实例化内部类

Question

我有一个实用工具方法,当从中删除不相关的逻辑时,简化的方法将如下所示:

public static <A extends Foo> List<A> getFooList(Class<A> clazz) {
   List<A> returnValue = new ArrayList<A>();
   for(int i=0; i < 5; i++) {
        A object = clazz.newInstance();
        returnValue.add(object);
   }

   return returnValue;
}

问题是,如果clazz是一个内部类,如果是Foo.Bar.class,那么该newInstance()方法即使Bar是公开的也不会起作用,因为它会抛出一个java.lang.InstantiationException.

有没有办法动态实例化内部类？

Answer 1

如果它真的是一个内部类而不是嵌套(静态)类,则有一个隐式构造函数参数,它是对外部类实例的引用.你不能Class.newInstance在那个阶段使用- 你必须得到适当的构造函数.这是一个例子:

import java.lang.reflect.*;

class Test
{
    public static void main(String[] args) throws Exception
    {
        Class<Outer.Inner> clazz = Outer.Inner.class;

        Constructor<Outer.Inner> ctor = clazz.getConstructor(Outer.class);

        Outer outer = new Outer();
        Outer.Inner instance = ctor.newInstance(outer);
    }
}

class Outer
{
    class Inner
    {
        // getConstructor only returns a public constructor. If you need
        // non-public ones, use getDeclaredConstructors
        public Inner() {}
    }
}

Answer 2

更通用的东西:

    public static <T> T createInstance(final Class<T> clazz) throws SecurityException, NoSuchMethodException,
            IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException {

            T instanceToReturn = null;
            Class< ? > enclosingClass = clazz.getEnclosingClass();

            if (enclosingClass != null) {
                Object instanceOfEnclosingClass = createInstance(enclosingClass);

                Constructor<T> ctor = clazz.getConstructor(enclosingClass);

                if (ctor != null) {
                    instanceToReturn = ctor.newInstance(instanceOfEnclosingClass);
                }
            } else {
                instanceToReturn = clazz.newInstance();
            }

            return instanceToReturn;
     }