java - Scala 相当于 Java java.lang.Class<T> 对象

Question

这个问题最好通过一个例子来解释:

在 Java 中，对于 JPA EntityManager，我可以执行以下操作(Account 是我的实体类):

Account result = manager.find(Account.class, primaryKey);

在 Scala 中，我天真的尝试是:

val result = manager.find(Account.class, primaryKey)

但是当我尝试在 Scala 中使用 Account.class 时，它似乎不喜欢这样。如何在 Scala 中为 Account 类指定 java.lang.Class 对象？

Answer 1

根据“The Scala Type System”，

val c = new C
val clazz = c.getClass              // method from java.lang.Object
val clazz2 = classOf[C]             // Scala method: classOf[C] ~ C.class
val methods = clazz.getMethods      // method from java.lang.Class<T>

The classOf[T] method returns the runtime representation for a Scala type. It is analogous to the Java expression T.class.
Using classOf[T] is convenient when you have a type that you want information about, while getClass is convenient for retrieving the same information from an instance of the type.

但是，classOf[T] 和 getClass 返回的值略有不同，反射(reflect)了 JVM 上类型删除的影响(在 getClass 的情况下)。

scala> classOf[C]
res0: java.lang.Class[C] = class C

scala> c.getClass
res1: java.lang.Class[_] = class C

这就是 following will not work 的原因:

val xClass: Class[X] = new X().getClass //it returns Class[_], nor Class[X]

val integerClass: Class[Integer] = new Integer(5).getClass //similar error

有一个ticket regarding the return type of getClass .

( James Moore 报告票证“现在”，即两年后的 2011 年 11 月，已修复。
在 2.9.1 中，getClass 现在执行以下操作:

scala> "foo".getClass 
       res0: java.lang.Class[_ <: java.lang.String] = class java.lang.String

)

回到 2009 年:

It would be useful if Scala were to treat the return from getClass() as a java.lang.Class[T] forSome { val T : C } where C is something like the erasure of the static type of the expression on which getClass is called

It would let me do something like the following where I want to introspect on a class but shouldn't need a class instance.
I also want to limit the types of classes I want to introspect on, so I use Class[_ <: Foo]. But this prevents me from passing in a Foo class by using Foo.getClass() without a cast.

注意:关于getClass，可能的解决方法是:

class NiceObject[T <: AnyRef](x : T) {
  def niceClass : Class[_ <: T] = x.getClass.asInstanceOf[Class[T]]
}

implicit def toNiceObject[T <: AnyRef](x : T) = new NiceObject(x)

scala> "Hello world".niceClass                                       
res11: java.lang.Class[_ <: java.lang.String] = class java.lang.String