[Spring] Servlet 정리하고 넘어가자!

Servlet 정리하고 넘어가자!

스프링 부트를 쓰다보면 잘 모르고 넘어가는 Spring MVC Flow! 이번기회에 한번 정리해보려고 합니다~

먼저 용어부터 익히자

• JSP : HTML문서에서 내부적으로 Java문법을 사용할 수 있게 해주는 Java Server Page언어
• Servlet : client 의 요청에 대한 결과를 response해주는 역할을 하는 자바 프로그램
• Servlet Container : 생성한 Servlet들을 관리해주는 컨테이너, Servlet을 보고 행동하는 수행자
  ex) tomcat
  
• Dispatcher-servlet : Servlet Container에서 Http요청을 가장먼저 받고 처리하는 Front Controller , 공통된 작업을 처리해준 뒤에 적절한 세부 컨트롤러에 작업을 위임함

DispatcherServlet

등록방식

1. Java configuration

public class MyWebApplicationInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) {

        // Load Spring web application configuration
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        context.register(AppConfig.class);

        // Create and register the DispatcherServlet
        DispatcherServlet servlet = new DispatcherServlet(context);
        ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
        registration.setLoadOnStartup(1);
        registration.addMapping("/app/*");
    }
}

2. web.xml

<web-app>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/app-context.xml</param-value>
    </context-param>

    <servlet>
        <servlet-name>app</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value></param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>app</servlet-name>
        <url-pattern>/app/*</url-pattern>
    </servlet-mapping>

</web-app>

동작방식(Spring Mvc Flow)

• Controller로 어떻게 매핑할까?
  • RequestMappingHandlerMapping이라는 Handler Mapping 사용 → Controller에 @RequestMapping으로 사용가능 ( 우리가 사용하는 @GetMapping등은 다 포함하고 있는 것이다)

• clinet 요청으로 받아온 정보들을 Controller에 어떻게 적합하게 넘겨줄까?
  • RequestMappingHandlerAdapter 이라는 Handler Adapter 사용 → 위와 같이 @RequestMapping으로 사용가능

• View name을 Controller가 넘겨주면 Dispatcher Servlet에 내장되어있는 여러 ViewResolver가 적절한 view를 찾아서 연결해준다.

  • view Name반환해주면

    @GetMapping("/customers/{customerId}")
        public String findCustomer(@PathVariable("customerId") UUID customerId, Model model){ // Model로 
            Optional<Customer> customer = customerService.getACustomer(customerId);
            if(customer.isPresent()){
                model.addAttribute("customer", customer.get());
                return "views/customer-details"; // View Name
            } else {
                return "views/404";
            }
        }

  • configureViewResolvers에 여러가지 등록

    static class ServletConfig implements WebMvcConfigurer, ApplicationContextAware {
            ApplicationContext applicationContext;
    
            @Override
            public void configureViewResolvers(ViewResolverRegistry registry) {
                // jsp
                registry.jsp().viewNames("jsp/*");
    
                // thymeleaf
                SpringResourceTemplateResolver springResourceTemplateResolver = new SpringResourceTemplateResolver();
                springResourceTemplateResolver.setApplicationContext(applicationContext);
    
                springResourceTemplateResolver.setPrefix("/WEB-INF/");
                springResourceTemplateResolver.setSuffix(".html");
                SpringTemplateEngine springTemplateEngine = new SpringTemplateEngine();
                springTemplateEngine.setTemplateResolver(springResourceTemplateResolver);
    
                ThymeleafViewResolver thymeleafViewResolver = new ThymeleafViewResolver();
                thymeleafViewResolver.setTemplateEngine(springTemplateEngine);
                thymeleafViewResolver.setOrder(1);
                thymeleafViewResolver.setViewNames(new String[]{"views/*"}); // view하위에 모든 template는 thymeleaf사용
                registry.viewResolver(thymeleafViewResolver);
            }

Root Application Context & Servlet Application Context

• ServletConfig (각각의 Dispatcher Servlet마다 개별적으로)

// Controller만
    @EnableWebMvc // spring mvc가 필요한 bean들 자동등록
    @Configuration
    @ComponentScan(basePackages = "com.programmers.demo.customer",
        includeFilters = @ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, value = CustomerController.class),
        useDefaultFilters = false)
    static class ServletConfig implements WebMvcConfigurer, ApplicationContextAware { // WebMvcConfigurer: MVC에 대해 특정한 설정을 해주기 위해
       ...
		}

• RootConfig ( 전체 ServletContext의 Servlet들이 root로 갖는)

// Service, Repository만
    @Configuration
    @EnableTransactionManagement
    @ComponentScan(basePackages = "com.programmers.demo.customer",
        excludeFilters = @ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, value = CustomerController.class)
    )
    static class RootConfig {

@Override
    public void onStartup(ServletContext servletContext) {

        // root applicationContext
        logger.info("Starting Server...");
        AnnotationConfigWebApplicationContext rootApplicationContext = new AnnotationConfigWebApplicationContext();
        rootApplicationContext.register(RootConfig.class);
        ContextLoaderListener loaderListener = new ContextLoaderListener(
            rootApplicationContext);
        servletContext.addListener(loaderListener);

        // servlet applicationContext
        AnnotationConfigWebApplicationContext webApplicationContext = new AnnotationConfigWebApplicationContext();
        webApplicationContext.register(ServletConfig.class);

        DispatcherServlet dispatcherServlet = new DispatcherServlet(webApplicationContext);
        Dynamic servletRegistration = servletContext.addServlet("test", dispatcherServlet); // 서블릿추가
        servletRegistration.addMapping("/"); // 모든 요청이 dispatcherServlet이 하게된다
        servletRegistration.setLoadOnStartup(-1); //default = -1 기다렸다가
    }

Web Server & WAS (Web Application Server)

• Web Server : 정적(html, css..)인 요청 처리
  • Nginx, Apache
• WAS : 동적(DB..)인 요청처리, JSP, Servlet구동환경 제공
  • Tomcat, JBoss

→ 두 기능을 분리해서 WAS의 부담을 줄여준다 → 시용감이 좋아짐

전체적인 흐름

1. Client에게 요청받기
2. Web Server가 정적인 요청처리
3. WAS가 web.xml을 참고해 Servlet에 대한 thread생성
4. HttpServletRequest와 HttpServletResponse 객체를 생성해 Servlet에게 전달, Service()호출

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            lastModified = this.getLastModified(req);
            if (lastModified == -1L) {
                this.doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader("If-Modified-Since");
                } catch (IllegalArgumentException var9) {
                    ifModifiedSince = -1L;
                }

                if (ifModifiedSince < lastModified / 1000L * 1000L) {
                    this.maybeSetLastModified(resp, lastModified);
                    this.doGet(req, resp);
                } else {
                    resp.setStatus(304);
                }
            }
        } else if (method.equals("HEAD")) {
            lastModified = this.getLastModified(req);
            this.maybeSetLastModified(resp, lastModified);
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }

    }

1. 요청에 맞게 doGet(), doPost()호출
2. 완료된 동적 페이지를 Response객체에 담아 WAS에 전달
3. WAS가 Response를 HttpResponse형태로 바꿔 Web Server로 전달
4. 스레드 종료, HttpServletRequest와 HttpServletResponse 객체 제거