Java ReflectionJava 리플렉션

// 3가지 방법
Class<?> c1 = String.class;
Class<?> c2 = "hello".getClass();
Class<?> c3 = Class.forName("java.lang.String");

// 기본 정보
System.out.println(c1.getName());        // java.lang.String
System.out.println(c1.getSimpleName());  // String
System.out.println(c1.getSuperclass());  // class java.lang.Object
System.out.println(Arrays.toString(c1.getInterfaces()));

class Person {
    private String name;
    public int age;
}

Class<?> cls = Person.class;
Field nameField = cls.getDeclaredField("name");
nameField.setAccessible(true);  // private 접근 허용

Person p = new Person();
nameField.set(p, "Alice");
System.out.println(nameField.get(p));  // Alice

Method method = String.class.getMethod("substring", int.class, int.class);
String result = (String) method.invoke("Hello World", 0, 5);
System.out.println(result);  // Hello

// 정적 메서드
Method parseInt = Integer.class.getMethod("parseInt", String.class);
int num = (int) parseInt.invoke(null, "42");

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@interface Validate { int min() default 0; int max() default 100; }

class Form {
    @Validate(min=1, max=50)
    String username;
}

// 런타임 어노테이션 읽기
for (Field f : Form.class.getDeclaredFields()) {
    Validate v = f.getAnnotation(Validate.class);
    if (v != null) {
        System.out.printf("%s: min=%d, max=%d%n", f.getName(), v.min(), v.max());
    }
}