18. Class 클래스

1. Class 클래스
    1) 자바의 모든 클래스와 인터페이스는 컴파일 후 .class 파일이 생성됨
    2) Class 클래스는 컴파일된 class 파일을 로드하여 객체를 동적 로드하고, 정보를 가져오는 메서드가 제공됨
    3) Class.forName("클래스 이름") 메서드로 클래스를 동적으로 로드 함
        - 실행(runtime)중에 데이터 타입을 binding하는 방법
        - 동적 로딩 시 오류가 발생하면 프로그램의 심각한 장애 발생가능 (컴파일 시에 타입이 정해지지 않으므로)
    4) reflection 프로그래밍
        - Class 클래스를 사용하여 클래스의 정보(생성자, 변수, 메서드) 등을 알 수 있고 인스턴스를 생성하고, 메서드를 호출하는 방식의 프로그래밍

 

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

public class ReflectionTest {

	public static void main(String[] args) throws ClassNotFoundException {
		
		Class clazz = Class.forName("java.lang.String");
		
		Constructor[] constructors = clazz.getConstructors();
		for(Constructor constructor : constructors) {
			System.out.println(constructor);
		}
		
		System.out.println();
		
		Method[] methods = clazz.getMethods();
		for(Method method : methods) {
			System.out.println(method);
		}
	}
}

---

public class Person {
	
	private String name;
	private int age;
	
	public Person() {}	// 기본 생성자

	public Person(String name) {
		// super();
		this.name = name;
	}

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

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}
	
	@Override
	public String toString() {
		
		return name;
	}
	
}

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

public class ClassTest {
	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
		// 1. new 연산자 class 생성
		Person person = new Person("Park");
		System.out.println(person);		// Park
		System.out.println();
		
		// 2. forName()
		Class clazz = Class.forName("kr.co.ezenac.classs.Person");
		Person person2 = (Person)clazz.newInstance();
		person2.setName("Lee");
		System.out.println(person2);	// Lee
		
		// 3. 
		Class clazz2 = person.getClass();
		Person person3 = (Person)clazz2.newInstance();
		System.out.println(person3);	// null
		
		Class[] parameterTypes = {String.class};
		Constructor constructor = clazz2.getConstructor(parameterTypes);
		
		Object[] arguments = {"Kim"};
		Person person4 = (Person)constructor.newInstance(arguments);
		System.out.println(person4);	// kim
	}
}