不能使用 getDeclaredFields() 来检索 Scala 类的字段 [英] Cannot use getDeclaredFields() to retrieve fields of a Scala class

问题描述

我正在尝试将 Java 库 (JOhm) 与 Scala 一起使用，并注意到当 lib 尝试使用诸如 model.getClass().getDeclaredFields().

I'm trying to use a Java library (JOhm) with Scala and noticed it fails when the lib tries to read the fields of my Scala classes with something like model.getClass().getDeclaredFields().

然后我决定尝试用 Scala 解释器中的简单示例做同样的事情:

Then I decided to try to do the same with simple examples in the Scala interpreter:

scala> import java.lang.reflect.Field;
import java.lang.reflect.Field

scala> class myClass(attribute1: String, attribute2: String, attribute3: String)
defined class myClass

scala> val myInstance = new myClass("value1", "value2", "value3")
myInstance: myClass = myClass@7055c39a

scala> myInstance.getClass().getDeclaredFields()
res0: Array[java.lang.reflect.Field] = Array()

确实，我们根本没有任何字段.

Indeed, we get no field at all.

现在，如果我试试这个会怎样:

Now, what if I try this:

scala> class myClass2(attribute1: String, attribute2: String, attribute3: String) { override def toString = this.attribute1 }
defined class myClass2

scala> val myInstance2 = new myClass2("value1", "value2", "value3")
myInstance2: myClass2 = value1

scala> myInstance2.getClass().getDeclaredFields()
res1: Array[java.lang.reflect.Field] = Array(private final java.lang.String myClass2.attribute1)

因此，如果使用类方法之一中的字段之一，则可以通过 getDeclaredFields() 找到它.我在这里错过了什么?

So if use one of the fields in one of the class' methods, it is found by getDeclaredFields(). What am I missing here?

推荐答案

您缺少的是构造函数参数不会自动提升到字段.

相反，它们只有在被使用时才会被提升.你使用了 attribute1 所以它变成了一个字段；你没有使用其他人，所以他们没有.

Rather, they are promoted only if they are used. You used attribute1 so it was turned into a field; you didn't use the others so they were not.

如果你将它们声明为 val 或 var，或者类是一个 case 类，它们也会被提升为字段(因为它们实际上会生成访问器方法，因此被使用).

If you declare them as val or var, or the class is a case class, they will also be promoted to fields (since they will actually have accessor methods generated, and thus are used).

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