JavaWeb——Filter过滤器和session监听器

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1. Filter：过滤器

通过Filter技术，对web服务器管理的所有web资源：例如Jsp, Servlet, 静态图片文件或静态 html 文件等进行拦截，从而实现一些特殊的功能。例如实现URL级别的权限访问控制、过滤敏感词汇、压缩响应信息等一些高级功能。

Filter的流程：Filter对用户请求进行预处理，接着将请求交给Servlet进行处理并生成响应，最后Filter再对服务器响应进行后处理。

==注意：过滤器是用来拦截请求和响应的，不能产生响应，而servlet是用来处理请求并产生响应的。==

　

2. Filter开发步骤

• 导包

  <dependency>
              <groupId>javax.servlet</groupId>
              <artifactId>javax.servlet-api</artifactId>
              <version>4.0.1</version>
          </dependency>
  
          <dependency>
              <groupId>javax.servlet.jsp</groupId>
              <artifactId>javax.servlet.jsp-api</artifactId>
              <version>2.3.3</version>
          </dependency>
  
          <dependency>
              <groupId>javax.servlet.jsp.jstl</groupId>
              <artifactId>jstl-api</artifactId>
              <version>1.2</version>
          </dependency>
  
          <dependency>
              <groupId>taglibs</groupId>
              <artifactId>standard</artifactId>
              <version>1.1.2</version>
          </dependency>
          <!--连接数据库-->
          <dependency>
              <groupId>mysql</groupId>
              <artifactId>mysql-connector-java</artifactId>
              <version>5.1.47</version>
          </dependency>
  

• 编写字符集编码过滤器

  • 实现类,实现Filter的接口，注意是javax.servlet包下的

  • 重写对应的方法

    package com.cheng.filter;
    
    import javax.servlet.*;
    import java.io.IOException;
    
    public class CharacterEncodingFilter implements Filter {
        //初始化  web服务器启动，就初始化，随时等待过滤对象的出现
        public void init(FilterConfig filterConfig) throws ServletException {
            System.out.println("CharacterEncodingFilter已经初始化");
    
        }
    
        //chain 链
        public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
            request.setCharacterEncoding("utf-8");
            response.setCharacterEncoding("utf-8");
            response.setContentType("text/html;charset=utf-8");
    
            System.out.println("CharacterEncodingFilter执行前。。。。。。");
            chain.doFilter(request,response);//让请求继续往下走，如果不写，程序到这里就会被拦截停止
            System.out.println("CharacterEncodingFilter执行后。。。。。。");
    
    
    
        }
        //销毁 web服务器关闭。过滤器销毁
        public void destroy() {
            System.out.println("CharacterEncodingFilter已经销毁");
    
        }
    }
    
    

  • 注册filter

     <!--注册filter-->
        <filter>
            <filter-name>CharacterEncodingFilter</filter-name>
            <filter-class>com.cheng.filter.CharacterEncodingFilter</filter-class>
        </filter>
        <filter-mapping>
            <filter-name>CharacterEncodingFilter</filter-name>
            <!--只要是/servlet下的任何请求都会经过这个过滤器-->
            <url-pattern>/servlet/*</url-pattern>
        </filter-mapping>
    

  测试filter

  测试类

  package com.cheng.servlet;
  
  import javax.servlet.ServletException;
  import javax.servlet.http.HttpServlet;
  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpServletResponse;
  import java.io.IOException;
  
  public class ShowServlet extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
  
          //请求的时候有乱码
          resp.getWriter().write("万里顾一程");
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
  
  

  注册servlet

  <servlet>
          <servlet-name>ShowServlet</servlet-name>
          <servlet-class>com.cheng.servlet.ShowServlet</servlet-class>
      </servlet>
  
     
      <servlet-mapping>
          <servlet-name>ShowServlet</servlet-name>
          <!--第一条路径，会经过filter-->
          <url-pattern>/servlet/show</url-pattern>
      </servlet-mapping>
      <servlet-mapping>
          <servlet-name>ShowServlet</servlet-name>
          <!--第二条路径，不会经过filter-->
          <url-pattern>/show</url-pattern>
      </servlet-mapping>
  

  启动服务器测试：

  经过过滤器

未经过过滤器

3. Filter实现权限拦截

用户登录界面login.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<form action="/servlet/login" method="post">
    用户名<input type="text" name="username">
    <input type="submit" value="登录">

</form>
</body>
</html>

登录用的Servlet,LoginServlet

package com.cheng.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class LoginServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取前端请求的参数
        String username = req.getParameter("username");
        if (username.equals("admin")){//登陆成功
            //在session里面保存一个数据,
            req.getSession().setAttribute("USER_SESSION",req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");//重定向
        }else{ //登录失败
            resp.sendRedirect("/error.jsp");

