5、JavaWeb總結：Servlet開發2（ServletConfig，ServletContext）

時間  2019-11-11
標籤 javaweb 總結 servlet 開發 servletconfig servletcontext 欄目 Java 简体版
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，可使用一個或多個<init-param>標籤爲servlet配置一些初始化參數。java

例如：mysql

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>admin</param-value>
 8     </init-param>
 9      <init-param>
10         <param-name>password</param-name>
11         <param-value>123456</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。程序員

例如：web

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
       String pwd = config.getInitParameter("password");
33         response.getWriter().print(paramVal +": " + pwd);
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：sql

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。瀏覽器

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app

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

public class ServletContextDemo1 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "ABC";
        /**
         * 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。
         */
        ServletContext context = this.getServletConfig().getServletContext();//得到ServletContext對象
        context.setAttribute("data", data);  //將data存儲到ServletContext對象中
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
}
import java.io.IOException;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletContextDemo2 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String data = (String) context.getAttribute("data");//從ServletContext對象中取出數據
        response.getWriter().print(data);
    }

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

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ide

：this

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：url

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 </web-app>

　　獲取Web應用的初始化參數，代碼以下：

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
 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
 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

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