如何在运行时使用 TypeTags 创建类型 T 的实例 [英] How to create an instance of type T at runtime with TypeTags
问题描述
以下是如何在运行时使用 Manifest
创建类型 T 的新实例:
Here below is how to create a new instance of type T at runtime with Manifest
:
trait MyTrait
class MyClass1(val name: String) extends MyTrait
class MyClass2(val name: String) extends MyTrait
class Test[T <: MyTrait] {
def createInstance[T](name: String)(implicit m: Manifest[T]): T = {
m.runtimeClass.getConstructors()(0)
.newInstance(name).asInstanceOf[T]
}
def doSomething() {
val myClass = createInstance("joe")
...
}
}
...
val test = new Test[MyClass1]
test.doSomething
上面的 createInstance
方法创建实现 MyTrait
的类之一的新实例,并使用给定的字符串调用构造函数.如何使用 TypeTag
实现相同的功能?
The createInstance
method above creates a new instance of one of the classes that implement MyTrait
and invokes the constructor with the given string. How do I implement the same with TypeTag
?
使用 scala.reflect.runtime._ 重新实现._
以下是按照 som-snytt 的建议重新实现的 Test
类:
Here below is class Test
reimplemented as suggested by som-snytt:
class Test[T <: MyTrait] {
import scala.reflect.runtime._
import scala.reflect.runtime.universe._
def createInstance[T: TypeTag](name: String): T = {
val tt = typeTag[T]
currentMirror.reflectClass(tt.tpe.typeSymbol.asClass).reflectConstructor(
tt.tpe.members.filter(m =>
m.isMethod && m.asMethod.isConstructor
).iterator.next.asMethod
)(name).asInstanceOf[T]
}
}
只要我在 Test
上调用 createInstance
它就可以工作:
As long as I call createInstance
on Test
it works:
val test = new Test[MyClass1]
val myClass = test.createInstance("hello") // this works
但是一旦我定义了一个从 Test
派生的新类...
But as soon as I define a new class derived from Test
...
class DerivedTest[T <: MyTrait] extends Test[T]
...然后我像这样在那个新类上调用 createInstance
...
... and I invoke createInstance
on that new class like this...
val test = new DerivedText[MyClass1]
val myClass = test.createInstance("hello") // this crashes
...我收到以下错误:
... I get the following error:
java.util.NoSuchElementException: next on empty iterator
at scala.collection.Iterator$$anon$2.next(Iterator.scala:39)
at scala.collection.Iterator$$anon$2.next(Iterator.scala:37)
at scala.collection.LinearSeqLike$$anon$1.next(LinearSeqLike.scala:62)
at DerivedTest$class.createInstance(<console>:27)
at $anon$1.createInstance(<console>:26)
at .<init>(<console>:28)
at .<clinit>(<console>)
at .<init>(<console>:7)
at .<clinit>(<console>)
at $print(<console>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
...
我错过了什么吗?
推荐答案
最终这里是实际工作的实现 - 请假设这个答案来自 som-snytt:
Eventually here is the implementation that actually works - please just assume this answer comes from som-snytt:
import scala.reflect.runtime._
import scala.reflect.runtime.universe._
class Test[T <: MyTrait : TypeTag] {
def createInstance(args: AnyRef*)(ctor: Int = 0): T = {
val tt = typeTag[T]
currentMirror.reflectClass(tt.tpe.typeSymbol.asClass).reflectConstructor(
tt.tpe.members.filter(m =>
m.isMethod && m.asMethod.isConstructor
).iterator.toSeq(ctor).asMethod
)(args: _*).asInstanceOf[T]
}
}
希望能帮到你.
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