Filter

public interface Filter {

  /**
   * Called by the web container to indicate to a filter that it is being
   * placed into service. The servlet container calls the init method exactly
   * once after instantiating the filter. The init method must complete
   * successfully before the filter is asked to do any filtering work.
   * <p>
   * The web container cannot place the filter into service if the init method
   * either:
   * <ul>
   * <li>Throws a ServletException</li>
   * <li>Does not return within a time period defined by the web
   *     container</li>
   * </ul>
   * The default implementation is a NO-OP.
   *
   * @param filterConfig The configuration information associated with the
   *                     filter instance being initialised
   *
   * @throws ServletException if the initialisation fails
   */
  public default void init(FilterConfig filterConfig) throws ServletException {
  }

  /**
   * The <code>doFilter</code> method of the Filter is called by the container
   * each time a request/response pair is passed through the chain due to a
   * client request for a resource at the end of the chain. The FilterChain
   * passed in to this method allows the Filter to pass on the request and
   * response to the next entity in the chain.
   * <p>
   * A typical implementation of this method would follow the following
   * pattern:- <br>
   * 1. Examine the request<br>
   * 2. Optionally wrap the request object with a custom implementation to
   * filter content or headers for input filtering <br>
   * 3. Optionally wrap the response object with a custom implementation to
   * filter content or headers for output filtering <br>
   * 4. a) <strong>Either</strong> invoke the next entity in the chain using
   * the FilterChain object (<code>chain.doFilter()</code>), <br>
   * 4. b) <strong>or</strong> not pass on the request/response pair to the
   * next entity in the filter chain to block the request processing<br>
   * 5. Directly set headers on the response after invocation of the next
   * entity in the filter chain.
   *
   * @param request  The request to process
   * @param response The response associated with the request
   * @param chain    Provides access to the next filter in the chain for this
   *                 filter to pass the request and response to for further
   *                 processing
   *
   * @throws IOException if an I/O error occurs during this filter's
   *                     processing of the request
   * @throws ServletException if the processing fails for any other reason
   */
  public void doFilter(ServletRequest request, ServletResponse response,
                       FilterChain chain) throws IOException, ServletException;

  /**
   * Called by the web container to indicate to a filter that it is being
   * taken out of service. This method is only called once all threads within
   * the filter's doFilter method have exited or after a timeout period has
   * passed. After the web container calls this method, it will not call the
   * doFilter method again on this instance of the filter. <br>
   * <br>
   *
   * This method gives the filter an opportunity to clean up any resources
   * that are being held (for example, memory, file handles, threads) and make
   * sure that any persistent state is synchronized with the filter's current
   * state in memory.
   *
   * The default implementation is a NO-OP.
   */
  public default void destroy() {
  }
}

• 스프링의 독자적인 기능이 아닌 자바 서블릿에서 제공하는 기능이다
• 스프링 프레임워크에서 필터로 인증 등 다양한 작업을 하는데 사용된다

위 그림은 스프링 프레임워크에서 요청에 대한 라이프사이클을 나타낸 그림이다. 스프링 프레임워크로 들어온 요청이 DispatcherServlet에 의해 컨트롤러에 매필된다. 필터는 요청이 DispatcherServlet에 의해 다루어지기 전과 후에 동작한다.

flowchart LR

요청[요청] <--> 필터1[필터] <--> 필터2[필터] <--> 필터3[필터] <--> 자원[자원]

또한 Filter는 FilterChain을 통해 여러 필터가 연쇄적으로 동작하게 할 수 있다

Filter는 어디에 쓰일까

필터는 주로 요청에 대한 인증, 권한 체크 등을 하는데에 쓰인다

구체적으로 들어온 요청이 DispatcherServlet에 전달되기 전에 헤더를 검사해 인증 토큰이 있는지 없는지, 올바른지 올바르지 않은지 등을 검사할 수 있다.

Filter의 사용법

클래스를 만들어서 인터페이스를 구현해서 만들 수 있다

Filter는 메소드 세 가지를 가지고 있다

1. init() : 필터가 생성될 때 수행되는 메소드
2. doFilter() : Request, Response가 필터를 거칠 때 수행되는 메소드
3. destroy() : 필터가 소멸될 때 수행되는 메소드

위 세가지 메소드를 구현하면 필터가 된다

public class SampleFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        log.info("SampleFilter Generated");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        log.info("SampleFilter Start");
        filterChain.doFilter(servletRequest, servletResponse);
        log.info("SampleFilter Finish");
    }

    @Override
    public void destroy() {
        log.info("SampleFilter destroy.");
    }
}

위처럼 만들면 된다. 그리고 Filter가 여러 개 만들어질 수 있다고 했는데 이는 FilterRegistrationBean의 setOrder() 메소드로 결정할 수 있다

@Configuration
public class FilterConfig {
  @Bean
  public void runSomeFilterRegistration() {
    someFilterRegistration();
  }

  private FilterRegistrationBean<SampleFilter> someFilterRegistration() {
    FilterRegistrationBean<SomeFilter> registration = new FilterRegistrationBean<>();
    registration.setFilter(new SampleFilter());
    registration.addUrlPatterns("/sample/*"); // url 패턴 설정
    registration.addInitParameter("param1", "param_value1"); // 파라미터 설정
    registration.setName("sample-filter"); // 필터명 설정
    registration.setOrder(1); // 순서 설정

    return registration;
  }
}