Filter(过滤器)常见应用

一、统一全站字符编码

　　通过配置参数charset指明使用何种字符编码,以处理Html Form请求参数的中文问题

package me.gacl.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;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
import javax.servlet.http.HttpServletResponse;

/**
* @ClassName: CharacterEncodingFilter
* @Description: 此过滤器用来解决全站中文乱码问题
* @author: 孤傲苍狼
* @date: 2014-8-31 下午11:09:37
*
*/ 
public class CharacterEncodingFilter implements Filter {

    private FilterConfig filterConfig = null;
    //设置默认的字符编码
    private String defaultCharset = "UTF-8";

    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
        
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        String charset = filterConfig.getInitParameter("charset");
        if(charset==null){
            charset = defaultCharset;
        }
        request.setCharacterEncoding(charset);
        response.setCharacterEncoding(charset);
        response.setContentType("text/html;charset="+charset);
        
        MyCharacterEncodingRequest requestWrapper = new MyCharacterEncodingRequest(request);
        chain.doFilter(requestWrapper, response);
    }

    public void init(FilterConfig filterConfig) throws ServletException {
        //得到过滤器的初始化配置信息
        this.filterConfig = filterConfig;
    }
    
    public void destroy() {

    }
}

/*
1.实现与被增强对象相同的接口 
2、定义一个变量记住被增强对象
3、定义一个构造器，接收被增强对象
4、覆盖需要增强的方法
5、对于不想增强的方法，直接调用被增强对象（目标对象）的方法
 */
 
class MyCharacterEncodingRequest extends HttpServletRequestWrapper{
    
    private HttpServletRequest request;
    public MyCharacterEncodingRequest(HttpServletRequest request) {
        super(request);
        this.request = request;
    }
    /* 重写getParameter方法
     * @see javax.servlet.ServletRequestWrapper#getParameter(java.lang.String)
     */
    @Override
    public String getParameter(String name) {
        
        try{
            //获取参数的值
            String value= this.request.getParameter(name);
            if(value==null){
                return null;
            }
            //如果不是以get方式提交数据的，就直接返回获取到的值
            if(!this.request.getMethod().equalsIgnoreCase("get")) {
                return value;
            }else{
                //如果是以get方式提交数据的，就对获取到的值进行转码处理
                value = new String(value.getBytes("ISO8859-1"),this.request.getCharacterEncoding());
                return value;
            }
        }catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

　　web.xml文件中的配置如下：

<filter>
      <filter-name>CharacterEncodingFilter</filter-name>
      <filter-class>me.gacl.web.filter.CharacterEncodingFilter</filter-class>
      <init-param>
          <param-name>charset</param-name>
          <param-value>UTF-8</param-value>
      </init-param>
  </filter>
  
  <filter-mapping>
      <filter-name>CharacterEncodingFilter</filter-name>
      <url-pattern>/*</url-pattern>
  </filter-mapping>

二、禁止浏览器缓存所有动态页面

　　有3 个HTTP 响应头字段都可以禁止浏览器缓存当前页面，它们在 Servlet 中的示例代码如下：

1 response.setDateHeader("Expires",-1);
2 response.setHeader("Cache-Control","no-cache");
3 response.setHeader("Pragma","no-cache");

　　并不是所有的浏览器都能完全支持上面的三个响应头，因此最好是同时使用上面的三个响应头。

• Expires数据头：值为GMT时间值，为-1指浏览器不要缓存页面
• Cache-Control响应头有两个常用值：
• no-cache指浏览器不要缓存当前页面。
• max-age:xxx指浏览器缓存页面xxx秒。

package me.gacl.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;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
* @ClassName: NoCacheFilter
* @Description: 禁止浏览器缓存所有动态页面
* @author: 孤傲苍狼
* @date: 2014-8-31 下午11:25:40
*
*/ 
public class NoCacheFilter implements Filter {


