JavaWeb学习-Servlet系列-8-ServletContext作为域对象和获取全局配置信息

女爷i 2022-02-03 18:47 226阅读 0赞

继续学习Servlet的知识，这篇来学习ServletContext接口。在一个web服务器中，一个应用只能有一个ServletContext对象，这个对象就代表当前整个应用。ServletContext主要有这几个功能：作为域对象，多个Servlet之间共享数据；获取全局配置信息；第三个应用就是获取资源路径。本篇我们来学习下第一个功能，作为域对象，能让多个Servlet之间共享数据。

1.项目准备

创建一个web动态项目，注意勾选创建web.xml文件，然后新建两个Servlet类继承HttpServer. 然后在web.xml中给Server1和Sevlet2配置映射。

ServletDemo1.java

package com.anthony.servlet;
    
    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 ServletDemo1 extends HttpServlet {
    
    	@Override
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		//调用GenericServlet类中的getServletContext()方法得到ServletContext对象
    		ServletContext appliction = this.getServletContext();
    		//向ServletContext对象中添加一个键值对
    		appliction.setAttribute("Name", "Anthony");
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    
    }

ServletDemo2.java

package com.anthony.servlet;
    
    import java.io.IOException;
    
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class ServletDemo2 extends HttpServlet {
    
    	@Override
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		//得到属性Name的value
    		String name = (String) this.getServletContext().getAttribute("Name");
    		System.out.println(name);
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    
    }

web.xml配置两个servlet映射。

<?xml version="1.0" encoding="UTF-8"?>
    <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
    
      <servlet>
        <servlet-name>servletDemo1</servlet-name>
        <servlet-class>com.anthony.servlet.ServletDemo1</servlet-class>
      </servlet>
      <servlet-mapping>
        <servlet-name>servletDemo1</servlet-name>
        <url-pattern>/demo1</url-pattern>
      </servlet-mapping>
      
      <servlet>
        <servlet-name>servletDemo2</servlet-name>
        <servlet-class>com.anthony.servlet.ServletDemo2</servlet-class>
      </servlet>
      <servlet-mapping>
        <servlet-name>servletDemo2</servlet-name>
        <url-pattern>/demo2</url-pattern>
      </servlet-mapping>
    
      <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
        <welcome-file>default.html</welcome-file>
        <welcome-file>default.htm</welcome-file>
        <welcome-file>default.jsp</welcome-file>
      </welcome-file-list>
    </web-app>

2.发布和测试

.在Eclipse上的server tab重新发布到服务器，然后浏览器开两个tab，分别打开下面url

[http://localhost:8080/Servlet01/demo1][http_localhost_8080_Servlet01_demo1]

[http://localhost:8080/Servlet01/demo2][http_localhost_8080_Servlet01_demo1]

运行第一个就运行了ServletDemo1中的appliction.setAttribute("Name", "Anthony");

运行第二个就执行了ServletDemo2中的String name = (String) this.getServletContext().getAttribute("Name");

所以Eclipse上日志控制台输出是：Anthony

这个就证明了ServletContext对象作为域对象，可以让多个Servlet对象之间共享数据。

获取全局配置信息

解释以下在web.xml中什么是全局配置信息，什么是局部配置信息

<?xml version="1.0" encoding="UTF-8"?>
    <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
      
      
      <context-param>
      	<param-name>encoding</param-name>
      	<param-value>GBK</param-value>
      </context-param>
      
      <servlet>
        <servlet-name>servletDemo3</servlet-name>
        <servlet-class>com.anthony.servlet.ServletDemo3</servlet-class>
      </servlet>
      <servlet-mapping>
        <servlet-name>servletDemo3</servlet-name>
        <url-pattern>/demo3</url-pattern>
      </servlet-mapping>
      <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
        <welcome-file>default.html</welcome-file>
        <welcome-file>default.htm</welcome-file>
        <welcome-file>default.jsp</welcome-file>
      </welcome-file-list>
    </web-app>

上面分别又一组全局配置信息(context-param标签范围) 和局部配置信息（servlet标签范围），本篇演示ServletContext获取context-parma标签内的全局配置信息

package com.anthony.servlet;
    
    import java.io.IOException;
    
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class ServletDemo3 extends HttpServlet {
    
    	@Override
    	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    		String encode = this.getServletContext().getInitParameter("encoding");
    		System.out.println(encode);
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    		doGet(req, resp);
    	}
    	
    }

重新部署到服务器，然后测试浏览器打开地址[http://localhost:8080/Servlet01/demo3][http_localhost_8080_Servlet01_demo3]

信息: Server startup in 559 ms
    GBK

由于前面我们学习过获取servlet中局部变量的方法，所以这里后去全局变量就很简单。

[http_localhost_8080_Servlet01_demo1]: http://localhost:8080/Servlet01/demo1
[http_localhost_8080_Servlet01_demo3]: http://localhost:8080/Servlet01/demo3