在Java中，当我不在内部类时如何访问外部类? [英] In Java, how do I access the outer class when I'm not in the inner class?

问题描述

如果我有一个内部类的实例，我怎样才能从不在内部类中的代码访问外部类?我知道在内部类中，我可以使用 Outer.this 来获取外部类，但是我找不到任何外部获取方式.

If I have an instance of an inner class, how can I access the outer class from code that is not in the inner class? I know that within the inner class, I can use Outer.this to get the outer class, but I can't find any external way of getting this.

例如:

public class Outer {
  public static void foo(Inner inner) {
    //Question: How could I write the following line without
    //  having to create the getOuter() method?
    System.out.println("The outer class is: " + inner.getOuter());
  }
  public class Inner {
    public Outer getOuter() { return Outer.this; }
  }
}

推荐答案

Outer$Inner 类的字节码将包含一个名为 this$0 的包范围字段输入 Outer.这就是非静态内部类在 Java 中的实现方式，因为在字节码级别没有内部类的概念.

The bytecode of the Outer$Inner class will contain a package-scoped field named this$0 of type Outer. That's how non-static inner classes are implemented in Java, because at bytecode level there is no concept of an inner class.

如果您真的愿意，您应该能够使用反射读取该字段.我从来没有必要这样做，所以你最好改变设计，这样就不需要了.

You should be able to read that field using reflection, if you really want to. I have never had any need to do it, so it would be best for you to change the design so that it's not needed.

以下是您的示例代码在使用反射时的样子.人，真丑.;)

Here is how your example code would look like when using reflection. Man, that's ugly. ;)

public class Outer {
    public static void foo(Inner inner) {
        try {
            Field this$0 = inner.getClass().getDeclaredField("this$0");
            Outer outer = (Outer) this$0.get(inner);
            System.out.println("The outer class is: " + outer);

        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

    public class Inner {
    }

    public void callFoo() {
        // The constructor of Inner must be called in 
        // non-static context, inside Outer.
        foo(new Inner()); 
    }

    public static void main(String[] args) {
        new Outer().callFoo();
    }
}

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