Servlet基础（续集）

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

  <servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping>

 一个Servlet可以指定多个映射路径

  <servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello2</url-pattern></servlet-mapping><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello3</url-pattern></servlet-mapping><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello4</url-pattern></servlet-mapping><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello5</url-pattern></servlet-mapping>

 一个Servlet可以指定通用映射路径

  <servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello/*</url-pattern></servlet-mapping>

 默认请求路径映射

 <servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/*</url-pattern></servlet-mapping>

 自定义后缀实现请求映射，注意点，*前面不能加映射的路径

          <servlet-mapping><servlet-name>hello</servlet-name><url-pattern>*.zzc</url-pattern></servlet-mapping>

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 {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {resp.setContentType("text/html");resp.setCharacterEncoding("UTF-8");PrintWriter writer = resp.getWriter();writer.print("<h1>404</h1>");}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doPost(req, resp);}
}//在servlet请求文件中写上：<servlet><servlet-name>error</servlet-name><servlet-class>com.kuang.servlet.ErrorServlet</servlet-class></servlet><servlet-mapping><servlet-name>error</servlet-name><url-pattern>/*</url-pattern></servlet-mapping>

 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；

    @Overrideprotected 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：usernameSystem.out.println("Hello");}@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//该步骤用于获取servlet01存放的contextServletContext 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);}<!--先访问hello--><servlet><servlet-name>hello</servlet-name><servlet-class>com.kuang.servlet.HelloServlet</servlet-class></servlet><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping>
<!--再访问getc--><servlet><servlet-name>getc</servlet-name><servlet-class>com.kuang.servlet.GetServlet</servlet-class></servlet><servlet-mapping><servlet-name>getc</servlet-name><url-pattern>/getc</url-pattern></servlet-mapping>

测试访问结果：

• 获取初始化参数

<!--配置一些web应用初始化参数--><context-param><param-name>url</param-name><param-value>jdbc:mysql://localhost:3306/mybatis</param-value></context-param>@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//该步骤用于获取servlet01存放的contextServletContext 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

    <servlet><servlet-name>sd4</servlet-name><servlet-class>com.kuang.servlet.ServletDome04</servlet-class></servlet><servlet-mapping><servlet-name>sd4</servlet-name><url-pattern>/sd4</url-pattern></servlet-mapping>@Overrideprotected 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);}@Overrideprotected 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

    <servlet><servlet-name>sd5</servlet-name><servlet-class>com.kuang.servlet.ServletDome05</servlet-class></servlet><servlet-mapping><servlet-name>sd5</servlet-name><url-pattern>/sd5</url-pattern></servlet-mapping>@Overrideprotected 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);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {}