如何在Java中使用反射实例化内部类? [英] How to instantiate an inner class with reflection in Java?

问题描述

我尝试实例化以下 Java 代码中定义的内部类:

I try to instantiate the inner class defined in the following Java code:

 public class Mother {
      public class Child {
          public void doStuff() {
              // ...
          }
      }
 }

当我尝试像这样获取 Child 的实例时

When I try to get an instance of Child like this

 Class<?> clazz= Class.forName("com.mycompany.Mother$Child");
 Child c = clazz.newInstance();

我收到此异常:

 java.lang.InstantiationException: com.mycompany.Mother$Child
    at java.lang.Class.newInstance0(Class.java:340)
    at java.lang.Class.newInstance(Class.java:308)
    ...

我错过了什么?

推荐答案

有一个额外的隐藏"参数，它是封闭类的实例.您需要使用 Class.getDeclaredConstructor 然后提供封闭类的实例作为参数.例如:

There's an extra "hidden" parameter, which is the instance of the enclosing class. You'll need to get at the constructor using Class.getDeclaredConstructor and then supply an instance of the enclosing class as an argument. For example:

// All exception handling omitted!
Class<?> enclosingClass = Class.forName("com.mycompany.Mother");
Object enclosingInstance = enclosingClass.newInstance();

Class<?> innerClass = Class.forName("com.mycompany.Mother$Child");
Constructor<?> ctor = innerClass.getDeclaredConstructor(enclosingClass);

Object innerInstance = ctor.newInstance(enclosingInstance);

或者，如果嵌套类实际上不需要引用封闭实例，请将其设为嵌套的 static 类:

Alternatively, if the nested class doesn't actually need to refer to an enclosing instance, make it a nested static class instead:

public class Mother {
     public static class Child {
          public void doStuff() {
              // ...
          }
     }
}

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