读取web.xml的配置参数

1.新建Servlet：

package com.wch.sp.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class RequestServlet extends HttpServlet {

	/**
	 * Constructor of the object.
	 */
	public RequestServlet() {
		super();
	}

	/**
	 * Destruction of the servlet. 

	 */
	public void destroy() {
		super.destroy(); 
	}
	
	
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		
		response.setCharacterEncoding("utf-8");
		request.setCharacterEncoding("utf-8");
		response.setContentType("text/html");
		
		PrintWriter out = response.getWriter();
		out.println("");
		out.println("");
		out.println("  ");
		out.println("  ");
		out.println(request.getRequestURI());
		String requestURI=request.getRequestURI();
		out.print("
");
		out.print("帐号:
");
		out.print("密码:
");
		out.println("");
		out.println("
");
		out.println("  ");
		out.println("");
		out.flush();
		out.close();
	}
	
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException
	{		
		response.setContentType("text/html");
		
		String username = request.getParameter("username");
		String password= request.getParameter("password");
		
		Enumeration params=this.getInitParameterNames();
		
		while(params.hasMoreElements())
		{
			String usernameParame = params.nextElement().toString();
			String passwordParame =this.getInitParameter(usernameParame);
			if(username.equals(usernameParame)&&password.equals(passwordParame))
			{
				request.getRequestDispatcher("/WEB-INF/noticde.html").forward(request, response);
				return;				
			}
		}
		
		this.doGet(request, response);
	}

	/**
	 * Initialization of the servlet. 

	 *
	 * @throws ServletException if an error occurs
	 */
	public void init() throws ServletException {
		// Put your code here
	}

}

2.配置web.xml文件：

  
    index.jsp
  
  
    
    FirstServlet
    com.wch.sp.servlet.FirstServlet
    
    
        message
        utf-8
    
    
    1

  
    RequestServlet
    com.wch.sp.servlet.RequestServlet
    
 		admin
 		admin
 	
 	
 		babyface
 		123
 	
  



    FirstServlet

    /servlet/firstServlet


  RequestServlet
  /servlet/RequestServlet  

3.在WEB-INF下面建立noticde.html

  
    
	
    
    
    
    
    

  
  
  
    HELLO
  

4.发布项目至服务器，启动服务器，访问： http://localhost:8080/Servlet/servlet/RequestServlet

结果为：

登录后：

总结：

         1.在web.xml配置文件中配置Servlet的时候，可以给Servlet配置初始化参数，使用标签类配置。配置完毕之后，Servlet提供方法：getInitParameterNames()或者getInitParameterName（String name）方法获取初始化参数值。这些初始化参数也可以使用ServletConfig对象获取，Servlet提供getServletConfig()方法获取ServletConfig对象。由ServletConfig取初始化参数与Servlet直接取方式一样。

        2.初始化参数的好处就是可以把某些变量拿到web.xml中配置，需要修改时只需要修改web.xml文件中重启服务器即可，而不是需要修改Servlet类

        3.由此扩展到上下文参数()：由于是配置在标签里的，只能由这个Servlet来读取，因此它不是全局参数，不能被其它的Servlet读取，这时可以配置上下文参数。获取context-param可以使用ServletContext对象，Servlet中可以通过getServletConfig（).getServletContext()来获取一个ServletContext对象，使用ServletContext的getInitParameter()方法来获取指定名称的参数，通过getInitParameters（）获取所有的context-param参数名称