case类private构造函数 - 需要readResolve实现 [英] case class private constructor - need for readResolve implementation

问题描述

我只是googling来找出如何创建一个case类与私人构造函数。以下是

I was just googling to find out how to create a case class with private constructor. Below is the correct way for doing this as described in

如何覆写在案例类别随播广告中套用

object A {
  def apply(s: String, i: Int): A =
    new A(s.toUpperCase, i) {} //abstract class implementation intentionally empty
}
abstract case class A private[A] (s: String, i: Int) {
  private def readResolve(): Object = //to ensure validation and possible singleton-ness, must override readResolve to use explicit companion object apply method
    A.apply(s, i)
  def copy(s: String = s, i: Int = i): A =
    A.apply(s, i)
}

下面是我的理解： -

Below is my understanding so far :-

如果我们声明一个case类抽象，那么编译器不会生成copy和apply方法的实现。

If we declare a case class abstract, then implementation for copy and apply method will not be generated by the compiler.

问题，我正在努力： -

Below is the question, that I am struggling with :-

为什么需要提供readResolve的实现？

Why it is required to provide implementation of readResolve ?

推荐答案

readResolve实现是通过编辑类的序列化副本来防止创建案例类的无效实例。

The readResolve implementation is there to prevent the creation of invalid instances of the case class by editing serialised copies of the class.

这是因为case类扩展了 Serializable ，因此可能最终得到序列化并写入文件（或DB，或任何位置）。此时，可以编辑文件/ DB /中的序列化副本以创建无效值（例如，使 s 小写）。在反序列化时，'live'实例将无效，除非重写序列化过程中使用的 readResolve 方法以防止这种情况。

It comes about because case classes extend Serializable, and so may end up getting serialised and written out to file (or DB, or wherever). At this point the serialised copy in the file/DB/wherever could be edited to create an invalid value (eg. making s lower case). On deserialising back, the 'live' instance will then be invalid, unless the readResolve method that is used in the deserialisation process is overriden to prevent this.

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