JavaWeb study notes 20/10/21HttpServletResponse

HttpServletResponse

The Web server receives the Http request from the client, and for this request, it creates an HttpServletRequest object representing the request, representing a corresponding HttpServletResponse;

• If you want to get the parameters requested by the client: find HttpServletRequest
• If you want to give the client some information: find HttpServletResponse

1. Simple classification

The method responsible for sending data to the browser

servletOutputStream getoutputstream() throws IOException;
PrintWriter getWriter() throws IOException;

The method responsible for sending response headers to the browser

void setCharacterEncoding(java.lang.String s);

void setContentLength(int i);

void setContentLengthLong(long l);

void setContentType(java.lang.String s);

void setDateHeader(java.lang.String s, long l);

void addDateHeader(java.lang.String s, long l);

void setHeader(java.lang.String s, java.lang.String s1);

void addHeader(java.lang.String s, java.lang.String s1);

void setIntHeader(java.lang.String s, int i);

void addIntHeader(java.lang.String s, int i);

Response status code

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;

2. Common applications

Output a message to the browser

download file

• To get the path of the downloaded file
• Downloaded file name
• Set the browser to support downloading
• Get the input stream of the downloaded file
• Create buffer
• Get OutputStream object
• Write FileOutputStream stream to buffer buffer
• Use OutputStream to output the data in the buffer to the client

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\\wswck\\IdeaProjects\\javaweb-02-servlet\\response\\src\\main\\resources\\姜子牙.png";
        System.out.println("下载文件的路径:"+realPath);
        //2.下载的文件名称
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        //3.设置让浏览器支持(Content-Disposition)下载所需,中文文件名URLEncoder.encode编码，否则可能乱码
        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buffer缓冲区,使用OutputStream将缓冲区中的数据输出到客户端
        while((len=in.read(buffer))>0){
    
    
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }

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

Verification code function

• Front end implementation
• Back-end implementation, need to use Java image class

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","3");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//笔
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.ITALIC,20));
        g.drawString(makeNum(),00,20);
        //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
        ImageIO.write(image,"jpg", resp.getOutputStream());
    }
        //生成随机数
        private String makeNum(){
    
    
        Random random = new Random();
        String num = random.nextInt(9999999) + "";
        StringBuffer sb = new StringBuffer();
        for(int i = 0; i < 7-num.length() ; i++){
    
    
            sb.append("0");
        }
            num = sb.toString() + num;
            return num;
        }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

Implement redirection

After B receives a request from client A for a web resource, B will notify client A to access another web resource C. This process is called redirection

Common scenario: user login

void sendRedirect(String var1) throws IOException;

test:

public class RedirectServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        /*
        resp.setHeader("Location","/r/img");
        resp.setStatus(302);
         */
        resp.sendRedirect("/response_war/img");//重定向
    }
}

Interview question: Could you talk about the difference between redirection and forwarding?

​ Similarities

• Page will jump

​ Difference

• When the request is forwarded, the url will not change
• When redirecting, the URL address bar will change

Simple implementation of login redirection

public class RedirectServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        /*
        resp.setHeader("Location","/r/img");
        resp.setStatus(302);
         */
        resp.sendRedirect("/response_war/img");//重定向
    }
}

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.println(username+":"+password);

        //重定向时候一定要注意，路径问题，否则404；
        resp.sendRedirect("/response_war/success.jsp");
    }
}

Front-end code

<html>
<body>
<h2>Hello World!</h2>
<%--这里提交的路径，需要寻找到项目的路径--%>
<%--${pageContext.request.contextPath}代表当前的项目--%>

<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>
    <input type="submit">

</form>
</body>
</html>

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