Servlet

1. servlet简介

• Servlet就是sun公司开发动态web的一门技术。
• Sun在这些API中提供一个接口叫做：Servlet，如果你想开发一个Servlet程序，只需要完成两个小步骤：
  1. 编写一个类，实现Servlet接口；
  2. 把开发好的Java类部署到web服务器中。
     把实现了Servlet接口Java程序叫做，Servlet

2. HelloServlet

Serlvet接口Sun公司有两个默认的实现类：HttpServlet，GenericServlet

1. 构建一个普通的Maven项目，删掉里面的src目录，以后我们的学习就在这个项目里面建立Moudel；这个空的工程就是Maven主工程；

2. 关于Maven父子工程的理解：
   父工程中会有：

   <modules>
       <module>servlet-01</module>
   </modules>

   子项目中会有：

   <parent>
       <artifactId>servlet</artifactId>
       <groupId>org.example</groupId>
       <version>1.0-SNAPSHOT</version>
   </parent>

   父项目中的jar包，子项目可以直接使用。反之，不可。

   Maven环境配置(搭配Tomcat 10)

   <dependency>
       <groupId>jakarta.servlet</groupId>
       <artifactId>jakarta.servlet-api</artifactId>
       <version>6.0.0</version>
   </dependency>
   <dependency>
       <groupId>jakarta.servlet.jsp</groupId>
       <artifactId>jakarta.servlet.jsp-api</artifactId>
       <version>3.1.1</version>
   </dependency>

3. Maven环境优化

4. 编写一个Servlet程序
   1.编写一个普通的类
   2.实现Servlet接口，这里我们直接继承HttpServlet类

   package com.hfuuwzy;
   
   import jakarta.servlet.ServletException;
   import jakarta.servlet.http.HttpServlet;
   import jakarta.servlet.http.HttpServletRequest;
   import jakarta.servlet.http.HttpServletResponse;
   
   import java.io.IOException;
   import java.io.PrintWriter;
   
   public class HelloServlet extends HttpServlet {
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           System.out.println("你好");
           PrintWriter writer = resp.getWriter();
           writer.print("hello,Servlet!!!");
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           super.doPost(req, resp);
       }
   }

5. 编写Servlet的映射
   为什么需要映射：我们写的是JAVA程序，但是要通过浏览器访问，而浏览器需要连接web服务器，所以我们需要在web服务中注册我们写的Servlet，还需给他一个浏览器能够访问的路径。

   <!--注册servlet-->
   <servlet>
       <servlet-name>hello</servlet-name>
       <servlet-class>com.hfuuwzy.HelloServlet</servlet-class>
   </servlet>
   <!--Servlet的请求路径-->
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello</url-pattern>
   </servlet-mapping>

6. 配置tomcat
   注意：配置项目发布路径就可以了

7. 启动测试

3. Servlet原理

4. Mapping问题

1. 一个Servlet可以指定一个映射路径
2. 一个Servlet可以指定多个映射路径
3. 一个Servlet可以指定通用映射路径
4. 指定一些后缀或者前缀等等…

5. 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，它代表了当前的web应用。

5.1 共享数据

在这个Servlet中保存的数据，可以在另一个Servlet中拿到

存入数据

public class HelloServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
//        this.getInitParameter() 初始化参数
//        this.getServletConfig() Servlet配置
//        this.getServletContext() Servlet配置上下文
        ServletContext servletContext = this.getServletContext();
        String username = "王朝阳"; // 数据
        servletContext.setAttribute("username",username); // 将一个数据保存在ServletContext中
    }
}

读取数据

@WebServlet(name = "getc",value = "/getc") // 此注解可以代替编写xml里的Servlet映射
public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");
        resp.setContentType("text/html;charset=utf-8"); // 设置读取的文本风格样式
        resp.getWriter().print("名字："+username);
    }
}

5.2 获取初始化参数

<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>

@WebServlet(name = "gp",value = "/gp")
public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }
}

5.3 请求转发

// 请求/sd4找到ServletDemo04，ServletDemo04进行请求转发到/gp，到/gp的页面
// (浏览器路径是sd4的路径，页面拿到的是/gp的数据)
@WebServlet(name = "sd4", value = "/sd4")
public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        System.out.println("进入了servlet04");
        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp"); // 转发的请求路径 
        requestDispatcher.forward(req, resp); // 调用forward实现请求转发
    }
}

5.4 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：都被打包到了同一个路径下：classes，我们俗称这个路径为classpath。

思路：需要一个文件流

@WebServlet(name = "sd5", value = "/sd5")
public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream inputStream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(inputStream);
        String user = prop.getProperty("username");
        String pwd = prop.getProperty("password");
        resp.getWriter().print(user+":"+pwd);
    }
}

6. HttpServletResponse

web服务器接收到客户端的http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse；|

• 如果要获取客户端请求过来的参数：找HttpServletRequest
• 如果要给客户端响应一些信息：找HttpServletResponse

1. 负责向浏览器发送数据的方法

public ServletOutputStream getOutputStream() throws IOException;
public PrintWriter getWriter() throws IOException;

