javaweb學習總結(六)——Servlet開發(二)

時間 2019-11-12

原文原文鏈接

1、ServletConfig講解

1.一、配置Servlet初始化參數

　　在Servlet的配置文件web.xml中，能夠使用一個或多個<init-param>標籤爲servlet配置一些初始化參數。html

例如：java

 1 <servlet>
 2     <servlet-name>ServletConfigDemo1</servlet-name>
 3     <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
 4     <!--配置ServletConfigDemo1的初始化參數 -->
 5     <init-param>
 6         <param-name>name</param-name>
 7         <param-value>gacl</param-value>
 8     </init-param>
 9      <init-param>
10         <param-name>password</param-name>
11         <param-value>123</param-value>
12     </init-param>
13     <init-param>
14         <param-name>charset</param-name>
15         <param-value>UTF-8</param-value>
16     </init-param>
17 </servlet>

1.二、經過ServletConfig獲取Servlet的初始化參數

　　當servlet配置了初始化參數後，web容器在建立servlet實例對象時，會自動將這些初始化參數封裝到ServletConfig對象中，並在調用servlet的init方法時，將ServletConfig對象傳遞給servlet。進而，咱們經過ServletConfig對象就能夠獲得當前servlet的初始化參數信息。mysql

例如：程序員

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 import java.util.Enumeration;
 5 import javax.servlet.ServletConfig;
 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 ServletConfigDemo1 extends HttpServlet {
12 
13     /**
14      * 定義ServletConfig對象來接收配置的初始化參數
15      */
16     private ServletConfig config;
17     
18     /**
19      * 當servlet配置了初始化參數後，web容器在建立servlet實例對象時，
20      * 會自動將這些初始化參數封裝到ServletConfig對象中，並在調用servlet的init方法時，
21      * 將ServletConfig對象傳遞給servlet。進而，程序員經過ServletConfig對象就能夠
22      * 獲得當前servlet的初始化參數信息。
23      */
24     @Override
25     public void init(ServletConfig config) throws ServletException {
26         this.config = config;
27     }
28 
29     public void doGet(HttpServletRequest request, HttpServletResponse response)
30             throws ServletException, IOException {
31         //獲取在web.xml中配置的初始化參數
32         String paramVal = this.config.getInitParameter("name");//獲取指定的初始化參數
33         response.getWriter().print(paramVal);
34         
35         response.getWriter().print("<hr/>");
36         //獲取全部的初始化參數
37         Enumeration<String> e = config.getInitParameterNames();
38         while(e.hasMoreElements()){
39             String name = e.nextElement();
40             String value = config.getInitParameter(name);
41             response.getWriter().print(name + "=" + value + "<br/>");
42         }
43     }
44 
45     public void doPost(HttpServletRequest request, HttpServletResponse response)
46             throws ServletException, IOException {
47         this.doGet(request, response);
48     }
49 
50 }

運行結果以下：web

2、ServletContext對象

　　WEB容器在啓動時，它會爲每一個WEB應用程序都建立一個對應的ServletContext對象，它表明當前web應用。
　　ServletConfig對象中維護了ServletContext對象的引用，開發人員在編寫servlet時，能夠經過ServletConfig.getServletContext方法得到ServletContext對象。
　　因爲一個WEB應用中的全部Servlet共享同一個ServletContext對象，所以Servlet對象之間能夠經過ServletContext對象來實現通信。ServletContext對象一般也被稱之爲context域對象。sql

3、ServletContext的應用

　　3.一、多個Servlet經過ServletContext對象實現數據共享

　　範例：ServletContextDemo1和ServletContextDemo2經過ServletContext對象實現數據共享瀏覽器

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 import javax.servlet.ServletContext;
 5 import javax.servlet.ServletException;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 
10 public class ServletContextDemo1 extends HttpServlet {
11 
12     public void doGet(HttpServletRequest request, HttpServletResponse response)
13             throws ServletException, IOException {
14         String data = "xdp_gacl";
15         /**
16          * ServletConfig對象中維護了ServletContext對象的引用，開發人員在編寫servlet時，
17          * 能夠經過ServletConfig.getServletContext方法得到ServletContext對象。
18          */
19         ServletContext context = this.getServletConfig().getServletContext();//得到ServletContext對象
20         context.setAttribute("data", data);  //將data存儲到ServletContext對象中
21     }
22 
23     public void doPost(HttpServletRequest request, HttpServletResponse response)
24             throws ServletException, IOException {
25         doGet(request, response);
26     }
27 }

