Behavioral_Chain Of Responsibility

✅

• use the handler chain to handle request

• client will only make request to the Interface Handler
• inside the chain of concrete handlers, it will handle the request
• 👍🏻 client will not know the logic behind request

• even if logic changes, client does not have to be modified

• 👍🏻 client(the class that makes the request) and the request handler is decoupled

• the handler will
• 1️⃣ handle the request

• 2️⃣ pass it to the next handler in the chain

• 👎🏻 difficult to debug, we do not know in which handler the error is occuring

👀 Chain of Responsibility

• abstract class Request Handler
• class PrintRequestHandler, LoggingRequestHandler, AuthRequestHandler extends Request Handler
• 🙋🏻‍♀️ Client: I want my request to be 1️⃣ check authentication, if ok 2️⃣ then log 3️⃣ then print

//step 1. Abstract class Request Handler
public abstract class RequestHandler {
    private RequestHandler nextHandler; //create chain of handlers

    public RequestHandler(RequestHandler nextHandler) {
        this.nextHandler = nextHandler;
    }

    public void handle(Request request){
        if(nextHandler != null){ //if next handler exsits
            nextHandler.handle(request); //call next handler
        }
    }
}

//step 2. create all the concrete handlers
public class AuthRequestHandler extends RequestHandler{
    public AuthRequestHandler(RequestHandler nextHandler) {
        super(nextHandler);
    }

    @Override
    public void handle(Request request) {
        System.out.println("Check authentication");
        super.handle(request);
    }
}

public class LoggingRequestHandler extends RequestHandler{
    //handle logic
}

public class PrintRequestHandler extends RequestHandler{
  //handle logic
}

• client code

public class Client {
    private RequestHandler requestHandler;

    public Client(RequestHandler requestHandler) {
        this.requestHandler = requestHandler;
    }

    public void doWork(){
        Request request = new Request("Hello request");
        requestHandler.handle(request); //client does not know what the handler will do
    }

    public static void main(String[] args) {
        //create chain of handlers
        RequestHandler chain = new AuthRequestHandler(new LoggingRequestHandler(new PrintRequestHandler(null)));
        Client client = new Client(chain);
        client.doWork();
    }
}

• result

Check authentication
Logging
Hello request

AuthRequestHandler 👉🏻 LoggingRequestHandler 👉🏻 PrintRequestHandler

👀 Simple example

• logger class

//step 1. Logger class(handler)
abstract class Logger {
    protected int level;
    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    public void logMessage(int level, String message){
    }

    protected abstract void write(String message);
}

//step 2. concrete handlers
class ConsoleLogger extends Logger {
}
class ErrorLogger extends Logger {
}

//step 3. create the chain
public class Main {
    public static void main(String[] args) {

        Logger errorLogger = new ErrorLogger(Logger.ERROR);
        Logger consoleLogger = new ConsoleLogger(Logger.INFO);

        errorLogger.setNextLogger(consoleLogger);

        errorLogger.logMessage(Logger.INFO, "This is an info message.");
        errorLogger.logMessage(Logger.ERROR, "This is an error message.");
    }
}

• client sends reqeust to errorLogger
• errorLogger calls consoleLogger

🛠️

• 👀 servlet filter
• @WebFilter를 추가하여 /hello controller에 들어오기 전에 미리 servelet component scan filter를 거치세요

// "/hello" 전에 이 WebFilter를 거쳐라
@WebFilter(urlPatterns = "/hello")
public class MyFilter implements Filter{
  @Override
  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException{
    chain.doFilter(request, response);
  }
}
//controller code
@RestController
public class HelloController{
  @GetMapping("/hello")
  public String hello(){
    return "hello";
  }
}

• 👀 Security config
• create filter chain
• to http.autorizeRequest().anyRequest().permitAll().addFilter()...

✅

1️⃣

2️⃣

3️⃣

4️⃣

• 1️⃣
• 2️⃣
• 3️⃣
• 4️⃣ 👍🏻 👎🏻

⭐️⭐️⭐️ EXAM ⭐️⭐️⭐️
❓
👉🏻