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

Posted 2020-08-23 技术让世界更精彩

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：

 1 <servlet>
 2     <description>This is the description of my J2EE component</description>
 3     <display-name>This is the display name of my J2EE component</display-name>
 4     <servlet-name>InitServlet2</servlet-name>
 5     <servlet-class>com.qls.counter2.InitServlet2</servlet-class>
 6     <load-on-startup>1</load-on-startup>//this is very important .you cannot ignore it .
 7   </servlet>
 8   <servlet>
 9     <description>This is the description of my J2EE component</description>
10     <display-name>This is the display name of my J2EE component</display-name>
11     <servlet-name>Login2</servlet-name>
12     <servlet-class>com.qls.counter2.Login2</servlet-class>
13   </servlet>
14   <servlet>
15     <description>This is the description of my J2EE component</description>
16     <display-name>This is the display name of my J2EE component</display-name>
17     <servlet-name>LoginOperation2</servlet-name>
18     <servlet-class>com.qls.counter2.LoginOperation2</servlet-class>
19   </servlet>
20   <servlet>
21     <description>This is the description of my J2EE component</description>
22     <display-name>This is the display name of my J2EE component</display-name>
23     <servlet-name>Ok2</servlet-name>
24     <servlet-class>com.qls.counter2.Ok2</servlet-class>
25   </servlet>

 //InitServlet2的代码如下：

 

 1 package com.qls.counter2;
 2 
 3 import java.io.BufferedReader;
 4 import java.io.BufferedWriter;
 5 import java.io.FileNotFoundException;
 6 import java.io.FileReader;
 7 import java.io.FileWriter;
 8 import java.io.IOException;
 9 import java.io.PrintWriter;
10 
11 import javax.servlet.ServletException;
12 import javax.servlet.http.HttpServlet;
13 import javax.servlet.http.HttpServletRequest;
14 import javax.servlet.http.HttpServletResponse;
15 /**
16  * 利用tomcat启动时调用init()方法，和关闭时调用destroy()方法的特点
17  * 把上次tomcat关闭时的数目写入到文件recordnum.txt中。
18  * 启动tomcat时从该文件recordnum.txt中读取数据。
19  * 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。
20  * 其实数据库的本质也是文件。
21  * @author 秦林森
22  *
23  */
24 public class InitServlet2 extends HttpServlet {
25 
26     
27     public void destroy() {
28         /**
29          * 利用tomcat关闭时调用该方法的特性
30          * 把ServletContext的属性值的写入文件recordnum.txt。
31          */
32         /**
33          * 获取文件recordnum.txt的全路径
34          */
35         System.out.println("destory被调用了");
36         String path = this.getServletContext().getRealPath("recordnum.txt");
37         try {
38             BufferedWriter out=new BufferedWriter(new FileWriter(path));
39             /**
40              * 把ServletContext中的数据写入到文件recordnum.txt中。
41              */
42             String num = (String) this.getServletConfig().getServletContext().getAttribute("pengta");
43             out.write(num);
44             out.close();
45         } catch (IOException e) {
46             // TODO Auto-generated catch block
47             e.printStackTrace();
48         }
49     
50         
51     }
52 
53     
54     public void doGet(HttpServletRequest request, HttpServletResponse response)
55             throws ServletException, IOException {
56 
57         response.setContentType("text/html;charset=utf-8");
58         PrintWriter out = response.getWriter();
59         
60     }
61 
62     
63     public void doPost(HttpServletRequest request, HttpServletResponse response)
64             throws ServletException, IOException {
65 
66         response.setContentType("text/html;charset=utf-8");
67         PrintWriter out = response.getWriter();
68         this.doGet(request, response);
69     }
70 
71     
72     public void init() throws ServletException {
73         System.out.println("init()被调用了");
74         /**
75          * 利用启动Tomcat时调用该方法的特性。
76          */
77         /**
78          * 获得recordnum.txt的全路径
79          */
80         String path = this.getServletConfig().getServletContext().getRealPath("recordnum.txt");
81         /**
82          * 从recordnum.txt中读取数据
83          */
84         try {
85             BufferedReader in = new BufferedReader(new FileReader(path));
86             String num = in.readLine();
87             this.getServletConfig().getServletContext().setAttribute("pengta", num);
88             in.close();//关流。
89         } catch (FileNotFoundException e) {
90             // TODO Auto-generated catch block
91             e.printStackTrace();
92         } catch (IOException e) {
93             // TODO Auto-generated catch block
94             e.printStackTrace();
95         }
96     }
97 
98 }