    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
        //把ServletRequest强转成HttpServletRequest
        HttpServletRequest request = (HttpServletRequest) req;
        //把ServletResponse强转成HttpServletResponse
        HttpServletResponse response = (HttpServletResponse) resp;
        //禁止浏览器缓存所有动态页面
        response.setDateHeader("Expires", -1);
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma", "no-cache");
        
        chain.doFilter(request, response);
    }

    public void init(FilterConfig filterConfig) throws ServletException {

    }
    
    public void destroy() {
        
    }
}

　　web.xml文件中的配置如下：

<filter>
      <filter-name>NoCacheFilter</filter-name>
      <filter-class>me.gacl.web.filter.NoCacheFilter</filter-class>
  </filter>
  
  <filter-mapping>
      <filter-name>NoCacheFilter</filter-name>
        <!--只拦截Jsp请求-->
      <servlet-name>*.jsp</servlet-name>
  </filter-mapping>

三、控制浏览器缓存页面中的静态资源

　　有些动态页面中引用了一些图片或css文件以修饰页面效果，这些图片和css文件经常是不变化的，所以为减轻服务器的压力，可以使用filter控制浏览器缓存这些文件，以提升服务器的性能。

package me.gacl.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;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
* @ClassName: CacheFilter
* @Description: 控制缓存的filter
* @author: 孤傲苍狼
* @date: 2014-9-1 下午9:39:38
*
*/ 
public class CacheFilter implements Filter {

    private FilterConfig filterConfig;

    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
    
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        
        //1.获取用户想访问的资源
        String uri = request.getRequestURI(); 
        
        //2.得到用户想访问的资源的后缀名
        String ext = uri.substring(uri.lastIndexOf(".")+1);
        
        //得到资源需要缓存的时间
        String time = filterConfig.getInitParameter(ext);
        if(time!=null){
            long t = Long.parseLong(time)*3600*1000;
            //设置缓存
            response.setDateHeader("expires", System.currentTimeMillis() + t);
        }
        
        chain.doFilter(request, response);

    }

    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
    }
    
    public void destroy() {
        
    }
}

　　web.xml文件中的配置如下：

<!-- 配置缓存过滤器 -->
   <filter>
      <filter-name>CacheFilter</filter-name>
      <filter-class>me.gacl.web.filter.CacheFilter</filter-class>
       <!-- 配置要缓存的web资源以及缓存时间，以小时为单位 -->
      <init-param>
          <param-name>css</param-name>
          <param-value>4</param-value>
      </init-param>
      <init-param>
          <param-name>jpg</param-name>
          <param-value>1</param-value>
      </init-param>
      <init-param>
          <param-name>js</param-name>
          <param-value>4</param-value>
      </init-param>
      <init-param>
          <param-name>png</param-name>
          <param-value>4</param-value>
      </init-param>
  </filter>
  <!-- 配置要缓存的web资源的后缀-->
  <filter-mapping>
      <filter-name>CacheFilter</filter-name>
      <url-pattern>*.jpg</url-pattern>
  </filter-mapping>
  
  <filter-mapping>
      <filter-name>CacheFilter</filter-name>
      <url-pattern>*.css</url-pattern>
  </filter-mapping>
  
  <filter-mapping>
      <filter-name>CacheFilter</filter-name>
      <url-pattern>*.js</url-pattern>
  </filter-mapping>
   <filter-mapping>
      <filter-name>CacheFilter</filter-name>
      <url-pattern>*.png</url-pattern>
  </filter-mapping>

四、实现用户自动登陆

　　思路是这样的：

　　1、在用户登陆成功后，发送一个名称为user的cookie给客户端，cookie的值为用户名和md5加密后的密码。
　　2、编写一个AutoLoginFilter，这个filter检查用户是否带有名称为user的cookie来，如果有，则调用dao查询cookie的用户名和密码是否和数据库匹配，匹配则向session中存入user对象（即用户登陆标记），以实现程序完成自动登陆。

　　核心代码如下：

　　处理用户登录的控制器：LoginServlet

package me.gacl.web.controller;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import me.gacl.dao.UserDao;
import me.gacl.domain.User;
import me.gacl.util.WebUtils;

public class LoginServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String username = request.getParameter("username");
        String password = request.getParameter("password");
        
