使用构造函数参数通过反射实例化对象 [英] instantiate object with reflection using constructor arguments

问题描述

我想弄清楚如何用反射实例化一个案例类对象.是否有任何支持?我最接近的是查看 scala.reflect.Invocation，但这似乎更适合执行作为对象一部分的方法.

I'm trying to figure out how to instantiate a case class object with reflection. Is there any support for this? The closest I've come is looking at scala.reflect.Invocation, but this seems more for executing methods that are a part of an object.

case class MyClass(id:Long, name:String)

def instantiate[T](className:String)(args:Any*) : T = { //your code here }

接近我正在寻找的 API.

Is close to the API I'm looking for.

任何帮助将不胜感激.

推荐答案

scala> case class Foo(id:Long, name:String)
defined class Foo

scala> val constructor = classOf[Foo].getConstructors()(0)
constructor: java.lang.reflect.Constructor[_] = public Foo(long,java.lang.String)

scala> val args = Array[AnyRef](new java.lang.Integer(1), "Foobar")
args: Array[AnyRef] = Array(1, Foobar)

scala> val instance = constructor.newInstance(args:_*).asInstanceOf[Foo]
instance: Foo = Foo(1,Foobar)

scala> instance.id
res12: Long = 1

scala> instance.name
res13: String = Foobar

scala> instance.getClass
res14: java.lang.Class[_] = class Foo

目前在 Scala 中没有太多的反射支持.但是您可以退回到 Java 反射 API.但是有一些障碍:

Currently there is not much reflection support in Scala. But you can fall back to th Java Reflection API. But there are some obstacles:

• 您必须创建一个 Array[AnyRef] 并在包装类(java.lang.Integer、java.lang.Character、java.lang.Double, ...)

newInstance(Object ... args) 得到一个 Object 的可变参数数组，所以你应该用 :_ 给类型推断器一个提示*

newInstance(Object ... args) gets an varargs array of Object, so you should give the type inferer a hint with :_*

newInstance(...) 返回一个 Object，因此您必须使用 asInstanceOf[T] 将其转换回>

newInstance(...) returns an Object so you have to cast it back with asInstanceOf[T]

我最接近你的 instantiate 函数是这样的:

The closest I could get to your instantiate function is this:

def instantiate(clazz: java.lang.Class[_])(args:AnyRef*): AnyRef = {
  val constructor = clazz.getConstructors()(0)
  return constructor.newInstance(args:_*).asInstanceOf[AnyRef]
}

val instance = instantiate(classOf[MyClass])(new java.lang.Integer(42), "foo")
println(instance)           // prints: MyClass(42,foo)
println(instance.getClass)  // prints: class MyClass

您无法从泛型类型获取 get 类.Java 擦除它(类型擦除).

You cannot get the get class from a generic type. Java erases it (type erasure).

2012 年 9 月 20 日

三年过去了，可以改进 instantiate 方法以返回正确类型的对象.

Three years on, the instantiate method can be improved to return a properly typed object.

def instantiate[T](clazz: java.lang.Class[T])(args:AnyRef*): T = {
  val constructor = clazz.getConstructors()(0)
  return constructor.newInstance(args:_*).asInstanceOf[T]
}

参见 http://www.nabble.com/How-do-I-get-the-class-of-a-Generic--td20873455.html

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