状态管理（之cookie、session、filter、listener）

#filter、listener

  # filter(过滤器)

1.作用

- 用来处理项目中的公共需求。很多请求都包含的业务

- 如：记录日志、过滤敏感词、权限检查

2.开发步骤

- 创建一个类，实现过滤器接口Filter

- 在web.xml里配置这个类

->: 过滤器由服务器自动调用

  # 实例

package web;

import java.io.IOException;

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

public class FindCostServlet extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
        System.out.println("查询资费");
    }

}

package web;

import java.io.IOException;

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

public class AddCostServlet extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
        System.out.println("增加资费");
    }

}

package web;

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;

public class LogFilter implements Filter {
    /**
     * 该方法是处理公共业务的方法。
     * filter相当于是Servlet们的管家，
     * tomcat在调用servlet前将请求提交给filter，filter有权让请求继续，也有权让请求终止。
     * tomcat就是调用doFilter方法让Filter统一处理请求的，调用前它会创建好request和response并传入，
     * 创建类型：RequestFacade和ResponseFacade
     */
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
            throws IOException, ServletException {
        System.out.println("在前面记日志");
        chain.doFilter(req, res);//请求继续，向后执行
        System.out.println("在后面记日志");
    }
    /**
     * 关闭tomcat时调用此方法
     */
    public void destroy() {
        System.out.println("销毁LogFilter");
    }
    
    /**
     * tomcat启动时会自动实例化Filter，然后调用其init()来初始化Filter。
     * 调用时会传入config，该对象和Filter是1对1关系，可以给Filter预置参数(web.xml)。
     * 该对象和ServletConfig用法完全一样。
     */
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("初始化LogFilter");
    }
    

}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5">
  <display-name>jsp6</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  
  <servlet>
      <servlet-name>findCost</servlet-name>
      <servlet-class>web.FindCostServlet</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>findCost</servlet-name>
      <url-pattern>/findCost</url-pattern>
  </servlet-mapping>
  
  <servlet>
      <servlet-name>addCost</servlet-name>
      <servlet-class>web.AddCostServlet</servlet-class>
  </servlet>
  <servlet-mapping>
      <servlet-name>addCost</servlet-name>
      <url-pattern>/addCost</url-pattern>
  </servlet-mapping>
  
  <filter>
      <filter-name>log</filter-name>
      <filter-class>web.LogFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>log</filter-name>
      <!-- 声明此Filter可以过滤哪些请求 -->
      <url-pattern>/*</url-pattern>
  </filter-mapping>
</web-app>

  # Listener

监听的部分由很多种，这里不多做赘述。

放一个例子参考，其他可百度。

package web;

import javax.servlet.ServletRequestAttributeEvent;
import javax.servlet.ServletRequestAttributeListener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;

public class MyListener implements ServletRequestListener,ServletRequestAttributeListener {
    //tomcat销毁request前自动调用此方法
    public void requestDestroyed(ServletRequestEvent e) {
        System.out.println("销毁request");
    }
    
    //tomcat 创建request前自动调用此方法
    public void requestInitialized(ServletRequestEvent e) {
        System.out.println("创建request");
        System.out.println(e.getServletRequest());
    }

    public void attributeAdded(ServletRequestAttributeEvent srae) {
        System.out.println("向request添加值");
    }

    public void attributeRemoved(ServletRequestAttributeEvent srae) {
        System.out.println("向request输出值");
    }

    public void attributeReplaced(ServletRequestAttributeEvent srae) {
        System.out.println("向request替换值");
    }

}

<listener>
      <listener-class>web.MyListener</listener-class>
  </listener>

package web;

import java.io.IOException;

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

public class FindCostServlet extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
        System.out.println("查询资费");
        req.setAttribute("attribute", "attribute");
    }

}