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。