Java中的Filter详解 --基础篇

Filter是Servlet规范中的一个高级特性，和Servlet不同的是，他们不处理客户端请求，只用于对request，response进行修改；

如果要自己实现一个自定义的Filter必须实现javax.servlet.Filter接口，接口中有三个方法：

package javax.servlet;

import java.io.IOException;

/**
 * A filter is an object that performs filtering tasks on either the
 * request to a resource (a servlet or static content), or on the response
 * from a resource, or both.
 * @since Servlet 2.3
 */

public interface Filter {

    /**
     * Called by the web container to indicate to a filter that it is
     * being placed into service.
     */
    public 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.
     */
    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.
     */
    public void destroy();

}

三个方法反应了Filter的生命周期，其中init(),destory()方法只在Web程序加载或者卸载的时候调用，doFilter()方法每次请求都会调用。

另外，Filter在web.xml中的简单配置如下：

<!-- Char encoding -->
 <filter >
       < filter-name> characterFilter </filter-name >
       < filter-class> space.ifei.wxchat.sys.CharacterFilter </filter-class >
<!--    <async-supported>true</async-supported> -->
<!--    <init- param> -->
<!--         <param-name>XXX</param-name> -->
<!--         <param-value>QQQ</param-value> -->
<!--    </init- param> -->
  </filter >
  <filter-mapping >
  <filter-name> characterFilter </filter-name > <!-- 同一个Filter，Filter的名字必须一致 -->
   <url-pattern> /* </url-pattern > <!-- Filter生效的 url规则，可以使用通配符 -->
<!--    <dispatcher>REQUEST</dispatcher> -->
<!--    <dispatcher>FORWARD</dispatcher> -->
  </filter-mapping >

其中：

“async-supported ”设置Servlet是否开启异步支持，默认不开启，详细见：http://www.ibm.com/developerworks/cn/java/j-lo-servlet30/

“init- param ”设置自定义Filter里面属性的值；

“dispatcher ”设置到达servlet的方式，方式有4种：REQUEST,FORWARD,INCLUDE,ERROR.

需要注意的是：在一个Web程序中可以配置多个Filter，Filter的执行顺序要分先后的