Reflection

Advanced ~25 min read

Reflection allows code to inspect other code (classes, interfaces, fields, and methods) at runtime, without knowing the names of the interfaces, fields, methods at compile time.

The `Class` Object

Every type in Java including primitive types has an associated Class object.

Class<?> c = Class.forName("java.util.ArrayList");
System.out.println(c.getName());

Inspecting Methods

Method[] methods = c.getMethods();
for (Method m : methods) {
    System.out.println(m.getName());
}

Accessing Private Fields (The Hacker Way)

Reflection can break encapsulation by accessing private fields.

Field f = myObj.getClass().getDeclaredField("privateField");
f.setAccessible(true); // Unlock it!
System.out.println(f.get(myObj));

Full Example

import java.lang.reflect.*;

class SecretClass {
    public String publicInfo = "I am public";
    private String secret = "I am PRIVATE stuff!";

private void spyMethod() {
        System.out.println("Spy method executed!");
    }
}

public class ReflectionExample {
    public static void main(String[] args) {
        try {
            SecretClass obj = new SecretClass();
            Class cls = obj.getClass();

System.out.println("Class Name: " + cls.getName());

// 1. Access Public Field
            System.out.println("Public: " + obj.publicInfo);

// 2. Access Private Field (Hack!)
            Field privateField = cls.getDeclaredField("secret");
            privateField.setAccessible(true); // THE KEY LINE
            System.out.println("Private (Hacked): " + privateField.get(obj));

// 3. Invoke Private Method
            Method privateMethod = cls.getDeclaredMethod("spyMethod");
            privateMethod.setAccessible(true);
            privateMethod.invoke(obj);

} catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output

Click Run to execute your code

Summary

Reflection is powerful but slow. Avoid it in performance-critical loops.
It can break security and design principles (like private encapsulation).
Extensively used by Frameworks (Spring, Hibernate, JUnit).

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The Class Object

Inspecting Methods

Accessing Private Fields (The Hacker Way)

Full Example

Summary

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The `Class` Object