Filter 并不是一个标准的Servlet ，它不能处理用户请求，也不能对客户端生成响应。
主要用于对HttpServletRequest 进行预处理，也可以对HttpServletResponse 进行后处理，是个典型的处理链。
Filter 有如下几个用处。
·在HttpServletRequest 到达Servlet 之前，拦截客户的HttpServletRequest 。
·根据需要检查HttpServletRequest ，也可以修改HttpServletRequest 头和数据。
·在HttpServletResponse 到达客户端之前，拦截HttpServletResponse 。
·根据需要检查HttpServletResponse ，也可以修改HttpServletResponse 头和数据。
Filter 有如下几个种类。
·用户授权的Filter: Filter 负责检查用户请求，根据请求过滤用户非法请求。
.日志Filter: 详细记录某些特殊的用户请求。
·负责解码的Filter: 包括对非标准编码的请求解码。
.能改变XML内容的XSLTFilter 等。
一个Filter 可负责拦截多个请求或响应:一个请求或响应也可被多个请求拦截。
创建一个Filter 只需两个步骤:
(I)创建Filter 处理类:
(2) 在web.xml 文件中配置Filter 。
创建Filter 必须实现javax.servle t. Filter 接口，在该接口中定义了三个方法。
void init(FilterConfig config): 用于完成Filter 的初始化。
void destroy(): 用于Filter 销毁前，完成某些资源的回收。
void doFilter(ServletRequest request, ServletResponse response,FilterChain chain): 实现过滤功能，该方法就是对每个请求及响应增加的额外处理。
public class SetCharacterEncodingFilter implements Filter ...{
// ----------------------------------------------------- Instance Variables
/** *//** * The default character encoding to set for requests that pass through * this filter.
*/ protected String encoding = "GBK"; /** *//** * The filter configuration object we are associated with. If this value * is null, this filter instance is not currently configured. */ protected FilterConfig filterConfig = null; /** *//** * Should a character encoding specified by the client be ignored? */ protected boolean ignore = true; // --------------------------------------------------------- Public Methods /** *//** * Take this filter out of service. */ public void destroy() ...{ this.encoding = null; this.filterConfig = null; } /** *//** * Select and set (if specified) the character encoding to be used to * interpret request parameters for this request. * * @param request The servlet request we are processing * @param result The servlet response we are creating * @param chain The filter chain we are processing * * @exception IOException if an input/output error occurs * @exception ServletException if a servlet error occurs */ public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException ...{ // Conditionally select and set the character encoding to be used if (ignore || (request.getCharacterEncoding() == null)) ...{ String encoding = selectEncoding(request); if (encoding != null) request.setCharacterEncoding(encoding); } chain.doFilter(request, response); } /** *//** * Place this filter into service. * * @param filterConfig The filter configuration object */ public void init(FilterConfig filterConfig) throws ServletException ...{ this.filterConfig = filterConfig; this.encoding = filterConfig.getInitParameter("encoding"); //System.out.println("encodeing=============" + this.encoding); String value = filterConfig.getInitParameter("ignore"); if (value == null) this.ignore = true; else if (value.equalsIgnoreCase("true")) this.ignore = true; else if (value.equalsIgnoreCase("yes")) this.ignore = true; else this.ignore = false; } // ------------------------------------------------------ Protected Methods /** *//** * Select an appropriate character encoding to be used, based on the * characteristics of the current request and/or filter initialization * parameters. If no character encoding should be set, return * <code>null</code>. * <p> * The default implementation unconditionally returns the value configured * by the <strong>encoding</strong> initialization parameter for this * filter. * * @param request The servlet request we are processing */ protected String selectEncoding(ServletRequest request) ...{ return (this.encoding); }
}//EOCweb.xml
<filter> <filter-name>Set Character Encoding</filter-name><!--filter名字--> <filter-class>com.infoer.util.SetCharacterEncodingFilter</filter-class><!--filter实现类--> <init-param> <param-name>encoding</param-name> <param-value>GBK</param-value> </init-param> </filter><!--拦截设置--> <filter-mapping> <filter-name>Set Character Encoding</filter-name> <url-pattern>*.jsp</url-pattern> </filter-mapping> <filter-mapping> <filter-name>Set Character Encoding</filter-name> <url-pattern>*.do</url-pattern> </filter-mapping>