【 JavaWeb 】- 8 ServletContext

    科技2025-05-31  39

    1 什么是 ServletContext 对象

    概念:代表整个 web 应用,可以和程序的容器(服务器)来通信。

    ServletContext 代表是一个web应用的环境(上下文)对象,ServletContext 对象内部封装是该 web 应用的信息 ServletContext 对象一个 web 应用只有一个。

    问题:一个 web 应用有几个 servlet 对象? ----多个

    2 ServletContext 对象的生命周期?

    创建:该web应用被加载(服务器启动或发布web应用(前提,服务器启动状态))销毁:web应用被卸载(服务器关闭,移除该web应用)

    3 怎么获得 ServletContext 对象

    1. 通过request对象获取 request.getServletContext(); 2. 通过HttpServlet获取 this.getServletContext(); package cn.ys.servletContext; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; @WebServlet("/servletContextDemo1") public class ServletContextDemo1 extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /* ServletContext对象获取: 1. 通过request对象获取 request.getServletContext(); 2. 通过HttpServlet获取 this.getServletContext(); */ //1. 通过request对象获取 ServletContext context1 = request.getServletContext(); //2. 通过HttpServlet获取 ServletContext context2 = this.getServletContext(); System.out.println(context1); System.out.println(context2); System.out.println(context1 == context2);//true } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request,response); } }

    4 ServletContext 的作用

    功能: 1. 获取MIME类型: * MIME类型:在互联网通信过程中定义的一种文件数据类型 * 格式: 大类型/小类型 text/html image/jpeg * 获取:String getMimeType(String file) 2. 域对象:共享数据 1. setAttribute(String name,Object value) 2. getAttribute(String name) 3. removeAttribute(String name) * ServletContext对象范围:所有用户所有请求的数据 3. 获取文件的真实(服务器)路径 1. 方法:String getRealPath(String path) String b = context.getRealPath("/b.txt");//web目录下资源访问 System.out.println(b); String c = context.getRealPath("/WEB-INF/c.txt");//WEB-INF目录下的资源访问 System.out.println(c); String a = context.getRealPath("/WEB-INF/classes/a.txt");//src目录下的资源访问 System.out.println(a);

    ① 获得web应用全局的初始化参数

    [外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-PIcdf7Z0-1602124049694)(/api/project/7821181/files/20940403/imagePreview)]

    ② 获得 web 应用中任何资源的绝对路径(重要) 方法:String path = context.getRealPath(相对于该web应用的相对地址);

    ③ ServletContext 是一个域对象(重要) 什么是域对象?什么是域? 存储数据的区域就是域对象

    注: ServletContext域对象的作用范围:整个web应用(所有的web资源都可以随意向 servletcontext域中存取数据,数据可以共享)

    域对象的通用的方法:

    setAtrribute(String name,Object obj);getAttribute(String name);removeAttribute(String name); package cn.ys.servletContext; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; @WebServlet("/servletContextDemo2") public class ServletContextDemo2 extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /* ServletContext功能: 1. 获取MIME类型: * MIME类型:在互联网通信过程中定义的一种文件数据类型 * 格式: 大类型/小类型 text/html image/jpeg * 获取:String getMimeType(String file) 2. 域对象:共享数据 3. 获取文件的真实(服务器)路径 */ //2. 通过HttpServlet获取 ServletContext context = this.getServletContext(); //3. 定义文件名称 String filename = "a.jpg"; //image/jpeg //4.获取MIME类型 String mimeType = context.getMimeType(filename); System.out.println(mimeType);//image/jpeg } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request,response); } } package cn.ys.servletContext; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; @WebServlet("/servletContextDemo3") public class ServletContextDemo3 extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /* ServletContext功能: 1. 获取MIME类型: 2. 域对象:共享数据 3. 获取文件的真实(服务器)路径 */ //2. 通过HttpServlet获取 ServletContext context = this.getServletContext(); //设置数据 context.setAttribute("msg", "haha"); } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request,response); } } package cn.ys.servletContext; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; @WebServlet("/servletContextDemo4") public class ServletContextDemo4 extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /* ServletContext功能: 1. 获取MIME类型: 2. 域对象:共享数据 3. 获取文件的真实(服务器)路径 */ //2. 通过HttpServlet获取 ServletContext context = this.getServletContext(); //获取数据 Object msg = context.getAttribute("msg"); System.out.println(msg); } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request,response); } } package cn.ys.servletContext; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.File; import java.io.IOException; @WebServlet("/servletContextDemo5") public class ServletContextDemo5 extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { /* ServletContext功能: 1. 获取MIME类型: 2. 域对象:共享数据 3. 获取文件的真实(服务器)路径 */ // 通过HttpServlet获取 ServletContext context = this.getServletContext(); // 获取文件的服务器路径 String b = context.getRealPath("/b.txt");//web目录下资源访问 System.out.println(b);///Users/yangshuo/IdeaProjects/JavaWeb/1-1-Servlet-7-ServletContext/src/main/webapp/b.txt // File file = new File(realPath); String c = context.getRealPath("/WEB-INF/c.txt");//WEB-INF目录下的资源访问 System.out.println(c);///Users/yangshuo/IdeaProjects/JavaWeb/1-1-Servlet-7-ServletContext/src/main/webapp/WEB-INF/c.txt String a = context.getRealPath("/WEB-INF/classes/a.txt");//src目录下的资源访问 System.out.println(a);///Users/yangshuo/IdeaProjects/JavaWeb/1-1-Servlet-7-ServletContext/src/main/webapp/WEB-INF/classes/a.txt } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request,response); } }

