是否可以使用 Java 反射从另一个类实例化私有(private)内部类。例如，如果我采用此代码

public class Main {
    public static void main(String[] args) {}
}

class OtherClass {
    private class Test {}
}

是否可以从 main 类中的 main 方法实例化并获得对 Test 的访问权限。

最佳答案

是的，您可以使用 Java 反射实例化私有(private)内部类。为此，您需要有一个外部类实例并调用内部类构造函数，该构造函数将在其第一个参数中使用外部类实例。

class OuterClass {
    private class InnerClass {
        {
            //this block is just to confirm that the inner object was created
            //it will be added to every constructor of this class
            System.out.println("inner object created");
        }
    }
}

当我们不知道私有(private)内部类的名称并且我们假设它有无参数构造函数时:

class Main {

    //no comment version
    public static Object giveMeInnerInstance() throws Exception{
        OuterClass outerObject = new OuterClass();
        Class<?> innerClass = OuterClass.class.getDeclaredClasses()[0];
        Constructor<?> constructor = innerClass.getDeclaredConstructors()[0];
        constructor.setAccessible(true);
        return constructor.newInstance(outerObject);
    }

    //commented version
    public static void main(String[] args) throws Exception {
        //we need an outer class object to use the inner object constructor
        //(the inner class object needs to know about its parent object)
        OuterClass outerObject = new OuterClass();

        //let's get the inner class 
        //(we know that the outer class has only one inner class, so we can use index 0)
        Class<?> innerClass = OuterClass.class.getDeclaredClasses()[0];
        //or if we know name of inner class we can use 
        //Class<?> innerClass = Class.forName("full.package.name.OuterClass$InnerClass")

        //since constructor so we could use it to pass instance of outer class and change 
        //its accessibility. We can use this code to get default constructor of InnerClass 
        //since we know that this is the only constructor here
        Constructor<?> constructor = innerClass.getDeclaredConstructors()[0];
        //we could also use 
        //Constructor<?> constructor = innerClass.getDeclaredConstructor(OuterClass.class);

        //the default constructor of the private class has same visibility that class has
        //so it is also private, so to be able to use it we need to make it accessible
        constructor.setAccessible(true);

        //now we are ready to create inner class instance
        Object innerObject = constructor.newInstance(outerObject);
    }
}

现在如果我们有类似的信息，我们可以让这段代码更清晰

• 内部类的名称，
• 构造函数参数

因此，我们可以通过使用内部类的名称来获取选定的内部类，而不是检查内部类列表并选择第一个

Class<?> inner = Class.forName("our.pack.age.OuterClass$InnerClass")
//                                                     ^^^^^^^^^^^

同样，我们可以通过调用 getDeclaredConstructor(outerType,rest,of,parameter,types) 来选择我们想要使用的构造函数，所以如果我们的内部类看起来像

class OuterClass {
    private class InnerClass {

        private int x;

        public InnerClass(int x) {
            this.x = x;
            System.out.println("inner object created");
        }

    }
}

我们的代码可以是

class ReflectionDemo {

    //no comment version
    public static Object giveMeInnerInstance() throws Exception{
        OuterClass outerObject = new OuterClass();
        Class<?> innerClass = Class.forName("com.stackoverflow.q14112166.OuterClass$InnerClass");
        Constructor<?> constructor = innerClass.getDeclaredConstructor(OuterClass.class, int.class);
        constructor.setAccessible(true);
        return constructor.newInstance(outerObject,42);
    }

    public static Object getFieldValue(Object obj, String fieldName) throws Exception{
        Class<?> clazz = obj.getClass();
        Field field = clazz.getDeclaredField(fieldName);
        field.setAccessible(true);
        return field.get(obj);
    }

    //lets test our code
    public static void main(String[] args) throws Exception {
        Object innerClassObject = giveMeInnerInstance();
        System.out.println(getFieldValue(innerClassObject, "x"));           
    }
}

输出:

inner object created
42

关于java - 用java反射实例化私有(private)内部类，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/14112166/