五、ServletContext
web容器在启动的时候，它会为每一个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用。
1.共享数据
在一个Servlet中保存的数据，可以在另一个Servlet中拿到。
public class HelloServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { //this.getInitParameter(); 初始化参数 //this.getServletConfig(); Servlet配置 //this.getServletContext(); Servlet上下文 ServletContext context = this.getServletContext(); String username = "英键"; //数据 context.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字为：username。值:username } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
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"); resp.setCharacterEncoding("utf-8"); resp.getWriter().print("名字为:"+username); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
web.xml：
<servlet> <servlet-name>hello</servlet-name> <servlet-class>com.yingjian.servlet.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>hello</servlet-name> <url-pattern>/hello</url-pattern> </servlet-mapping> <servlet> <servlet-name>getcontext</servlet-name> <servlet-class>com.yingjian.servlet.GetServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>getcontext</servlet-name> <url-pattern>/getcontext</url-pattern> </servlet-mapping>
2.获取初始化参数
web.xml：
<context-param> <param-name>url</param-name> <param-value>jdbc:mysql://localhost:3306/mybatis</param-value> </context-param>
@Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { ServletContext context = this.getServletContext(); String url = context.getInitParameter("url"); resp.getWriter().print(url); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
3.请求转发
request and dispatcher
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ServletDemo04"); /* 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 */ context.getRequestDispatcher("/gp").forward(req,resp); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
4.读取资源文件
思路：需要一个文件流
public class ServletDemo05 extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { //InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/abc1.properties"); InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/yingjian/servlet/abc.properties"); Properties prop = new Properties(); prop.load(is); String user = prop.getProperty("username"); String pwd = prop.getProperty("password"); resp.getWriter().print(user + " " + pwd); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
在java目录下新建abc.properties，在resources目录下新建abc1.properties
发现：都被打包到了同一个路径下：classes，俗称classpath（类路径）
六、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，一个代表响应的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.状态码与方法
- 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;
负责向浏览器发送数据的方法
ServletOutputStream getOutputStream() throws IOException; 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);
2.常见应用
（1）向浏览器输出消息
（2）下载文件
步骤：
要获取下载文件的路径
设置下载的文件名
设置想办法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
将FileOutputStream流写入到buffer缓冲区
使用OutputStream讲缓冲区中的数据输出到客户端
代码示例：
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 = "E:\\IntelliJ IDEA2020.3.2\\IdeaProjects\\JavaWeb\\javaweb-02-servlet\\response\\target\\classes\\练字.png"; System.out.println("下载文件的路径：" + 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(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(realPath); //5. 创建缓冲区 int len = 0; byte[] buffer = new byte[1024]; //6. 获取OutputStream ServletOutputStream out = resp.getOutputStream(); //7. 讲FileOutputStream流写入到buffer缓冲区 while ((len=in.read(buffer))>0){ out.write(buffer,0,len); } in.close(); out.close(); //8. 使用OutputStream讲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
后端实现，需要用到java的图片类，生产一个图片
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/png"); //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("Program","no-cache"); //把图片写个浏览器 ImageIO.write(image,"jpg",resp.getOutputStream()); } //生产随机数 private String makeNum(){ Random random = new Random(); String num = random.nextInt(8888888) + ""; StringBuffer sb = new StringBuffer(); for (int i = 0; i < 7-num.length(); i++) { sb.append("0"); } String s = sb.toString() + num; return num; } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
4.实现重定向
- 一个web资源收到客户端请求后，它会通知客户端去访问另外一个web资源，这个过程叫重定向。
- 常见场景：用户登录
void sendRedirect(String var1) throws IOException;
代码示例：
public class RedirectServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { /* resp.setHeader("Location","/response/red"); resp.setStatus(302); */ //重定向时候一定要注意路径问题，否则404 resp.sendRedirect("/response/img"); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
5.简单实现登录重定向
public class RequestTest extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { System.out.println("进入这个请求了"); //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/success.jsp"); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } }
登录页面：
<%@ page contentType="text/html; charset=UTF-8" language="java" %> <html> <body> <h2>Hello World!</h2> <%--这里提交的路径，需要找到项目的路径--%> <%--${pageContext.request.contextPath}代表当前的项目--%> <form action="${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>
登录成功页面：
<%@ page contentType="text/html;charset=UTF-8" language="java" %> <html> <head> <title>Title</title> </head> <body> <h1>Success!</h1> </body> </html>
七、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
- 步骤：获取参数，请求转发
public class LoginServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doPost(req, resp); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { req.setCharacterEncoding("utf-8"); resp.setCharacterEncoding("utf-8"); /* req.getParameter 获取前端传递的参数 */ String username = req.getParameter("username"); String password = req.getParameter("password"); String[] hobby = req.getParameterValues("hobby"); System.out.println("==============================="); System.out.println(username); System.out.println(password); System.out.println(Arrays.toString(hobby)); System.out.println("==============================="); System.out.println(req.getContextPath()); //通过请求转发 ‘/'代表当前的web应用，这里与response中的sendRedirect写的路径有所不同 req.getRequestDispatcher("/success.jsp").forward(req,resp); } }
登录页面：
<%@ page contentType="text/html;charset=UTF-8" language="java" %> <html> <head> <title>登录</title> </head> <body> <h1>登录</h1> <div style="text-align:left;"> <%-- 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="hobby" value="女孩">女孩 <input type="checkbox" name="hobby" value="代码">代码 <input type="checkbox" name="hobby" value="唱歌">唱歌 <input type="checkbox" name="hobby" value="电影">电影 <br> <input type="submit"> </form> </div> </body> </html>
登录成功页面：
<html> <head> <title>Title</title> </head> <body> <h1>登录成功!</h1> </body> </html>
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会发生变化
后记
Java全栈学习路线可参考：【Java全栈学习路线】最全的Java学习路线及知识清单，Java自学方向指引，内含最全Java全栈学习技术清单~