    5 案例:文件下载

    * 文件下载需求: 1. 页面显示超链接 2. 点击超链接后弹出下载提示框 3. 完成图片文件下载 * 分析: 1. 超链接指向的资源如果能够被浏览器解析,则在浏览器中展示,如果不能解析,则弹出下载提示框。不满足需求 2. 任何资源都必须弹出下载提示框 3. 使用响应头设置资源的打开方式: * content-disposition:attachment;filename=xxx * 步骤: 1. 定义页面,编辑超链接href属性,指向Servlet,传递资源名称filename 2. 定义Servlet 1. 获取文件名称 2. 使用字节输入流加载文件进内存 3. 指定response的响应头: content-disposition:attachment;filename=xxx 4. 将数据写出到response输出流 * 问题: * 中文文件问题 * 解决思路: 1. 获取客户端使用的浏览器版本信息 2. 根据不同的版本信息,设置filename的编码方式不同

    package cn.ys.utils; import sun.misc.BASE64Encoder; import java.io.UnsupportedEncodingException; import java.net.URLEncoder; public class DownLoadUtils { public static String getFileName(String agent, String filename) throws UnsupportedEncodingException { if (agent.contains("MSIE")) { // IE浏览器 filename = URLEncoder.encode(filename, "utf-8"); filename = filename.replace("+", " "); } else if (agent.contains("Firefox")) { // 火狐浏览器 BASE64Encoder base64Encoder = new BASE64Encoder(); filename = "=?utf-8?B?" + base64Encoder.encode(filename.getBytes("utf-8")) + "?="; } else { // 其它浏览器 filename = URLEncoder.encode(filename, "utf-8"); } return filename; } } package cn.ys.download; import cn.ys.utils.DownLoadUtils; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.FileInputStream; import java.io.IOException; @WebServlet("/downloadServlet") public class DownloadServlet extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //1.获取请求参数,文件名称 String filename = request.getParameter("filename"); //2.使用字节输入流加载文件进内存 //2.1找到文件服务器路径 ServletContext servletContext = this.getServletContext(); String realPath = servletContext.getRealPath("/img/" + filename); //2.2用字节流关联 FileInputStream fis = new FileInputStream(realPath); //3.设置response的响应头 //3.1设置响应头类型:content-type String mimeType = servletContext.getMimeType(filename);//获取文件的mime类型 response.setHeader("content-type",mimeType); //3.2设置响应头打开方式:content-disposition //解决中文文件名问题 //1.获取user-agent请求头、 String agent = request.getHeader("user-agent"); //2.使用工具类方法编码文件名即可 filename = DownLoadUtils.getFileName(agent, filename); response.setHeader("content-disposition","attachment;filename="+filename); //4.将输入流的数据写出到输出流中 ServletOutputStream sos = response.getOutputStream(); byte[] buff = new byte[1024 * 8]; int len = 0; while((len = fis.read(buff)) != -1){ sos.write(buff,0,len); } fis.close(); } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request,response); } }

    download.html

    <!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> </head> <body> <a href="/day15/img/1.jpg">图片1</a> <a href="/day15/img/1.avi">视频</a> <hr> <a href="/day15/downloadServlet?filename=九尾.jpg">图片1</a> <a href="/day15/downloadServlet?filename=1.avi">视频</a> </body> </html>

    register.html

    <!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> <script> /* 分析: 点击超链接或者图片,需要换一张 1.给超链接和图片绑定单击事件 2.重新设置图片的src属性值 */ window.onload = function(){ //1.获取图片对象 var img = document.getElementById("checkCode"); //2.绑定单击事件 img.onclick = function(){ //加时间戳 var date = new Date().getTime(); img.src = "/day15/checkCodeServlet?"+date; } } </script> </head> <body> <img id="checkCode" src="/day15/checkCodeServlet" /> <a id="change" href="">看不清换一张?</a> </body> </html>
    Processed: 0.009, SQL: 8