        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

注销用的Servlet,LogoutServlet

package com.cheng.servlet;

import com.cheng.util.Constant;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class LogoutServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Object user_session = req.getSession().getAttribute(Constant.USER_SESSION);
        if (user_session!=null){//如果user_session不为空，这说明处于登录状态
            req.getSession().removeAttribute("USER_SESSION");//移除这个数据，sessionID就被移除了
            resp.sendRedirect("/login.jsp");//重新回到登录页面

        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

注册servlet

    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.cheng.servlet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/servlet/login</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>LogoutServlet</servlet-name>
        <servlet-class>com.cheng.servlet.LogoutServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LogoutServlet</servlet-name>
        <url-pattern>/servlet/logout</url-pattern>
    </servlet-mapping>

用户主页面（登录成功后的页面）success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>欢迎来到主页面</h1>
<p><a href="/servlet/logout">注销</a></p>
</body>
</html>

登录错误页面error.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>错误页面</h1>
<p>错误情况：用户名,密码错误或权限不足</p>
</body>
</html>

过滤器Filter,保证只能通过登录的方式进入主页面

package com.cheng.filter;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class SysFilter implements Filter {
    public void init(FilterConfig filterConfig) throws ServletException {

    }

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

        //ServletRequest转化为HttpServletRequest,通过HttpServletRequest才能拿到session
        HttpServletRequest request1 = (HttpServletRequest) request;
        HttpServletResponse response1 = (HttpServletResponse) response;
        Object user_session = request1.getSession().getAttribute("USER_SESSION");
        if (user_session==null){//如果USER_SESSION里的值为空，则重定向到错误页面，不能再进入用户主页面
            response1.sendRedirect("/error.jsp");
        }
        chain.doFilter(request,response);
    }
    public void destroy() {

    }
}

注册过滤器

    <filter>
        <filter-name>SysFilter</filter-name>
        <filter-class>com.cheng.filter.SysFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>SysFilter</filter-name>
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>

启动服务器测试

1.通过登录界面登录到主页，输入admin

• 登陆成功

• 如果输入的不是admin，则登录失败

2.注销页面，在主页面点击注销,自动返回登录页面

3.尝试在登录页面直接访问主页面，结果直接返回到错误页面，说明权限拦截成功

4. session监听器

监听器就是一个实现特定接口（接口有N中）的普通java程序，这个程序专门用于监听另一个java对象的方法调用或属性改变，当被监听对象发生上述事件后，监听器某个方法将立即被执行。

实现在线人数监听器

实现类,实现了session监听器的接口，重写方法

package com.cheng.listener;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class OnlineCountListener implements HttpSessionListener {
    //创建session的监听
    //一旦创建一个session就会触发一次下面的事件
    public void sessionCreated(HttpSessionEvent se) {
        //HttpSessionEvent代表触发事件的对象
        HttpSession session = se.getSession();
        ServletContext servletContext = session.getServletContext();//获得上下文
        System.out.println(se.getSession().getId());
        //设置在线人数
        Integer onlineCount = (Integer) servletContext.getAttribute("OnlineCount");
        //判断，当在线人数为0时，让在线人数为1
        if (onlineCount==null){
            onlineCount = new Integer(1);
        }else{//当在线人数不为0时，count+1
            int count = onlineCount.intValue();
            onlineCount = new Integer(count+1);
        }
        servletContext.setAttribute("OnlineCount",onlineCount);

    }
    //销毁session的监听
    //一旦销毁一个session就会触发一次下面的事件
    public void sessionDestroyed(HttpSessionEvent se) {
        HttpSession session = se.getSession();
        ServletContext servletContext = session.getServletContext();//获得上下文
        Integer onlineCount = (Integer) servletContext.getAttribute("OnlineCount");
        if (onlineCount==null){
            onlineCount = new Integer(0);
        }else{
            int count = onlineCount.intValue();
            onlineCount = new Integer(count-1);
        }
        servletContext.setAttribute("OnlineCount",onlineCount);
    }
}

注册监听器

<!--注册监听器-->
    <listener>
        <listener-class>com.cheng.listener.OnlineCountListener</listener-class>
    </listener>

访问的页面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
<h1>当前有<span style="color:blue"><%= this.getServletConfig().getServletContext().getAttribute("OnlineCount")%></span>人在线</h1>
  </body>
</html>

启动服务器测试，每打开一个浏览器访问该页面，在线人数就+1