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 import javax.servlet.ServletContext;
 5 import javax.servlet.ServletException;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 
10 public class ServletContextDemo2 extends HttpServlet {
11 
12     public void doGet(HttpServletRequest request, HttpServletResponse response)
13             throws ServletException, IOException {
14         ServletContext context = this.getServletContext();
15         String data = (String) context.getAttribute("data");//從ServletContext對象中取出數據
16         response.getWriter().print("data="+data);
17     }
18 
19     public void doPost(HttpServletRequest request, HttpServletResponse response)
20             throws ServletException, IOException {
21         doGet(request, response);
22     }
23 }

　　先運行ServletContextDemo1，將數據data存儲到ServletContext對象中，而後運行ServletContextDemo2就能夠從ServletContext對象中取出數據了，這樣就實現了數據共享，以下圖所示：緩存

　　3.二、獲取WEB應用的初始化參數

　　在web.xml文件中使用<context-param>標籤配置WEB應用的初始化參數，以下所示：服務器

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
 3     http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
 4     <display-name></display-name>
 5     <!-- 配置WEB應用的初始化參數 -->
 6     <context-param>
 7         <param-name>url</param-name>
 8         <param-value>jdbc:mysql://localhost:3306/test</param-value>
 9     </context-param>
10 
11     <welcome-file-list>
12         <welcome-file>index.jsp</welcome-file>
13     </welcome-file-list>
14 </web-app>

　　獲取Web應用的初始化參數，代碼以下：app

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 import javax.servlet.ServletContext;
 5 import javax.servlet.ServletException;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 
10 
11 public class ServletContextDemo3 extends HttpServlet {
12 
13     public void doGet(HttpServletRequest request, HttpServletResponse response)
14             throws ServletException, IOException {
15 
16         ServletContext context = this.getServletContext();
17         //獲取整個web站點的初始化參數
18         String contextInitParam = context.getInitParameter("url");
19         response.getWriter().print(contextInitParam);
20     }
21 
22     public void doPost(HttpServletRequest request, HttpServletResponse response)
23             throws ServletException, IOException {
24         doGet(request, response);
25     }
26 
27 }

運行結果：

　　3.三、用servletContext實現請求轉發

ServletContextDemo4

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 import javax.servlet.RequestDispatcher;
 6 import javax.servlet.ServletContext;
 7 import javax.servlet.ServletException;
 8 import javax.servlet.http.HttpServlet;
 9 import javax.servlet.http.HttpServletRequest;
10 import javax.servlet.http.HttpServletResponse;
11 
12 public class ServletContextDemo4 extends HttpServlet {
13 
14     public void doGet(HttpServletRequest request, HttpServletResponse response)
15             throws ServletException, IOException {
16         String data = "<h1><font color='red'>abcdefghjkl</font></h1>";
17         response.getOutputStream().write(data.getBytes());
18         ServletContext context = this.getServletContext();//獲取ServletContext對象
19         RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//獲取請求轉發對象(RequestDispatcher)
20         rd.forward(request, response);//調用forward方法實現請求轉發
21     }
22 
23     public void doPost(HttpServletRequest request, HttpServletResponse response)
24             throws ServletException, IOException {
25     }
26 }

