重回JavaWeb-Servlet

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：
  • 编写一个类，实现Servlet接口
  • 把开发好的Java类部署到web服务器中
    把实现了Servlet接口的Java程序就叫做，Servlet

HelloServlet

• 构建一个Maven项目
  
• 编写HelloServlet继承HttpServlet

    public class HelloServlet extends HttpServlet {
  
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            this.doPost(req, resp);
        }
  
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            PrintWriter writer = resp.getWriter();
            writer.print("Hello");
            writer.write("World");
        }
    }
  

• 编写Servlet映射

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

• 配置Tomcat
• 启动测试

ServletContext对象

web容器在启动时，会为每个web程序创建一个对应的ServletContext对象，他代表了当前的web应用；
- 共享数据
我在这个Servlet中保存的数据，可以再另外一个Servlet中拿到；
-

1. 下载文件
   - 获取文件路径
   - 下载的文件名是啥
   - 想办法设置让浏览器能够支持下载我们需要的东西
   - 获取下载文件的输入流
   - 创建缓冲区
   - 获取OutputStream对象
   - 将FileOutputStream流写入到buffer缓冲区
   - 使用OutputStream将缓冲区的数据输出到客户端

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;
public class DownloadServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String realPath = this.getServletContext().getRealPath("/WEB-INF/classes/面经--倾听潮汐.pdf");
        System.out.println("下载路径: " + realPath);
        String fileName = realPath.substring(realPath.lastIndexOf('\\') + 1);
        resp.setContentType("text/html; charset=utf-8");
        resp.setHeader("Content-disposition","attachment;filename="+ URLEncoder.encode(fileName, "UTF-8"));
        FileInputStream inputStream = new FileInputStream(realPath);

        int len = 0;
        byte[] bytes = new byte[1024];
        ServletOutputStream outputStream = resp.getOutputStream();
        while ((len = inputStream.read(bytes)) > 0){
            outputStream.write(bytes, 0, len);
        }

        outputStream.close();
        inputStream.close();
    }
}

mport javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

public class ImageServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器5秒刷新一次
        resp.setHeader("refresh", "5");

        //在内存中创建图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D graphics = (Graphics2D) image.getGraphics(); //笔
        //画笔颜色
        graphics.setColor(Color.white);
        graphics.fillRect(0, 0, 80, 20);
        //给图片写数据
        graphics.setColor(Color.BLUE);
        graphics.setFont(new Font(null, Font.BOLD, 20));
        graphics.drawString(randomNum(), 0, 20);
        //高速浏览器，这个请求用图片方式打开
        resp.setContentType("image/png");
        //网站存在缓存， 不让网站有缓存
        resp.setDateHeader("expires", -1);
        resp.setHeader("Cache-Control", "no-cache");

        //把图片写给浏览器
        ImageIO.write(image, "png",resp.getOutputStream());
    }

    private String randomNum(){
        Random random = new Random();
        String num = random.nextInt(9999) + "";
        StringBuffer stringBuffer = new StringBuffer();
        for (int i = 0; i < 4 - num.length(); i++) {
            stringBuffer.append("0");
        }
        return stringBuffer.toString() + num;
    }
}

https://blog.csdn.net/qq_36791569/article/details/80152638

重定向

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        resp.sendRedirect("/");
        resp.setHeader("location", "/");
        resp.setStatus(302);
    }

面试题：重定向和请求转发的区别

Cookie

/保存用户上一次访问的时间
public class CookieServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html; charset=utf-8");
        PrintWriter out = resp.getWriter();
        //服务器端从客户端获取
        Cookie[] cookies = req.getCookies();
        //判断是否存在
        if (cookies != null && cookies.length != 0){

            for (int i = 0; i < cookies.length; i++) {
                if (cookies[i] != null){
                    if ("lastLogin".equals(cookies[i].getName())) {
                        out.write("上一次访问的时间是" + cookies[i].getValue());
                    }
                }
            }
        }else {
            out.write("欢迎登陆");
        }
            //服务器给客户端响应一个cookie
        Cookie lastLogin = new Cookie("lastLogin", new Date().getTime() + "");
        lastLogin.setMaxAge(3);
        resp.addCookie(lastLogin);
    }
}

Session

/保存用户上一次访问的时间
public class CookieServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html; charset=utf-8");
        PrintWriter out = resp.getWriter();
        //服务器端从客户端获取
        Cookie[] cookies = req.getCookies();
        //判断是否存在
        if (cookies != null && cookies.length != 0){

            for (int i = 0; i < cookies.length; i++) {
                if (cookies[i] != null){
                    if ("lastLogin".equals(cookies[i].getName())) {
                        out.write("上一次访问的时间是" + cookies[i].getValue());
                    }
                }
            }
        }else {
            out.write("欢迎登陆");
        }
            //服务器给客户端响应一个cookie
        Cookie lastLogin = new Cookie("lastLogin", new Date().getTime() + "");
        lastLogin.setMaxAge(3);
        resp.addCookie(lastLogin);
    }
}