JavaWeb之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HtttpServletResponse

简单分类

负责向浏览器发送数据的方法

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

负责向浏览器发送响应头的方法

    void setCharacterEncoding(String var1);

    void setContentLength(int var1);

    void setContentLengthLong(long var1);

    void setContentType(String var1);

    void setDateHeader(String var1, long var2);

    void addDateHeader(String var1, long var2);

    void setHeader(String var1, String var2);

    void addHeader(String var1, String var2);

    void setIntHeader(String var1, int var2);

    void addIntHeader(String var1, int var2);

响应的状态码

    int SC_CONTINUE = 100;
    int SC_SWITCHING_PROTOCOLS = 101;
    int SC_OK = 200;
    int SC_CREATED = 201;
    int SC_ACCEPTED = 202;
    int SC_NON_AUTHORITATIVE_INFORMATION = 203;
    int SC_NO_CONTENT = 204;
    int SC_RESET_CONTENT = 205;
    int SC_PARTIAL_CONTENT = 206;
    int SC_MULTIPLE_CHOICES = 300;
    int SC_MOVED_PERMANENTLY = 301;
    int SC_MOVED_TEMPORARILY = 302;
    int SC_FOUND = 302;
    int SC_SEE_OTHER = 303;
    int SC_NOT_MODIFIED = 304;
    int SC_USE_PROXY = 305;
    int SC_TEMPORARY_REDIRECT = 307;
    int SC_BAD_REQUEST = 400;
    int SC_UNAUTHORIZED = 401;
    int SC_PAYMENT_REQUIRED = 402;
    int SC_FORBIDDEN = 403;
    int SC_NOT_FOUND = 404;
    int SC_METHOD_NOT_ALLOWED = 405;
    int SC_NOT_ACCEPTABLE = 406;
    int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
    int SC_REQUEST_TIMEOUT = 408;
    int SC_CONFLICT = 409;
    int SC_GONE = 410;
    int SC_LENGTH_REQUIRED = 411;
    int SC_PRECONDITION_FAILED = 412;
    int SC_REQUEST_ENTITY_TOO_LARGE = 413;
    int SC_REQUEST_URI_TOO_LONG = 414;
    int SC_UNSUPPORTED_MEDIA_TYPE = 415;
    int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
    int SC_EXPECTATION_FAILED = 417;
    int SC_INTERNAL_SERVER_ERROR = 500;
    int SC_NOT_IMPLEMENTED = 501;
    int SC_BAD_GATEWAY = 502;
    int SC_SERVICE_UNAVAILABLE = 503;
    int SC_GATEWAY_TIMEOUT = 504;
    int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

Response下载文件

1. 向浏览器输出消息
2. 下载文件
   1. 要获取下载文件的路径
   2. 下载的文件名是啥？
   3. 设置想办法浏览器能够支持下载我们需要的东西
   4. 获取下载文件的输入流
   5. 创建缓冲区
   6. 获取OUtputStream对象
   7. 将FileOutputStream流写入到buffer缓冲区
   8. 使用OutputStream将缓冲区中的数据输出到客户端

public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

//        a. 要获取下载文件的路径
        String realPath = this.getServletContext().getRealPath("WEB-INF/classes/img/头像.jpg");
        System.out.println("下载文件的路径："+realPath);
//        b. 下载的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
//        c. 设置想办法浏览器能够支持下载我们需要的东西，中文需要进行URL编码
        resp.setHeader("Content-Disposition", "attachment;filename=" + URLEncoder.encode(fileName, "UTF-8"));
//        d. 获取下载文件的输入流
        FileInputStream fileInputStream = new FileInputStream(realPath);
//        e. 创建缓冲区
        int  len = 0;
        byte[] bytes = new byte[1024];
//        f. 获取OUtputStream对象
        ServletOutputStream outputStream = resp.getOutputStream();
//        g. 将FileOutputStream流写入到buffer缓冲区
        while ((len = fileInputStream.read(bytes)) != -1) {
            outputStream.write(bytes, 0, len);
        }
//        h. 使用OutputStream将缓冲区中的数据输出到客户端
        fileInputStream.close();
        outputStream.close();
    }

}

web.xml

    <servlet>
        <servlet-name>file</servlet-name>
        <servlet-class>com.cike.www.FileServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>file</servlet-name>
        <url-pattern>/file</url-pattern>
    </servlet-mapping>·

访问浏览器可以成果看见下载文件

Response验证码实现

验证码怎么来的？

• 前端实现
• 后端实现，需要用到 Java 的图片类，生成一个图片

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        如何让浏览器5秒自动刷新1次；
        resp.setHeader("refresh","5");

//        在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
//        得到图片
        Graphics g = (Graphics2D) image.getGraphics();  //笔
//        设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
//        给图片写入数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        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，不让浏览器存在缓存
         resp.setDateHeader("Expires",-1);
         resp.setHeader("Cache-Control","no-cache");
         resp.setHeader("Pragma","no-cache");
         resp.setHeader("Cache-Control","max-age=0");

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


    }
//    生成随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(99999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i< -num.length(); i++) {
            sb.append("0");
        }
        return sb.toString() + num;
    }

}

web.xml

    <servlet>
        <servlet-name>img</servlet-name>
        <servlet-class>com.cike.www.ImageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>img</servlet-name>
        <url-pattern>/img</url-pattern>
    </servlet-mapping>

Response重定向

B一个web资源收到客户端请求后，B他会通知A客户端去访问另外一个web资源C

，这个过程叫重定向

常见场景：

• 用户登录

    void sendRedirect(String var1) throws IOException

测试：

public class RedirectServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        /*
        resp.setHeader("Location","/response/img");
        resp.setStatus(HttpServletResponse.SC_FOUND);

        */
//        设置的路径为网站访问的真实路径，也可以是Tomcat配置的路径为开始
        resp.sendRedirect("/response/img");
    }
}

web.xml

    <servlet>
        <servlet-name>redirect</servlet-name>
        <servlet-class>com.cike.www.RedirectServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>redirect</servlet-name>
        <url-pattern>/redirect</url-pattern>
    </servlet-mapping>

面试题：请你聊一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不会产生变化
  • 重定向的时候，url地址栏会发生变化

附加登录小实验-登录重定向Success

index.jsp

<%--
  Created by IntelliJ IDEA.
  User: Love_MengYao
  Date: 2025/11/6
  Time: 11:02
  To change this template use File | Settings | File Templates.
--%>




<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Admin</title>
</head>
<body>
<h1>Hello World!</h1>

<%--这里提交的路径，需要找到项目的路径--%>
<%--${pageContext.request.contextPath}代表当前项目的路径--%>
<form action="${pageContext.request.contextPath}/login" method="get">
    用户名：<input type="text" name="username"> <br>
    密码：<input type="password" name="password"> <br>
    登录：<input type="submit">
</form>

</body>
</html>

success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Success</title>
</head>
<body>
<h1>Success</h1>

</body>
</html>

RequestTest.java

public class RequestTest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        处理请求
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.printf(username + ":" + password);

//        重定向一定要注意路径，否则显示404
        resp.sendRedirect("/response/success.jsp");
    }
}

web.xml

    <servlet>
        <servlet-name>request</servlet-name>
        <servlet-class>com.cike.www.RequestTest</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>request</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>