Servlets Filter 是Servlet 2.3 规范中新增加的，它是截取用户从客户端提交的请求，在还没有到达需要访问的资源时运行的一个类。它操纵来自客户端的请求，在资源还没有初发送到客户端前截取响应，并处理这些还没有发送到客户端的响应。
Filters 有许多的应用场合。Servlet 2.3 规范建议如下的地方可以应用Filter：
authentication filters
logging and auditing filters
image conversion filters
data compression filters
encryption filters
tokenizing filters
filters that trigger resource access events
XSL/T filters that transform XML content
MIME-type chain filters
例：
package jasso.web.filter;
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;
/**
*
* @author
*/
public class SetCharacterEncodingFilter implements Filter {
private String encoding;
/**
* Called by the web container to indicate to a filter that it is being
* taken out of service.
*
* @todo Implement this javax.servlet.Filter method
*/
public void destroy() {
}
/**
* 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.
*
* @param request
* ServletRequest
* @param response
* ServletResponse
* @param chain
* FilterChain
* @throws IOException
* @throws ServletException
* @todo Implement this javax.servlet.Filter method
*/
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
request.setCharacterEncoding(encoding);
chain.doFilter(request, response);
}
/**
* @see javax.servlet.Filter#init(FilterConfig)
*/
public void init(FilterConfig config) throws ServletException {
encoding = config.getInitParameter("encoding");
}
}
web.xml部分
......
<filter>
<filter-name>encodingFilter</filter-name>
<filter-class>my.web.filter.SetCharacterEncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>Shift_JIS</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>encodingFilter</filter-name>
<servlet-name>action</servlet-name>
</filter-mapping>
<servlet>
<servlet-name>action</servlet-name>
<servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
<init-param>
<param-name>config</param-name>
<param-value>/WEB-INF/config/struts-config.xml,/WEB-INF/config/struts-config-contents.xml</param-value>
</init-param>
<init-param>
<param-name>debug</param-name>
<param-value>2</param-value>
</init-param>
<init-param>
<param-name>detail</param-name>
<param-value>2</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
----------------------------------------------
javax.servlet.filter的用法
如何实现一个过滤器呢？
1.所在的类实现Filter接口
public interface Filter
public void init(FilterConfig filterConfig)throws ServletException
过滤器初始化是在容器启动时自动初始化
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain)throws IOException,ServletExcepton
在标准Servlet中的doGet或doPost 方法中，传递参数为：HttpServletRequest、HttpServletResponse
过滤器销毁
public void destroy()
package cn.mldn.lxh.filter
import java.io.*;
import javax.servlet.*;
public class FirstFilter implements Filter
{
public void init(FilterConfig config)throws ServletException
{
System.out.println("过滤器初始化");
}
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain) throws IOException,ServletException
{
System.out.println(过滤doFilter);
}
public void destroy()
{
System.out.println(过滤器销毁);
}
}
web.xml文件配置
<filter>
<filter-name> first </filter-name>
<filter-class> cn.mldn.lxh.filter.FirstFilter </filter-class>
</filter>
<filter-mapping>
<filter-name> first </filter-name>
与servlet中的url-pattern不一样，此处表示对那个页面进行过滤，如果全部都过滤，则"/*"
<url-pattern> /* </url-pattern>
</filter-mapping>
如果过滤器要将内容传递到目的地，则需要FilterChain,将请求继续向下转发
过滤器也会执行两次：FilterChain这前执行一次，之后再执行一次
1.过滤非法文字：
package cn.mldn.lxh.filter;
import java.io.*;
import javax.servlet.*;
public class CharFilter implements Filter
{
public void init(FilterConfig filterConfig)throws ServletException
{
//System.out.println("init");
}
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain)throws IOException,ServletException
{
String content=request.getParameter("content");
//如果indexOf返回-1则表示没有查到所要的内容
if(content!=null)
if(content.indexOf("AAA")==-1)
{
chain.doFilter(request,response);
}
esle
{
System.out.println("有非法文字");
//如果需要的话，此处依然可以使用RequestDispatcher进行跳转
}
}
else
{
chain.doFilter(request,response);
}
}
public void destroy()
{
//System.out.println("过滤器销毁");
}
<filter>
<filter-name> char </filter-name>
<filter-class> cn.mldn.lxh.filter.CharFilter </filter-class>
</filter>
<filter-mapping>
<filter-name> char </filter-name>
<url-pattern> /* </url-pattern>
</filter-mapping>
2.对内容进行统一的编码
setCharacterEncoding("GB2312");
package cn.mldn.lxh.filter;
import java.io.*;
import javax.servlet.*;
public class EncodingFilter implements Filter
{
public void init(FilterConfig filterConfig)throws ServletException
{
//System.out.println("过滤器初始化");
}
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain)throws IOException,ServletException
{
try{
request.setCharacterEncoding("GB2312");
}
catch(Exception e)
{}
chain.doFilter(request,response);
}
public void destroy()
{
//System.out.println("销毁");
}
}
<filter>
<filter-name> encoding </filter-name>
<filter-class> </filter-class>
</filter>
<filter-mapping>
<filter-name> encoding </filter-name>
<url-pattern> /* </url-pattern>
</filter-mapping>
3.登陆验证
package cn.mldn.lxh.filter;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class LoginFilter implements Filter
{
public void init(FilterCoinfig filterConfig)throws ServletException
{
//System.out.println("过滤器初始化");
}
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain)throws IOException,ServletException
{
//Session属于http范畴，把以ServletRequest对象需要先转换成HttpServletRequest对象
HttpServletRequest req=(HttpServletRequest)request;
HttpSession session =req.getSession();
//如果session不为空，则可以浏览其它页面
if(session.getAttribute("uname")!=null)
{
chain.doFilter(request,response);
}
else
{
//通过requestDispatcher跳转到登陆面
request.getRequestDispatcher("login.jsp").forward(request,response);
}
}
public void destroy()
{
//System,out.println("销毁");
}
}
<filter>
<filter-name> login </filter-name>
<filter-class> cn.mldn.lxh.filter.LoginFilter </filter-class>
<init-param>
<param-name> 参数名称 </param-name>
<param-value> 参数值 </param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name> login </filter-name>
<url-pattern> /* </url-pattern>
</filter-mapping>
读取web.xml中的参数的方法：
public void init(FilterConfig arg0) throws ServletException {
String 参数值= arg0.getInitParameter("参数名称");
}