[Visitor]방문자 패턴

Visitor Pattern

기존 코드를 변경하지 않고 새로운 기능을 추가하는 방법
더블 디스패치(Double dispatch)를 활용할 수 있다.

 

BEFORE

- 예시를 위해 지저분하지만 아래와 같이 코드를 써본다. 

Client

public class Client {

    public static void main(String[] args) {
        Shape rectangle = new Rectangle();
        Device device = new Phone();
        rectangle.printTo(device);
    }
}

Shape - Rectangle, Triangle

public interface Shape {

    void printTo(Device device);

}



public class Rectangle implements Shape {

    @Override
    public void printTo(Device device) {
        if (device instanceof Phone) {
            System.out.println("print Rectangle to phone");
        } else if (device instanceof Watch) {
            System.out.println("print Rectangle to watch");
        }
    }
}



public class Triangle implements Shape {

    @Override
    public void printTo(Device device) {
        if (device instanceof Phone) {
            System.out.println("print Triangle to Phone");
        } else if (device instanceof Watch) {
            System.out.println("print Triangle to Watch");
        }
    }

}

Device - Phone, Watch

public interface Device {
}



public class Phone implements Device{
}


public class Watch implements Device{
}

 

 

AFTER

Client

public class Client {

    public static void main(String[] args) {
        Shape rectangle = new Rectangle();
        Device device = new Pad();
        rectangle.accept(device);
    }
}

Shape - Rectangle, Triangle, Circle

public interface Shape {

    void accept(Device device);

}



public class Rectangle implements Shape {


    @Override
    public void accept(Device device) {
        device.print(this);
    }
}



public class Triangle implements Shape {


    @Override
    public void accept(Device device) {
        device.print(this);
    }
}



public class Circle implements Shape {

    @Override
    public void accept(Device device) {
        device.print(this);
    }
}

Device - Phone, Watch, Pad

public interface Device {
    void print(Circle circle);

    void print(Rectangle rectangle);

    void print(Triangle triangle);
}



public class Phone implements Device {

    @Override
    public void print(Circle circle) {
        System.out.println("Print Circle to Phone");
    }

    @Override
    public void print(Rectangle rectangle) {
        System.out.println("Print Rectangle to Phone");

    }

    @Override
    public void print(Triangle triangle) {
        System.out.println("Print Triangle to Phone");
    }
}




public class Watch implements Device {
    @Override
    public void print(Circle circle) {
        System.out.println("Print Circle to Watch");
    }

    @Override
    public void print(Rectangle rectangle) {
        System.out.println("Print Rectangle to Watch");
    }

    @Override
    public void print(Triangle triangle) {
        System.out.println("Print Triangle to Watch");
    }
}




public class Pad implements Device {
    @Override
    public void print(Circle circle) {
        System.out.println("Print Circle to Pad");
    }

    @Override
    public void print(Rectangle rectangle) {
        System.out.println("Print Rectangle to Pad");
    }

    @Override
    public void print(Triangle triangle) {
        System.out.println("Print Triangle to Pad");
    }
}

 

 

[패턴 복습]

장점	단점
- 기존 코드를 변경하지 않고 새로운 코드를 추가할 수 있다. - 추가 기능을 한 곳에 모아둘 수 있다.	- 복잡하다. - 새로운 Element를 추가하거나 제거할 때 모든 Visitor 코드를 변경해야 한다.

 

[실무에서 쓰이는 경우]

- 자바
    - FileVisitor, SimpleFileVisitor
    - AnnotationValueVisitor
    - ElementVisitor
- 스프링
    - BeanDefinitionVisitor