ServletContextDemo5

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 import javax.servlet.ServletException;
 5 import javax.servlet.http.HttpServlet;
 6 import javax.servlet.http.HttpServletRequest;
 7 import javax.servlet.http.HttpServletResponse;
 8 
 9 public class ServletContextDemo5 extends HttpServlet {
10 
11     public void doGet(HttpServletRequest request, HttpServletResponse response)
12             throws ServletException, IOException {
13         response.getOutputStream().write("servletDemo5".getBytes());
14     }
15 
16     public void doPost(HttpServletRequest request, HttpServletResponse response)
17             throws ServletException, IOException {
18         this.doGet(request, response);
19     }
20 
21 }

　　運行結果：

　　訪問的是ServletContextDemo4，瀏覽器顯示的倒是ServletContextDemo5的內容，這就是使用ServletContext實現了請求轉發

　　3.四、利用ServletContext對象讀取資源文件

　　項目目錄結構以下：

代碼範例：使用servletContext讀取資源文件

  1 package gacl.servlet.study;
  2 
  3 import java.io.FileInputStream;
  4 import java.io.FileNotFoundException;
  5 import java.io.IOException;
  6 import java.io.InputStream;
  7 import java.text.MessageFormat;
  8 import java.util.Properties;
  9 import javax.servlet.ServletException;
 10 import javax.servlet.http.HttpServlet;
 11 import javax.servlet.http.HttpServletRequest;
 12 import javax.servlet.http.HttpServletResponse;
 13 
 14 /**
 15  * 使用servletContext讀取資源文件
 16  * 
 17  * @author gacl
 18  * 
 19  */
 20 public class ServletContextDemo6 extends HttpServlet {
 21 
 22     public void doGet(HttpServletRequest request, HttpServletResponse response)
 23             throws ServletException, IOException { 
 24         /**
 25          * response.setContentType("text/html;charset=UTF-8");目的是控制瀏覽器用UTF-8進行解碼；
 26          * 這樣就不會出現中文亂碼了
 27          */
 28         response.setHeader("content-type","text/html;charset=UTF-8");
 29         readSrcDirPropCfgFile(response);//讀取src目錄下的properties配置文件
 30         response.getWriter().println("<hr/>");
 31         readWebRootDirPropCfgFile(response);//讀取WebRoot目錄下的properties配置文件
 32         response.getWriter().println("<hr/>");
 33         readPropCfgFile(response);//讀取src目錄下的db.config包中的db3.properties配置文件
 34         response.getWriter().println("<hr/>");
 35         readPropCfgFile2(response);//讀取src目錄下的gacl.servlet.study包中的db4.properties配置文件
 36         
 37     }
 38 
 39     /**
 40      * 讀取src目錄下的gacl.servlet.study包中的db4.properties配置文件
 41      * @param response
 42      * @throws IOException
 43      */
 44     private void readPropCfgFile2(HttpServletResponse response)
 45             throws IOException {
 46         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");
 47         Properties prop = new Properties();
 48         prop.load(in);
 49         String driver = prop.getProperty("driver");
 50         String url = prop.getProperty("url");
 51         String username = prop.getProperty("username");
 52         String password = prop.getProperty("password");
 53         response.getWriter().println("讀取src目錄下的gacl.servlet.study包中的db4.properties配置文件：");
 54         response.getWriter().println(
 55                 MessageFormat.format(
 56                         "driver={0},url={1},username={2},password={3}", 
 57                         driver,url, username, password));
 58     }
 59 
 60     /**