2. 响应的状态码

   public static final int SC_CONTINUE = 100;
    /**
     * Status code (200) indicating the request succeeded normally.
     */
    public static final int SC_OK = 200;
    /**
     * Status code (302) indicating that the resource has temporarily
     * moved to another location, but that future references should
     * still use the original URI to access the resource.
     *
     * This definition is being retained for backwards compatibility.
     * SC_FOUND is now the preferred definition.
     */
    public static final int SC_MOVED_TEMPORARILY = 302;
    /**
    * Status code (302) indicating that the resource reside
    * temporarily under a different URI. Since the redirection might
    * be altered on occasion, the client should continue to use the
    * Request-URI for future requests.(HTTP/1.1) To represent the
    * status code (302), it is recommended to use this variable.
    */
    public static final int SC_FOUND = 302;
    /**
     * Status code (304) indicating that a conditional GET operation
     * found that the resource was available and not modified.
     */
    public static final int SC_NOT_MODIFIED = 304;
    /**
     * Status code (404) indicating that the requested resource is not
     * available.
     */
    public static final int SC_NOT_FOUND = 404;
    /**
     * Status code (500) indicating an error inside the HTTP server
     * which prevented it from fulfilling the request.
     */
    public static final int SC_INTERNAL_SERVER_ERROR = 500;
    /**
     * Status code (502) indicating that the HTTP server received an
     * invalid response from a server it consulted when acting as a
     * proxy or gateway.
     */
    public static final int SC_BAD_GATEWAY = 502;
    // ...

常见应用

1. 向浏览器输出消息

2. 下载文件

@WebServlet(name = "file",value = "/file")
public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 1.要获取下载文件的路径
        String realPath = "C:\\Users\\WZY\\Desktop\\大二下\\Web程序设计\\servlet\\response\\src\\main\\resources\\薇尔莉特.png";
        System.out.println("下载文件的路径是：" + realPath);
        // 2.下载的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        // 3.设置想办法让浏览器能够支持下载我们需要的东西
        resp.setHeader("Content-Disposition", "attachment;filename=" + URLEncoder.encode(fileName, "utf-8"));
        // 4.获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath);
        // 5.创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        // 6.获取OutputStream对象
        ServletOutputStream out = resp.getOutputStream();
        // 7.将FileOutputStream流写入到bufer缓冲区
        while ((len = in.read(buffer)) != -1) {
            out.write(buffer, 0, len);
        }
        in.close();
        out.close();
    }

    // 8.使用OutputStream将缓冲区中的数据输出到客户端！
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

3. 验证码功能

@WebServlet(name = "img",value = "/img")
public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 让浏览器3秒刷新一次
        resp.setHeader("refresh", "3");
        // 在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);// 宽、高、颜色
        // 得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();// 得到一只2D的笔
        // 设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0, 0, 80, 20);// 填充颜色
        // 换个背景颜色
        g.setColor(Color.BLUE);
        // 设置字体样式：粗体，20
        g.setFont(new Font(null, Font.BOLD, 20));
        // 画一个字符串（给图片写数据）
        g.drawString(makeNum(), 0, 20);
        // 告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        // 网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires", -1);
        resp.setHeader("Cache-Control", "no-cache");
        resp.setHeader("Pragma", "no-cache");
        // 把图片写给浏览器
        boolean write = ImageIO.write(image, "jpg", resp.getOutputStream());
    }

    // 生成随机数
    private String makeNum() {
        Random random = new Random();
        String num = random.nextInt(9999999) + "";// 随机数，最大七位，[0,9999999)
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7 - num.length(); i++) {// 不足七位，则添加0
            sb.append("0");
        }
        num = sb.toString() + num;// 不足七位，在随机数前面添加0
        return num;
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

4. 实现请求重定向

   @WebServlet(name = "Red", value = "/red")
   public class RedirectServlet extends HttpServlet {
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   /*
           resp.setHeader("Location","/res/image");
           resp.setStatus(302);
   */
           resp.sendRedirect("/res/img");
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           doGet(req, resp);
       }
   }

7. HttpServletRequest

HttpServletRequest代表客户端的请求，用户通过Http协议访问服务器，HTTP请求中的所有信息会被封装到HttpServletRequest，通过这个HttpServletRequest的方法，获得客户端的所有信息。

1. 获取前端传递的参数
   

2. 请求转发
   前端：

   <%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>
   <!DOCTYPE html>
   <html>
   <head>
       <title>登录</title>
   </head>
   <body>
   <h1>登录</h1>
   <div >
       <%--这里表单表示的意思：以post方式提交表单，提交到我们的login请求--%>
       <form action="${pageContext.request.contextPath}/login" method="post">
           用户名：<input type="text" name="username"><br>
           密码：<input type="password" name="password"><br>
           爱好：
           <input type="checkbox" name="hobbys" value="代码"> 代码
           <input type="checkbox" name="hobbys" value="唱歌"> 唱歌
           <input type="checkbox" name="hobbys" value="女孩"> 女孩
           <input type="checkbox" name="hobbys" value="电影"> 电影
           <br>
           <input type="submit" name="提交">
       </form>
   </div>
   </body>
   </html>

   后端：

   @WebServlet(name = "LoginServlet",value = "/login")
   public class LoginServlet extends HttpServlet {
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           String username = req.getParameter("username");
           String password = req.getParameter("password");
           String[] hobbys = req.getParameterValues("hobbys");
           System.out.println("------------------------");
           System.out.println(username);
           System.out.println(password);
           System.out.println(Arrays.toString(hobbys));
           // 通过请求转发
   //        resp.sendRedirect("/req/success.jsp");
           req.getRequestDispatcher("/success.jsp").forward(req,resp); // 转发 / 代表当前的web应用，直接跟路径名称就行，与重定向不一样
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           doGet(req, resp);
       }
   }

面试题：请你聊聊重定向和转发的区别？
相同点：页面都会实现跳转
不同点：

• 请求转发的时候，url地址栏不会产生变化。307
• 重定向时候，url地址栏会发生变化。302