What is the Reflection API in Java and what can you do with it? Java में Reflection API क्या है और इससे क्या कर सकते हैं?

Reflection lets a program inspect and manipulate classes, methods, fields, and constructors at RUNTIME, even without knowing their names at compile time - the foundation of many frameworks (Spring, Hibernate, JUnit).

import java.lang.reflect.*;

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    private void greet() {
        System.out.println('Hello, ' + name);
    }
}

public class ReflectionDemo {
    public static void main(String[] args) throws Exception {
        Class<?> clazz = Class.forName('Person');  // or Person.class

        // Inspecting fields
        Field[] fields = clazz.getDeclaredFields();
        for (Field field : fields) {
            System.out.println('Field: ' + field.getName() + ' - ' + field.getType());
        }

        // Inspecting methods
        Method[] methods = clazz.getDeclaredMethods();
        for (Method method : methods) {
            System.out.println('Method: ' + method.getName());
        }

        // Creating an instance dynamically
        Constructor<?> constructor = clazz.getConstructor(String.class, int.class);
        Object person = constructor.newInstance('John', 30);

        // Accessing a PRIVATE field via reflection
        Field nameField = clazz.getDeclaredField('name');
        nameField.setAccessible(true);  // bypasses access control
        String name = (String) nameField.get(person);
        System.out.println('Name: ' + name);  // John

        // Calling a PRIVATE method via reflection
        Method greetMethod = clazz.getDeclaredMethod('greet');
        greetMethod.setAccessible(true);
        greetMethod.invoke(person);  // Hello, John
    }
}

// Real-world uses: Spring's dependency injection, JUnit finding @Test methods,
// JSON libraries (Gson/Jackson) mapping fields, ORM frameworks like Hibernate