五、JavaWeb总结：Servlet开发2（ServletConfig，ServletContext）-白红宇

五、JavaWeb总结：Servlet开发2（ServletConfig，ServletContext）

阅读量：6551 次

发布时间：2019-06-24

本文共 7828 字，大约阅读时间需要 26 分钟。

一、ServletConfig讲解

1.1、配置Servlet初始化参数

　　在Servlet的配置文件web.xml中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。

例如：

1 
    
      2     
     
      ServletConfigDemo1
      3     
     
      gacl.servlet.study.ServletConfigDemo1
      4     
      5     
     
       6         
      
       name
       7         
      
       admin
       8     
      9      
     
      10         
      
       password
      11         
      
       123456
      12     
     13     
     
      14         
      
       charset
      15         
      
       UTF-8
      16     
     17

1.2、通过ServletConfig获取Servlet的初始化参数

　　当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，将ServletConfig对象传递给servlet。进而，我们通过ServletConfig对象就可以得到当前servlet的初始化参数信息。

例如：

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      * 定义ServletConfig对象来接收配置的初始化参数15      */16     private ServletConfig config;17     18     /**19      * 当servlet配置了初始化参数后，web容器在创建servlet实例对象时，20      * 会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，21      * 将ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以22      * 得到当前servlet的初始化参数信息。23      */24     @Override25     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         //获取在web.xml中配置的初始化参数32         String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数       String pwd = config.getInitParameter("password");33         response.getWriter().print(paramVal +": " + pwd);36         //获取所有的初始化参数37         Enumeration
    
      e = config.getInitParameterNames();38         while(e.hasMoreElements()){39             String name = e.nextElement();40             String value = config.getInitParameter(name);41             response.getWriter().print(name + "=" + value + "
     
");42         }43     }44 45     public void doPost(HttpServletRequest request, HttpServletResponse response)46             throws ServletException, IOException {47         this.doGet(request, response);48     }49 50 }

运行结果如下：

二、ServletContext对象

　WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。

　ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。

　由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

3.1、多个Servlet通过ServletContext对象实现数据共享

　　范例：ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享

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对象中维护了ServletContext对象的引用，开发人员在编写servlet时，         * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。         */        ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象        context.setAttribute("data", data);  //将data存储到ServletContext对象中    }    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");//从ServletContext对象中取出数据        response.getWriter().print(data);    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        doGet(request, response);    }}

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，然后运行ServletContextDemo2就可以从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示

：

3.2、获取WEB应用的初始化参数

　　在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数，如下所示：

1 
     2 
    
      4     
      5     
      6     
     
       7         
      
       url
       8         
      
       jdbc:mysql://localhost:3306/test
       9     
     10

　　获取Web应用的初始化参数，代码如下：

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         //获取整个web站点的初始化参数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 }

运行结果：

3.3、用servletContext实现请求转发

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 = "abcdefghjkl
";17         response.getOutputStream().write(data.getBytes());18         ServletContext context = this.getServletContext();//获取ServletContext对象19         RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)20         rd.forward(request, response);//调用forward方法实现请求转发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 }

　　运行结果：

访问的是ServletContextDemo4，浏览器显示的却是ServletContextDemo5的内容，这就是使用ServletContext实现了请求转发。

3.4、利用ServletContext对象读取资源文件

转载于:https://my.oschina.net/tianshuo/blog/733758

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