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 {
    }