ClassTag、Manifest、ClassManifest、TypeTag-优快云博客

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def main(args: Array[String]) {
//从理论上讲，创建一个泛型Array是不可能的，但是可以借助Manifest实现
//用Manifest[T]的隐式值，保存运行时T的信息，在实际运行时，作为一个参数作用在方法运行的上下文中
def arrayMake[T : Manifest](first : T, second : T) = {
val r = new Array[T](2); r(0) = first; r(1) = second; r
}
arrayMake(1,2).foreach(println)
def mkArray[T : ClassTag](elems: T*) = Array[T](elems: _*)
mkArray(42, 13).foreach(println)
mkArray("Japan","Brazil","Germany").foreach(println)
def manif[T](x: List[T])(implicit m: Manifest[T]) = {//原生写法，隐式参数
if (m <:< manifest[String])
println("List strings")
else
println("Some other type")
}
manif(List("Spark", "Hadoop")) //运行时，会传入Manifest[String]隐式值
manif(List(1, 2))
manif(List("Scala", 3))
val m = manifest[A[String]]
println(m)
val cm = classManifest[A[String]]
println(cm)
}

由于Manifest在依赖路径判断上存在问题，所以，Scala推出来ClassTag、TypeTag用来取代Manifest、classManifest
在Spark中使用最多的就是ClassTag，ClassTag源码注释：
* A `ClassTag[T]` stores the erased class of a given type `T`, accessible via the `runtimeClass`
* field. This is particularly useful for instantiating `Array`s whose element types are unknown
* at compile time.
* ClassTag存储T被擦出的class信息，在不知道元素类型而初始化Array时特别有用
*
* `ClassTag`s are a weaker special case of [[scala.reflect.api.TypeTags#TypeTag]]s, in that they
* wrap only the runtime class of a given type, whereas a `TypeTag` contains all static type
* information. That is, `ClassTag`s are constructed from knowing only the top-level class of a
* type, without necessarily knowing all of its argument types. This runtime information is enough
* for runtime `Array` creation.
* ClassTag比TypeTag弱，它只包含运行时class信息，TypeTag包含所有static类型信息

来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/28912557/viewspace-1973327/，如需转载，请注明出处，否则将追究法律责任。

转载于:http://blog.itpub.net/28912557/viewspace-1973327/