过滤器Filter学习的一些体会

过滤器是一个程序，它先于与之相关的servlet或JSP页面运行在服务器上。过滤器可附加到一个或多个servlet或JSP页面上，并且可以检查进入这些资源的请求信息
使用Filter开发的基本步骤:

1:建立实现Filter接口的类,包括3个方法:doFilter,init,destroy;

2:在doFilter中放入过滤行为,包括3个参数:ServletRequest,ServletResponse,FilterChain

3:调用FilterChain的doFilter方法

4:在web-xml中部署相关的filter和filter mapping 文件

一般用到2个实例:字符编码和登录

下面用字符编码做例:

在eclipse中写出一个简单的FILTER接口实现的类

package com.sun.servlet;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

public class SetCharacterCodeFilter implements Filter {
String encoding = "";

public void destroy() {
System.out.println("Set Character Code Filter destroy...");

}

public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
request.setCharacterEncoding(encoding);

chain.doFilter(request, response);
System.out.println("Set Character Code Filter succeed!!");
}

public void init(FilterConfig config) throws ServletException {
encoding = config.getInitParameter("charset");
System.out.println("Set Character Code Filter init ...charset="
+ encoding);

}

}
然后在配置web.xml文件

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<filter>
<filter-name>SetCharacter</filter-name>//自己命名的name
<filter-class>com.sun.servlet.SetCharacterCodeFilter</filter-class>
<init-param>
<param-name>charset</param-name>//设置初始化参数
<param-value>gbk</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>SetCharacter</filter-name>//保持一致
<url-pattern>/webbooks/*</url-pattern>//本人图书馆项目目录,使用了路径映射
</filter-mapping>

//下面的是JDBC的连接的一段配置信息

<resource-ref>
<description>DB Connection</description>
<res-ref-name>jdbc/webbookdb</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>

</web-app>