Servlet原理

Servlet是由Web服务器调用，Web服务器在收到浏览器请求之后，会：

Mapping问题

一个Servlet可以指定一个映射路径

  
    hello
    /hello
  

 一个Servlet可以指定多个映射路径

  
    hello
    /hello
  
  
    hello
    /hello2
  
  
    hello
    /hello3
  
  
    hello
    /hello4
  
  
    hello
    /hello5
  

 一个Servlet可以指定通用映射路径

  
    hello
    /hello/*
  

 默认请求路径映射

 
     hello
     /*
 

 自定义后缀实现请求映射，注意点，*前面不能加映射的路径

          
            hello
            *.zzc
          

errorservlet例子：

package com.kuang.servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
public class ErrorServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("UTF-8");
        PrintWriter writer = resp.getWriter();
        writer.print("
404
");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

//在servlet请求文件中写上：
  
    error
    com.kuang.servlet.ErrorServlet
  
  
    error
    /*
  

 优先级问题：指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求；

ServletContext

web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前web应用；

• 共享数据：我在这个Servlet中保存的数据，可以在另外一个servlet中拿到；

  

      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
  //        this.getInitParameter()    初始化参数
  //        this.getServletConfig()    Servlet配置
  //        this.getServletContext()   Servlet上下文
          ServletContext context = this.getServletContext();
          String username = "zzc";//数据
          context.setAttribute("username", username);//将一个数据保存在了ServletContext中，名字为：username、值：username
          System.out.println("Hello");
      }
  
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          //该步骤用于获取servlet01存放的context
          ServletContext context = this.getServletContext();
          String username = (String) context.getAttribute("username");
          //下面两个是用来识别中文汉字，以至于不会出现乱码
          resp.setContentType("text/html");
          resp.setCharacterEncoding("UTF-8");
          //注意在获取context前，首先需要让servlet01放入数据，我们再访问servlet02的网络
          resp.getWriter().println("名字"+username);
      }
  
  
      
        hello
        com.kuang.servlet.HelloServlet
      
      
        hello
        /hello
      
  
      
          getc
          com.kuang.servlet.GetServlet
      
      
          getc
          /getc
      

  测试访问结果：

  

  • 获取初始化参数

        
            url
            jdbc:mysql://localhost:3306/mybatis
        
     @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            //该步骤用于获取servlet01存放的context
            ServletContext context = this.getServletContext();
            String url = context.getInitParameter("url");
            resp.getWriter().print(url);
        }

    • 请求转发

       他路径是sd4，但是请求了/gp，所以就显示了gp的内容

          
              sd4
              com.kuang.servlet.ServletDome04
          
          
              sd4
              /sd4
          
      
          @Override
          protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              ServletContext context = this.getServletContext();
              RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//转发的请求路径
              //调用forward实现请求转发；
              requestDispatcher.forward(req, resp);
          }
          @Override
          protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              super.doPost(req, resp);
          }
      }

      

      转发概念：

      重定向概念：

      

      • 读取资源文件

        Properties

        • 在java目录下新建properties
        • 在resources目录下新建properties

          发现：都被打包到同一路径下：classes，我们俗称这个路径为classpath

          

          

          思路：构建一个 文件流

              
                  sd5
                  com.kuang.servlet.ServletDome05
              
              
                  sd5
                  /sd5
              
          
              @Override
              protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                  //该步骤用于获取所需东西，然后把资源变成一个流
                  InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");//第一个/表示模块路径（当前web项目）
                  Properties prop= new Properties();
                  //load()加载数据
                  prop.load(is);
                  String user = prop.getProperty("username");
                  String pwd = prop.getProperty("password");
                  resp.getWriter().print(user+":"+pwd);
              }
              @Override
              protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              }