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>

上面的代码片显示Spring中的管理类是CharacterEncodingFilter下面我们进入到这个类的内部，首先看下继承关系

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

最终继承的类是**javax.servlet.Filter**，这个类初始化调用的是`public final void init(FilterConfig filterConfig)函数， `参数filterConfig是工程启动的时候传递的。当请求产生的时候调用类的`public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)`方法。这个方法是在**OncePerRequestFilter**类中重写的，

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，这个是用户直接交互的类中需要实现的抽象方法，也就是可以在这里定义自己想要的行为。具体到代码实在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);    }

代码写的很清楚，只是根据配置参数信息将request和response设置相应的编码格式。
这里在重点说一下FilterChain这个类，俗名过滤链，这个类的作用就是对当前的请求执行一系列的过滤操作，是一个接口，这里直接说实现类VirtualFilterChain这个类是作为class CompositeFilter implements Filter的私有内部类实现的。类的里面是以List<? extends Filter>的数据结构模拟链的顺序执行。上代码：

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);            }        }    }