JavaWeb学习-Servlet系列-7-Servlet获取配置信息

桃扇骨 2022-02-03 13:23 288阅读 0赞

本篇来学习下Servlet获取配置信息，前面我们学习Servlet的生命周期方法，知道有一个getServletConfig()和一个init(servlet config)，ServletConfig是一个对象，通过这个对象，我们可以获取在web.xml中的配置信息，这些信息包括变量和变量的值。获取ServletConfig对象的方法有好几种。

1.在web.xml中的servlet对象添加一个变量

<?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>servletDemo3</servlet-name>
        <servlet-class>com.anthony.servlet.ServletDemo3</servlet-class>
        <init-param>
        	<param-name>encoding</param-name>
        	<param-value>UTF-8</param-value>
        </init-param>
      </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>

2.通过ServletConfig对象获取这变量的值

我们自定义的ServletDemo3中重写了init()方法，从init()方法拿到了服务器创建的ServletConfig对象，然后我们在doGet()方法中获取web.xml中的变量的值。

package com.anthony.servlet;
    
    import java.io.IOException;
    
    import javax.servlet.ServletConfig;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class ServletDemo3 extends HttpServlet {
    	
    	private ServletConfig config;
    
    	@Override
    	public void init(ServletConfig config) throws ServletException {
    		this.config = config;
    	}
    
    	@Override
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		System.out.println("*****ServletDemo3********");
    		String encodingType = config.getInitParameter("encoding");
    		System.out.println(encodingType);
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    
    }

重新部署到tomcat服务器上，然后浏览器打开 http://localhost:8080/Servlet01/demo3

在Eclipse的日志控制台输出

*****ServletDemo3********
    UTF-8

**第二种方法得到config的对象**

这种方法是通过使用继承父类GenericServlet的方法得到ServletConfig对象。

package com.anthony.servlet;
    
    import java.io.IOException;
    
    import javax.servlet.ServletConfig;
    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 request, HttpServletResponse response) throws ServletException, IOException {
    		System.out.println("*****ServletDemo3********");
    		String encodingType = super.getInitParameter("encoding");
    		System.out.println(encodingType);
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    
    }

重新发布到tomcat，同样可以打印出变量的值。

**第三种方法得到ServletConfig对象**

第三种方法是通过我们写的子类去调用getServletConfig方法，得到config对象，然后再调用getInitParameter方法。

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 request, HttpServletResponse response) throws ServletException, IOException {
    		String encodingType = this.getServletConfig().getInitParameter("encoding");
    		System.out.println(encodingType);
    	}
    
    	@Override
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    
    }

重新部署，测试依然得到encoding的值。以上就是三种获取ServletConfig对象然后获取web.xml中变量的值的方法。