 61      * 讀取src目錄下的db.config包中的db3.properties配置文件
 62      * @param response
 63      * @throws FileNotFoundException
 64      * @throws IOException
 65      */
 66     private void readPropCfgFile(HttpServletResponse response)
 67             throws FileNotFoundException, IOException {
 68         //經過ServletContext獲取web資源的絕對路徑
 69         String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");
 70         InputStream in = new FileInputStream(path);
 71         Properties prop = new Properties();
 72         prop.load(in);
 73         String driver = prop.getProperty("driver");
 74         String url = prop.getProperty("url");
 75         String username = prop.getProperty("username");
 76         String password = prop.getProperty("password");
 77         response.getWriter().println("讀取src目錄下的db.config包中的db3.properties配置文件：");
 78         response.getWriter().println(
 79                 MessageFormat.format(
 80                         "driver={0},url={1},username={2},password={3}", 
 81                         driver,url, username, password));
 82     }
 83 
 84     /**
 85      * 經過ServletContext對象讀取WebRoot目錄下的properties配置文件
 86      * @param response
 87      * @throws IOException
 88      */
 89     private void readWebRootDirPropCfgFile(HttpServletResponse response)
 90             throws IOException {
 91         /**
 92          * 經過ServletContext對象讀取WebRoot目錄下的properties配置文件
 93          * 「/」表明的是項目根目錄
 94          */
 95         InputStream in = this.getServletContext().getResourceAsStream("/db2.properties");
 96         Properties prop = new Properties();
 97         prop.load(in);
 98         String driver = prop.getProperty("driver");
 99         String url = prop.getProperty("url");
100         String username = prop.getProperty("username");
101         String password = prop.getProperty("password");
102         response.getWriter().println("讀取WebRoot目錄下的db2.properties配置文件：");
103         response.getWriter().print(
104                 MessageFormat.format(
105                         "driver={0},url={1},username={2},password={3}", 
106                         driver,url, username, password));
107     }
108 
109     /**
110      * 經過ServletContext對象讀取src目錄下的properties配置文件
111      * @param response
112      * @throws IOException
113      */
114     private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {
115         /**
116          * 經過ServletContext對象讀取src目錄下的db1.properties配置文件
117          */
118         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");
119         Properties prop = new Properties();
120         prop.load(in);
121         String driver = prop.getProperty("driver");
122         String url = prop.getProperty("url");
123         String username = prop.getProperty("username");
124         String password = prop.getProperty("password");
125         response.getWriter().println("讀取src目錄下的db1.properties配置文件：");
126         response.getWriter().println(
127                 MessageFormat.format(
128                         "driver={0},url={1},username={2},password={3}", 
129                         driver,url, username, password));
130     }
131 
132     public void doPost(HttpServletRequest request, HttpServletResponse response)
133             throws ServletException, IOException {
134         this.doGet(request, response);
135     }
136 
137 }

