ServletContext结合Servlet接口中的init()方法和destroy()方法的运用----网站计数器

我们一般知道Servlet接口中的init()方法在tomcat启动时调用，destroy()方法在tomcat关闭时调用。那么这两个方法到底在实际开发中有什么作用呢？这就是这个随笔主要讲的内容。

思路：网站计数器，如果想在服务器关闭时，保留以前的访问量，最简单的方式是把访问数据放在数据库中，但这样会增加服务器的压力。所以ServletContext结合Servlet接口中的init()方法和destroy()方法的一起使用便可以做一个网站计数器。

把数据写在一个.txt文件中，在init（）方法中把文件的数据读取出来，并添加到ServletContext中。在destroy（）方法，把ServletContext中的数据取出来写入到文件中。

具体代码如下：本代码共包括的类，以及文件的位置见下图中的红色矩形框：

web.xml中的代码如下：

 <servlet>
     <description>This is the description of my J2EE component</description>
     <display-name>This is the display name of my J2EE component</display-name>
     <servlet-name>InitServlet2</servlet-name>
     <servlet-class>com.qls.counter2.InitServlet2</servlet-class>
     <load-on-startup>1</load-on-startup>//this is very important .you cannot ignore it .
   </servlet>
   <servlet>
     <description>This is the description of my J2EE component</description>
     <display-name>This is the display name of my J2EE component</display-name>
     <servlet-name>Login2</servlet-name>
     <servlet-class>com.qls.counter2.Login2</servlet-class>
   </servlet>
   <servlet>
     <description>This is the description of my J2EE component</description>
     <display-name>This is the display name of my J2EE component</display-name>
     <servlet-name>LoginOperation2</servlet-name>
     <servlet-class>com.qls.counter2.LoginOperation2</servlet-class>
   </servlet>
   <servlet>
     <description>This is the description of my J2EE component</description>
     <display-name>This is the display name of my J2EE component</display-name>
     <servlet-name>Ok2</servlet-name>
     <servlet-class>com.qls.counter2.Ok2</servlet-class>
   </servlet>

//InitServlet2的代码如下：

 package com.qls.counter2;

 import java.io.BufferedReader;
 import java.io.BufferedWriter;
 import java.io.FileNotFoundException;
 import java.io.FileReader;
 import java.io.FileWriter;
 import java.io.IOException;
 import java.io.PrintWriter;

 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 /**
  * 利用tomcat启动时调用init()方法，和关闭时调用destroy()方法的特点
  * 把上次tomcat关闭时的数目写入到文件recordnum.txt中。
  * 启动tomcat时从该文件recordnum.txt中读取数据。
  * 这样就避免了把这些数据不停保留在数据库中了，增加服务器的运行压力。
  * 其实数据库的本质也是文件。
  * @author 秦林森
  *
  */
 public class InitServlet2 extends HttpServlet {

     public void destroy() {
         /**
          * 利用tomcat关闭时调用该方法的特性
          * 把ServletContext的属性值的写入文件recordnum.txt。
          */
         /**
          * 获取文件recordnum.txt的全路径
          */
         System.out.println("destory被调用了");
         String path = this.getServletContext().getRealPath("recordnum.txt");
         try {
             BufferedWriter out=new BufferedWriter(new FileWriter(path));
             /**
              * 把ServletContext中的数据写入到文件recordnum.txt中。
              */
             String num = (String) this.getServletConfig().getServletContext().getAttribute("pengta");
             out.write(num);
             out.close();
         } catch (IOException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }

     }

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();

     }

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         this.doGet(request, response);
     }

     public void init() throws ServletException {
         System.out.println("init()被调用了");
         /**
          * 利用启动Tomcat时调用该方法的特性。
          */
         /**
          * 获得recordnum.txt的全路径
          */
         String path = this.getServletConfig().getServletContext().getRealPath("recordnum.txt");
         /**
          * 从recordnum.txt中读取数据
          */
         try {
             BufferedReader in = new BufferedReader(new FileReader(path));
             String num = in.readLine();
             this.getServletConfig().getServletContext().setAttribute("pengta", num);
             in.close();//关流。
         } catch (FileNotFoundException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         } catch (IOException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
     }

 }

//Login2 的代码如下：

 package com.qls.counter2;

 import java.io.IOException;
 import java.io.PrintWriter;

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

 public class Login2 extends HttpServlet {

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         out.println("<h1>统计网站浏览的次数的登录界面</h1>");
         out.println("<br/>");
         out.println("<form action='/servletContext/LoginOperation2' method='post'>");
         out.println("<br/>");
         out.println("用户名：<input type='text' name='username'/>");
         out.println("<br/>");
         out.println("密码：<input type='password' name='password'/>");
         out.println("<br/>");
         out.println("<input type='submit' value='登录'/>");
         out.println("</form>");
     }

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         this.doGet(request, response);
     }

 }

//LoginOperation2的代码如下：

 package com.qls.counter2;

 import java.io.IOException;
 import java.io.PrintWriter;

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

 public class LoginOperation2 extends HttpServlet {

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         /**
          * 获取在登录界面输入的密码：
          */
         String password = request.getParameter("password");
         //获取ServletContext中的数目
         String ouyangfeng = (String) this.getServletConfig().getServletContext().getAttribute("pengta");
         /**
          * 由于每个用户访问该网站都有一个唯一的密码和用户名，现假设该用户的密码是123，只要密码是123就能登录进去。
          */
         if (password.equals("123")) {

                 this.getServletConfig().getServletContext().setAttribute("pengta", (Integer.parseInt(ouyangfeng)+1)+"");

                 response.sendRedirect("/servletContext/Ok2");
         }else{
             request.getRequestDispatcher("/Login2").forward(request, response);
         }
     }

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         this.doGet(request, response);
     }

 }

//Ok2 的代码如下：

 package com.qls.counter2;

 import java.io.IOException;
 import java.io.PrintWriter;

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

 public class Ok2 extends HttpServlet {

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         out.println("访问该网站的次数是："+this.getServletContext().getAttribute("pengta"));
     }

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

         response.setContentType("text/html;charset=utf-8");
         PrintWriter out = response.getWriter();
         this.doGet(request, response);
     }

 }

//recordnum.txt的代码如下：

recordnum.txt的内容只有一个0.如下图所示：