使用Java反射实例化私有内部类

Question

是否可以使用Java反射从另一个类实例化私有内部类。例如，如果我使用此代码

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

class OtherClass {
    private class Test {}
}

是否可以实例化并从类main中的main方法获得对Test的访问权。

Answer 1

[使用反射时，您会发现该内部类的构造函数将外部类的实例作为附加参数（总是第一个）。

请参阅以下问题以获取相关信息：

• Instantiating inner class

• How can I instantiate a member class through reflection on Android

• In Java, how do I access the outer class when I'm not in the inner class?

示例：

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class OuterClass {

    private class InnerClass {

    }

    public OuterClass() {
        super();
    }

    public static void main(String[] args) {
        // instantiate outer class
        OuterClass outer = new OuterClass();

        // List all available constructors.
        // We must use the method getDeclaredConstructors() instead
        // of getConstructors() to get also private constructors.
        for (Constructor<?> ctor : OuterClass.InnerClass.class
                .getDeclaredConstructors()) {
            System.out.println(ctor);
        }

        try {
            // Try to get the constructor with the expected signature.
            Constructor<InnerClass> ctor = OuterClass.InnerClass.class
                    .getDeclaredConstructor(OuterClass.class);
            // This forces the security manager to allow a call
            ctor.setAccessible(true);

            // the call
            try {
                OuterClass.InnerClass inner = ctor.newInstance(outer);
                System.out.println(inner);
            } catch (InstantiationException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IllegalArgumentException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        } catch (NoSuchMethodException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (SecurityException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

Answer 2

是的，您可以使用Java反射实例化一个私有内部类。为此，您需要有一个外部类的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");
        }
    }
}

当我们不知道私有内部类的名称，并且假定它没有参数的构造函数时：

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，我们可以使此代码更清晰

• 内部类名称，
• 构造函数参数

因此，不用检查内部类的列表并选择第一个类，我们可以使用]通过其名称来选择内部类。

Class<?> inner = Class.forName("our.pack.age.OuterClass$InnerClass")
//                                                     ^^^^^^^^^^^
类似地，我们可以通过调用getDeclaredConstructor(outerType,rest,of,parameter,types)来选择要使用的构造函数，以便内部类看起来像]
class OuterClass {
    private class InnerClass {

        private int x;

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

    }
}
我们的代码可能是
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