運行結果以下：

代碼範例：使用類裝載器讀取資源文件

  1 package gacl.servlet.study;
  2 
  3 import java.io.FileOutputStream;
  4 import java.io.IOException;
  5 import java.io.InputStream;
  6 import java.io.OutputStream;
  7 import java.text.MessageFormat;
  8 import java.util.Properties;
  9 
 10 import javax.servlet.ServletException;
 11 import javax.servlet.http.HttpServlet;
 12 import javax.servlet.http.HttpServletRequest;
 13 import javax.servlet.http.HttpServletResponse;
 14 
 15 /**
 16  * 用類裝載器讀取資源文件
 17  * 經過類裝載器讀取資源文件的注意事項:不適合裝載大文件，不然會致使jvm內存溢出
 18  * @author gacl
 19  *
 20  */
 21 public class ServletContextDemo7 extends HttpServlet {
 22 
 23     public void doGet(HttpServletRequest request, HttpServletResponse response)
 24             throws ServletException, IOException {
 25         /**
 26          * response.setContentType("text/html;charset=UTF-8");目的是控制瀏覽器用UTF-8進行解碼；
 27          * 這樣就不會出現中文亂碼了
 28          */
 29         response.setHeader("content-type","text/html;charset=UTF-8");
 30         test1(response);
 31         response.getWriter().println("<hr/>");
 32         test2(response);
 33         response.getWriter().println("<hr/>");
 34         //test3();
 35         test4();
 36         
 37     }
 38     
 39     /**
 40      * 讀取類路徑下的資源文件
 41      * @param response
 42      * @throws IOException
 43      */
 44     private void test1(HttpServletResponse response) throws IOException {
 45         //獲取到裝載當前類的類裝載器
 46         ClassLoader loader = ServletContextDemo7.class.getClassLoader();
 47         //用類裝載器讀取src目錄下的db1.properties配置文件
 48         InputStream in = loader.getResourceAsStream("db1.properties");
 49         Properties prop = new Properties();
 50         prop.load(in);
 51         String driver = prop.getProperty("driver");
 52         String url = prop.getProperty("url");
 53         String username = prop.getProperty("username");
 54         String password = prop.getProperty("password");
 55         response.getWriter().println("用類裝載器讀取src目錄下的db1.properties配置文件：");
 56         response.getWriter().println(
 57                 MessageFormat.format(
 58                         "driver={0},url={1},username={2},password={3}", 
 59                         driver,url, username, password));
 60     }
 61 
 62     /**
 63      * 讀取類路徑下面、包下面的資源文件
 64      * @param response
 65      * @throws IOException
 66      */
 67     private void test2(HttpServletResponse response) throws IOException {
 68         //獲取到裝載當前類的類裝載器
 69         ClassLoader loader = ServletContextDemo7.class.getClassLoader();
 70         //用類裝載器讀取src目錄下的gacl.servlet.study包中的db4.properties配置文件
 71         InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties");
 72         Properties prop = new Properties();
 73         prop.load(in);
 74         String driver = prop.getProperty("driver");
 75         String url = prop.getProperty("url");
 76         String username = prop.getProperty("username");
 77         String password = prop.getProperty("password");
 78         response.getWriter().println("用類裝載器讀取src目錄下的gacl.servlet.study包中的db4.properties配置文件：");
 79         response.getWriter().println(
 80                 MessageFormat.format(
 81                         "driver={0},url={1},username={2},password={3}", 
 82                         driver,url, username, password));
 83     }
 84     
 85     /**
 86      * 經過類裝載器讀取資源文件的注意事項:不適合裝載大文件，不然會致使jvm內存溢出
 87      */
 88     public void test3() {
 89         /**
 90          * 01.avi是一個150多M的文件，使用類加載器去讀取這個大文件時會致使內存溢出：
 91          * java.lang.OutOfMemoryError: Java heap space
 92          */
 93         InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");
 94         System.out.println(in);
 95     }
 96     
 97     /**
 98      * 讀取01.avi,並拷貝到e:\根目錄下
 99      * 01.avi文件太大，只能用servletContext去讀取
100      * @throws IOException
101      */
102     public void test4() throws IOException {
103         // path=G:\Java學習視頻\JavaWeb學習視頻\JavaWeb\day05視頻\01.avi
104         // path=01.avi
105         String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");
106         /**
107          * path.lastIndexOf("\\") + 1是一個很是絕妙的寫法
108          */
109         String filename = path.substring(path.lastIndexOf("\\") + 1);//獲取文件名
110         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi");
111         byte buffer[] = new byte[1024];
112         int len = 0;
113         OutputStream out = new FileOutputStream("e:\\" + filename);
114         while ((len = in.read(buffer)) > 0) {
115             out.write(buffer, 0, len);
116         }
117         out.close();
118         in.close();
119     }
120 
121     public void doPost(HttpServletRequest request, HttpServletResponse response)
122             throws ServletException, IOException {
123 
124         this.doGet(request, response);
125     }
126 
127 }

　　運行結果以下：

4、在客戶端緩存Servlet的輸出

　　對於不常常變化的數據，在servlet中能夠爲其設置合理的緩存時間值，以免瀏覽器頻繁向服務器發送請求，提高服務器的性能。例如：

 1 package gacl.servlet.study;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.ServletException;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 
10 public class ServletDemo5 extends HttpServlet {
11 
12     public void doGet(HttpServletRequest request, HttpServletResponse response)
13             throws ServletException, IOException {
14         String data = "abcddfwerwesfasfsadf";
15         /**
16          * 設置數據合理的緩存時間值，以免瀏覽器頻繁向服務器發送請求，提高服務器的性能
17          * 這裏是將數據的緩存時間設置爲1天
18          */
19         response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000); 20         response.getOutputStream().write(data.getBytes());
21     }
22 
23     public void doPost(HttpServletRequest request, HttpServletResponse response)
24             throws ServletException, IOException {
25 
26         this.doGet(request, response);
27     }
28 
29 }