五、JavaWeb总结:Servlet开发2(ServletConfig,ServletContext)
五、JavaWeb总结:Servlet开发2(ServletConfig,ServletContext)
AAASSSSddd 发表于1年前
五、JavaWeb总结:Servlet开发2(ServletConfig,ServletContext)
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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 <servlet>
 2     <servlet-name>ServletConfigDemo1</servlet-name>
 3     <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
 4     <!--配置ServletConfigDemo1的初始化参数 -->
 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>

1.2、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     @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         //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<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 }

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 <?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应用的初始化参数 -->
 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>

  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 = "<h1><font color='red'>abcdefghjkl</font></h1>";
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

 

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