Java Reflection从字段获取实例 [英] Java Reflection get the Instance from a Field

问题描述

是否可以从字段中获取实例?
这是示例代码:

is there any way to get the Instance from a Field?
Here's a sample code:

public class Apple {
    // ... a bunch of stuffs..
}

public class Person {
    @MyAnnotation(value=123)
    private Apple apple;
}

public class AppleList {
    public add(Apple apple) {
        //...
    }
}

public class Main {
    public static void main(String args[]) {
        Person person = new Person();
        Field field = person.getClass().getDeclaredField("apple");

        // Do some random stuffs with the annotation ...

        AppleList appleList = new AppleList();

        // Now I want to add the "apple" instance into appleList, which I think
        // that is inside of field.

        appleList.add( .. . // how do I add it here? is it possible?

        // I can't do .. .add( field );
        // nor .add( (Apple) field );
    }
}

我需要使用反射，因为我将其与注释一起使用.这只是一个样本"，实际上是通过从类中获取方法AppleList.add(Apple apple)并随后调用它来调用方法AppleList.add(Apple apple).

I need to use Reflection, because I'm using it with annotations. This is just a "sample", the method AppleList.add(Apple apple) is actually called by getting the method from the class, and then invoking it.

并这样做，例如:method.invoke( appleList, field );

原因:java.lang.IllegalArgumentException: argument type mismatch

* EDIT * 这对于正在寻找同一事物的人可能会有所帮助.

*EDIT* This might be helpful for someone who's looking for the same thing.

如果Person类具有两个或多个Apple变量:

if the class Person, had 2 or more Apple variables:

public class Person {
    private Apple appleOne;
    private Apple appleTwo;
    private Apple appleThree;
}

当我获得该字段时，例如:

when I get the Field, like:

Person person = new Person();
// populate person
Field field = person.getClass().getDeclaredField("appleTwo");
// and now I'm getting the instance...
Apple apple = (Apple) field.get( person );
// this will actually get me the instance "appleTwo"
// because of the field itself...

首先，通过单独查看以下行:(Apple) field.get( person );
让我认为它将去并获得一个与Apple类匹配的实例.
这就是为什么我想知道:它将返回哪个苹果公司?"

at the beginning, by looking at the line alone: (Apple) field.get( person );
made me think that it would go and get an Instance which matches Apple class.
that's why I wondered: "which Apple will it return?"

推荐答案

该字段本身不是苹果，而是一个字段.由于它是一个 instance 字段，因此需要一个声明类的实例才能获取值.您想要:

The field isn't an apple itself - it's just a field. As it's an instance field, you need an instance of the declaring class before you can get a value. You want:

Apple apple = (Apple) field.get(person);

当然，对于person实例，在字段中填充 后的

....

... after the apple field is populated for the instanced referred to be person, of course.

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