Scala等效于Java java.lang.Class< T>目的 [英] Scala equivalent of Java java.lang.Class<T> Object
问题描述
问题最好用一个例子来解释:
The question is best explained by an example:
在Java中,对于JPA EntityManager,我可以做下面的事情(Account是我的Entity类):
In Java for a JPA EntityManager, I can do the following(Account is my Entity class):
Account result = manager.find(Account.class, primaryKey);
在Scala中,我的天真尝试是:
In Scala, my naive attempt is:
val result = manager.find(Account.class, primaryKey)
但是当我尝试在Scala中使用 Account.class
时,似乎不喜欢这样。如何在Scala中为Account类指定java.lang.Class对象?
But when I try to use Account.class
in Scala, it seems to not like this. How can I specify the java.lang.Class object for the Account class in Scala?
推荐答案
根据 Scala类型系统,
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>
classOf [T]
方法返回Scala类型的运行时表示。它类似于Java表达式T.class
。
使用classOf [T]
方便,当你有一个类型,你想要的信息,而getClass
方便从类型的实例检索相同的信息。
The
classOf[T]
method returns the runtime representation for a Scala type. It is analogous to the Java expressionT.class
.
UsingclassOf[T]
is convenient when you have a type that you want information about, whilegetClass
is convenient for retrieving the same information from an instance of the type.
但是, classOf [T]
和 getClass
不同的值,反映类型擦除对JVM的影响,在getClass的情况下。
However, classOf[T]
and getClass
return slightly different values, reflecting the effect of type erasure on the JVM, in the case of getClass.
scala> classOf[C]
res0: java.lang.Class[C] = class C
scala> c.getClass
res1: java.lang.Class[_] = class C
是为什么无法正常工作:
val xClass: Class[X] = new X().getClass //it returns Class[_], nor Class[X]
val integerClass: Class[Integer] = new Integer(5).getClass //similar error
有关 getClass 的返回类型的机票code> 。
( James Moore 报告票是现在,即2011年11月,两年后,固定。
在2.9.1中, getClass
现在有:
(James Moore reports that the ticket is "now", ie Nov. 2011, two years later, fixed.
In 2.9.1, getClass
now does:
scala> "foo".getClass
res0: java.lang.Class[_ <: java.lang.String] = class java.lang.String
)
回到2009年:
如果Scala将getClass一个java.lang.Class [T] forSome {val T:C}其中C类似于调用getClass的表达式的静态类型的擦除
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
它会让我做一些类似下面的事情,我想内省一个类,但不应该需要一个类实例。
我也想限制类的类型,我想introspect,所以我使用类[_< ;: Foo]。但这阻止我通过使用Foo.getClass()没有传递一个Foo类。
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:
Note: regarding getClass
, a possible workaround would be:
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
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