如何从main()方法访问静态内部类中私有类的私有方法

Alright, let's tackle this problem head-on. Since both the StarLord private inner class (inside Avengers) and its getGamora() method are marked as private, standard Java syntax won't let us access them directly. We need to use reflection to bypass these access modifiers. Here's a working implementation:

Full Working Code #

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

public class Thanos {
    public static void main(String[] args) throws Exception {
        // 1. Instantiate the static inner class Avengers
        Avengers avngrs = new Thanos.Avengers();
        
        // 2. Retrieve the private inner class StarLord from Avengers
        Class<?> starLordClass = Avengers.class.getDeclaredClass("Thanos.Avengers.StarLord");
        
        // 3. Get the constructor of StarLord (requires an Avengers instance for non-static inner class)
        Constructor<?> starLordConstructor = starLordClass.getDeclaredConstructor(Avengers.class);
        starLordConstructor.setAccessible(true); // Allow access to private constructor
        Object starLordInstance = starLordConstructor.newInstance(avngrs);
        
        // 4. Fetch the private getGamora() method
        Method getGamoraMethod = starLordClass.getDeclaredMethod("getGamora");
        getGamoraMethod.setAccessible(true); // Allow access to private method
        
        // 5. Invoke the method to print the desired output
        getGamoraMethod.invoke(starLordInstance);
    }

    static class Avengers {
        private class StarLord {
            private void getGamora() {
                System.out.println("Access me to get a soul for a soul.");
            }
        }
    }
}

Breakdown of Key Steps #

• Step 1: We start by creating an instance of Avengers (the static inner class). Since Avengers is static, we don't need a Thanos instance to instantiate it.
• Step 2: Using getDeclaredClass(), we access the private StarLord inner class. This method lets us retrieve classes that aren't publicly visible, unlike getClasses() which only returns public inner classes.
• Step 3: Non-static inner classes (like StarLord) have an implicit constructor parameter that takes their outer class instance (Avengers). We fetch this constructor, enable access to it with setAccessible(true), then create a StarLord instance by passing our Avengers object.
• Step 4: We get the private getGamora() method using getDeclaredMethod(), then call setAccessible(true) to override the private access restriction.
• Step 5: Finally, we invoke the method on our StarLord instance, which outputs the required message.

Note on Reflection #

Reflection is a powerful tool, but it should be used sparingly. It bypasses Java's built-in access control and can make code harder to maintain, debug, and secure. Use it only when you have no other alternative (like in this specific scenario).

内容的提问来源于stack exchange，提问作者kapishreshth