javaweb中Filter(过滤器)的常见应用

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
*/
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：

• response.setDateHeader("Expires",-1);
• response.setHeader("Cache-Control","no-cache");
• 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
*/
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
  * @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
  * @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，希望对大家的学习有所帮助。