Java中的Filter詳解 --基礎篇

Filter是Servlet規范中的一個高級特性，和Servlet不同的是，他們不處理客戶端請求，只用於對request，response進行修改；

 

如果要自己實現一個自定義的Filter必須實現javax.servlet.Filter接口，接口中有三個方法：

 

 

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();
 
}

　　

三個方法反應了Filter的生命周期，其中init(),destory()方法只在Web程序加載或者卸載的時候調用，doFilter()方法每次請求都會調用。

 

另外，Filter在web.xml中的簡單配置如下：

 

 

<!-- 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 > <!-- 同一個Filter，Filter的名字必須一致 -->
   <url-pattern> /* </url-pattern > <!-- Filter生效的 url規則，可以使用通配符 -->
<!--    <dispatcher>REQUEST</dispatcher> -->
<!--    <dispatcher>FORWARD</dispatcher> -->
  </filter-mapping >
 

　　

 

 

其中：

“async-supported ”設置Servlet是否開啟異步支持，默認不開啟，詳細見： http://www.ibm.com/developerworks/cn/java/j-lo-servlet30/

“init- param ”設置自定義Filter里面屬性的值；

“dispatcher ”設置到達servlet的方式，方式有4種：REQUEST,FORWARD,INCLUDE,ERROR.

 

 

需要注意的是：在一個Web程序中可以配置多個Filter，Filter的執行順序要分先后的