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；
负责向浏览器发送数据的方法↓
public PrintWriter getWriter() throws IOException;
public ServletOutputStream getOutputStream() throws IOException;
负责向浏览器发送响应头的方法↓
public void setCharacterEncoding(String charset);
public void setContentLength(int len);
public void setContentLengthLong(long len);
public void setContentType(String type);
public void setDateHeader(String name, long date);
public void addDateHeader(String name, long date);
public void setHeader(String name, String value);
public void addHeader(String name, String value);
public void setIntHeader(String name, int value);
public void addIntHeader(String name, int value);
响应的状态码↓
public static final int SC_CONTINUE = 100;
public static final int SC_SWITCHING_PROTOCOLS = 101;
public static final int SC_OK = 200;
public static final int SC_CREATED = 201;
public static final int SC_ACCEPTED = 202;
public static final int SC_NON_AUTHORITATIVE_INFORMATION = 203;
public static final int SC_NO_CONTENT = 204;
public static final int SC_RESET_CONTENT = 205;
public static final int SC_PARTIAL_CONTENT = 206;
public static final int SC_MULTIPLE_CHOICES = 300;
public static final int SC_MOVED_PERMANENTLY = 301;
public static final int SC_MOVED_TEMPORARILY = 302;
public static final int SC_FOUND = 302;
public static final int SC_SEE_OTHER = 303;
public static final int SC_NOT_MODIFIED = 304;
public static final int SC_USE_PROXY = 305;
public static final int SC_TEMPORARY_REDIRECT = 307;
public static final int SC_BAD_REQUEST = 400;
public static final int SC_UNAUTHORIZED = 401;
public static final int SC_PAYMENT_REQUIRED = 402;
public static final int SC_FORBIDDEN = 403;
public static final int SC_NOT_FOUND = 404;
public static final int SC_METHOD_NOT_ALLOWED = 405;
public static final int SC_NOT_ACCEPTABLE = 406;
public static final int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
public static final int SC_REQUEST_TIMEOUT = 408;
public static final int SC_CONFLICT = 409;
public static final int SC_GONE = 410;
public static final int SC_LENGTH_REQUIRED = 411;
public static final int SC_PRECONDITION_FAILED = 412;
public static final int SC_REQUEST_ENTITY_TOO_LARGE = 413;
public static final int SC_REQUEST_URI_TOO_LONG = 414;
public static final int SC_UNSUPPORTED_MEDIA_TYPE = 415;
public static final int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
public static final int SC_EXPECTATION_FAILED = 417;
public static final int SC_INTERNAL_SERVER_ERROR = 500;
public static final int SC_NOT_IMPLEMENTED = 501;
public static final int SC_BAD_GATEWAY = 502;
public static final int SC_SERVICE_UNAVAILABLE = 503;
public static final int SC_GATEWAY_TIMEOUT = 504;
public static final int SC_HTTP_VERSION_NOT_SUPPORTED = 505;
①获取下载文件的路径
②命名下载文件名
③设置浏览器能够支持下载
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Buffer缓冲区
⑧使用OutputStream将缓冲区中的数据输出到客户端
@WebServlet("/file")
public class FileServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//1.获取下载文件的路径
String realPath = this.getServletContext().getRealPath("/WEB-INF/classes/a.jpg");// ‘/‘表示web根目录
System.out.println("下载的文件路径：" + realPath);
//2.获取要下载的文件名
String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
//3.设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西,中文文件名URLEncoder.encode编码，否则有可能乱码
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(new File(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)) != -1) {
out.write(buffer,0,len);
}
in.close();
out.close();
}
}
@WebServlet("/yzm")
public class yzmServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//如何让浏览器3秒自动刷新一次;
// 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.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/jpg");
//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;
}
}
B一个web资源收到客户端A请求后，B会通知A去访问另外一个web资源C，这个过程叫重定向。
@WebServlet("/redir")
public class RedirectServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
/*
resp.setHeader("Location","/r/img");
resp.setStatus(302); /重定向
responseDemo_war：项目名
*/
resp.sendRedirect("/responseDemo_war/yzm");
}
}
常见场景：
@WebServlet("/login")
public class LoginServlet 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("/responseDemo_war/success.jsp");
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req,resp);
}
}
index.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<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>
success.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
<h1>Success</h1>
</body>
</html>
原文：https://www.cnblogs.com/saxonsong/p/14808033.html