 //Login2 的代码如下：

 1 package com.qls.counter2;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.ServletException;
 7 import javax.servlet.http.HttpServlet;
 8 import javax.servlet.http.HttpServletRequest;
 9 import javax.servlet.http.HttpServletResponse;
10 
11 public class Login2 extends HttpServlet {
12 
13     
14     public void doGet(HttpServletRequest request, HttpServletResponse response)
15             throws ServletException, IOException {
16 
17         response.setContentType("text/html;charset=utf-8");
18         PrintWriter out = response.getWriter();
19         out.println("<h1>统计网站浏览的次数的登录界面</h1>");
20         out.println("<br/>");
21         out.println("<form action=‘/servletContext/LoginOperation2‘ method=‘post‘>");
22         out.println("<br/>");
23         out.println("用户名：<input type=‘text‘ name=‘username‘/>");
24         out.println("<br/>");
25         out.println("密码：<input type=‘password‘ name=‘password‘/>");
26         out.println("<br/>");
27         out.println("<input type=‘submit‘ value=‘登录‘/>");
28         out.println("</form>");
29     }
30 
31     
32     public void doPost(HttpServletRequest request, HttpServletResponse response)
33             throws ServletException, IOException {
34 
35         response.setContentType("text/html;charset=utf-8");
36         PrintWriter out = response.getWriter();
37         this.doGet(request, response);
38     }
39 
40     
41 
42 }

 //LoginOperation2的代码如下：

 1 package com.qls.counter2;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.ServletException;
 7 import javax.servlet.http.HttpServlet;
 8 import javax.servlet.http.HttpServletRequest;
 9 import javax.servlet.http.HttpServletResponse;
10 
11 public class LoginOperation2 extends HttpServlet {
12 
13     
14     public void doGet(HttpServletRequest request, HttpServletResponse response)
15             throws ServletException, IOException {
16 
17         response.setContentType("text/html;charset=utf-8");
18         PrintWriter out = response.getWriter();
19         /**
20          * 获取在登录界面输入的密码：
21          */
22         String password = request.getParameter("password");
23         //获取ServletContext中的数目
24         String ouyangfeng = (String) this.getServletConfig().getServletContext().getAttribute("pengta");
25         /**
26          * 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。
27          */
28         if (password.equals("123")) {
29         
30                 this.getServletConfig().getServletContext().setAttribute("pengta", (Integer.parseInt(ouyangfeng)+1)+"");
31             
32                 response.sendRedirect("/servletContext/Ok2");
33         }else{
34             request.getRequestDispatcher("/Login2").forward(request, response);
35         }
36     }
37 
38     
39     public void doPost(HttpServletRequest request, HttpServletResponse response)
40             throws ServletException, IOException {
41 
42         response.setContentType("text/html;charset=utf-8");
43         PrintWriter out = response.getWriter();
44         this.doGet(request, response);
45     }
46 
47 }

 

 //Ok2 的代码如下：

 1 package com.qls.counter2;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.ServletException;
 7 import javax.servlet.http.HttpServlet;
 8 import javax.servlet.http.HttpServletRequest;
 9 import javax.servlet.http.HttpServletResponse;
10 
11 public class Ok2 extends HttpServlet {
12 
13     
14     public void doGet(HttpServletRequest request, HttpServletResponse response)
15             throws ServletException, IOException {
16 
17         response.setContentType("text/html;charset=utf-8");
18         PrintWriter out = response.getWriter();
19         out.println("访问该网站的次数是："+this.getServletContext().getAttribute("pengta"));
20     }
21 
22     
23     public void doPost(HttpServletRequest request, HttpServletResponse response)
24             throws ServletException, IOException {
25 
26         response.setContentType("text/html;charset=utf-8");
27         PrintWriter out = response.getWriter();
28         this.doGet(request, response);
29     }
30 
31 }

 //recordnum.txt的代码如下：

recordnum.txt的内容只有一个0.如下图所示：