        UserDao dao = new UserDao();
        User user = dao.find(username, password);
        if(user==null){
            request.setAttribute("message", "用户名或密码不对！！");
            request.getRequestDispatcher("/message.jsp").forward(request, response);
            return;
        }
        request.getSession().setAttribute("user", user);
        //发送自动登陆cookie给客户端浏览器进行存储
        sendAutoLoginCookie(request,response,user);
        request.getRequestDispatcher("/index.jsp").forward(request, response);
    }

    /**
    * @Method: sendAutoLoginCookie
    * @Description: 发送自动登录cookie给客户端浏览器
    * @Anthor:孤傲苍狼
    *
    * @param request
    * @param response
    * @param user
    */ 
    private void sendAutoLoginCookie(HttpServletRequest request, HttpServletResponse response, User user) {
        if (request.getParameter("logintime")!=null) {
            int logintime = Integer.parseInt(request.getParameter("logintime"));
            //创建cookie,cookie的名字是autologin，值是用户登录的用户名和密码，用户名和密码之间使用.进行分割，密码经过md5加密处理
            Cookie cookie = new Cookie("autologin",user.getUsername() + "." + WebUtils.md5(user.getPassword()));
            //设置cookie的有效期
            cookie.setMaxAge(logintime);
            //设置cookie的有效路径
            cookie.setPath(request.getContextPath());
            //将cookie写入到客户端浏览器
            response.addCookie(cookie);
        }
    }
    
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request, response);
    }

}

　　处理用户自动登录的过滤器：AutoLoginFilter

package me.gacl.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;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import me.gacl.dao.UserDao;
import me.gacl.domain.User;
import me.gacl.util.WebUtils;

public class AutoLoginFilter implements Filter {

    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
        
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        //如果已经登录了，就直接chain.doFilter(request, response)放行
        if(request.getSession().getAttribute("user")!=null){
            chain.doFilter(request, response);
            return;
        }
        
        //1.得到用户带过来的authlogin的cookie
        String value = null;
        Cookie cookies[] = request.getCookies();
        for(int i=0;cookies!=null && i<cookies.length;i++){
            if(cookies[i].getName().equals("autologin")){
                value = cookies[i].getValue();
            }
        }
        
        //2.得到 cookie中的用户名和密码 
        if(value!=null){
            String username = value.split("\\.")[0];
            String password = value.split("\\.")[1];
            
            //3.调用dao获取用户对应的密码
            UserDao dao = new UserDao();
            User user = dao.find(username);
            String dbpassword = user.getPassword();
            
            //4.检查用户带过来的md5的密码和数据库中的密码是否匹配,如匹配则自动登陆
            if(password.equals(WebUtils.md5(dbpassword))){
                request.getSession().setAttribute("user", user);
            }
        }
        
        chain.doFilter(request, response);
    }
    
    public void destroy() {
        
    }

    public void init(FilterConfig filterConfig) throws ServletException {

    }
}

　　如果想取消自动登录，那么可以在用户注销时删除自动登录cookie，核心代码如下：

package me.gacl.web.controller;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CancelAutoLoginServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //移除存储在session中的user
        request.getSession().removeAttribute("user");
        //移除自动登录的cookie
        removeAutoLoginCookie(request,response);
        //注销用户后跳转到登录页面
        request.getRequestDispatcher("/login.jsp").forward(request, response);
    }

    /**
    * @Method: removeAutoLoginCookie
    * @Description: 删除自动登录cookie，
    *     JavaWeb中删除cookie的方式就是新创建一个cookie，新创建的cookie与要删除的cookie同名，
    *     设置新创建的cookie的cookie的有效期设置为0，有效路径与要删除的cookie的有效路径相同
    * @Anthor:孤傲苍狼
    *
    * @param request
    * @param response
    */ 
    private void removeAutoLoginCookie(HttpServletRequest request, HttpServletResponse response) {
        //创建一个名字为autologin的cookie
        Cookie cookie = new Cookie("autologin","");
         //将cookie的有效期设置为0，命令浏览器删除该cookie
        cookie.setMaxAge(0);
        //设置要删除的cookie的path
        cookie.setPath(request.getContextPath());
        response.addCookie(cookie);
    }
    
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
}

　　以上就是过滤器的几个常见应用场景。