  def widget = new Cog
}

class FlyWheel extends Whatsit[Sprocket] {
  def widget = new Sprocket
}

比较Scala和Java.我想了解一下有多少LOC来获取GenericInterfaces并在Scala中找到你想要的东西,但后来我结束了这个练习.

package findwidgets

import reflect._
import reflect.runtime.universe._
import reflect.runtime.currentMirror
import scala.collection.JavaConverters._
import java.util.ServiceLoader

object Test extends App {
  import widgets.{ Widget,Whatsit,Cog }
  val ws = (ServiceLoader load classOf[Widget]).asScala
  for (w <- ws) {
    Console println s"Turn a ${w.getClass} by ${w.turn}"
  }
  val need = typeOf[Whatsit[Cog]]
  for (x <- (ServiceLoader load classOf[Whatsit[Cog]]).asScala) {
    val im = currentMirror reflect x
    if (im.symbol.toType weak_<:< need)
      Console println s"$x is what I need"
    else
      Console println s"$x is not what I need,I'm looking for a $need"
    // java says:
    if (classOf[Whatsit[Cog]] isAssignableFrom x.getClass)
      Console println s"Um,OK,I'll take the $x"
    else
      Console println s"${classOf[Whatsit[Cog]]} isn't ass'able from ${x.getClass}"
  }
}