Spring 配置 工程Web.xml中encodingFilter

在web.xml中的配置如下

 <filter>
    <filter-name>encodingFilter</filter-name>
    <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
    <init-param>
      <param-name>encoding</param-name>
      <param-value>GBK</param-value>
    </init-param>
  </filter>

上面的代碼片顯示Spring中的管理類是CharacterEncodingFilter下面我們進入到這個類的內部，首先看下繼承關係

public class CharacterEncodingFilter extends OncePerRequestFilter 
public abstract class OncePerRequestFilter extends GenericFilterBean
public abstract class GenericFilterBean implements
        Filter, BeanNameAware, EnvironmentAware, ServletContextAware, InitializingBean, DisposableBean

最終繼承的類是**javax.servlet.Filter**，這個類初始化調用的是`public final void init(FilterConfig filterConfig)函數， `參數filterConfig是工程啓動的時候傳遞的。當請求產生的時候調用類的`public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)`方法。這個方法是在**OncePerRequestFilter**類中重寫的，

public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {

        if (!(request instanceof HttpServletRequest) || !(response instanceof HttpServletResponse)) {
            throw new ServletException("OncePerRequestFilter just supports HTTP requests");
        }
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;

        String alreadyFilteredAttributeName = getAlreadyFilteredAttributeName();
        if (request.getAttribute(alreadyFilteredAttributeName) != null || shouldNotFilter(httpRequest)) {
            // Proceed without invoking this filter...
            filterChain.doFilter(request, response);
        }
        else {
            // Do invoke this filter...
            request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);
            try {
                doFilterInternal(httpRequest, httpResponse, filterChain);
            }
            finally {
                // Remove the "already filtered" request attribute for this request.
                request.removeAttribute(alreadyFilteredAttributeName);
            }
        }
    }

首先看到doFilterInternal方法，這個是用戶直接交互的類中需要實現的抽象方法，也就是可以在這裏定義自己想要的行爲。具體到代碼實在CharacterEncodingFilter中

protected void doFilterInternal(
            HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {

        if (this.encoding != null && (this.forceEncoding || request.getCharacterEncoding() == null)) {
            request.setCharacterEncoding(this.encoding);
            if (this.forceEncoding) {
                response.setCharacterEncoding(this.encoding);
            }
        }
        filterChain.doFilter(request, response);
    }

代碼寫的很清楚，只是根據配置參數信息將request和response設置相應的編碼格式。
這裏在重點說一下FilterChain這個類，俗名過濾鏈，這個類的作用就是對當前的請求執行一系列的過濾操作，是一個接口，這裏直接說實現類VirtualFilterChain這個類是作爲class CompositeFilter implements Filter的私有內部類實現的。類的裏面是以List<? extends Filter>的數據結構模擬鏈的順序執行。上代碼：

private static class VirtualFilterChain implements FilterChain {
        private final FilterChain originalChain;
        private final List<? extends Filter> additionalFilters;
        private int currentPosition = 0;

        private VirtualFilterChain(FilterChain chain, List<? extends Filter> additionalFilters) {
            this.originalChain = chain;
            this.additionalFilters = additionalFilters;
        }

        public void doFilter(final ServletRequest request, final ServletResponse response) throws IOException,
                ServletException {
            if (currentPosition == additionalFilters.size()) {
                originalChain.doFilter(request, response);
            } else {
                currentPosition++;
                Filter nextFilter = additionalFilters.get(currentPosition - 1);
                nextFilter.doFilter(request, response, this);
            }
        }

    }