Java学习笔记（七十 一）—— SerlvetContext

概念

• SerlvetContext对象代表整个web应用，可以和程序的容器（服务器）来通信

获取

• 通过request对象获取
  request.getServletContext();

• 通过HttpServlet获取
  this.getServletContext();

  		// ServletContext对象获取
          ServletContext servletContext1 = request.getServletContext();
          ServletContext servletContext2 = this.getServletContext();
          System.out.println(servletContext1); // org.apache.catalina.core.ApplicationContextFacade@429db069
          System.out.println(servletContext2); // org.apache.catalina.core.ApplicationContextFacade@429db069
          System.out.println(servletContext1==servletContext2); // true

功能

获取MIME类型：

• MIME类型：在互联网通信过程中定义的一中文件数据类型
  • 格式：大类型/小类型 text/html
• 获取：String getMimeType(String file)

          // ServletContext对象获取
          ServletContext servletContext = request.getServletContext();
          // 定义文件名称
          String filename="a.jpg";
          String mimeType = servletContext.getMimeType(filename);
          System.out.println(mimeType); // image/jpeg

域对象：共享数据

1. setAttribute(String name,Object value)
2. getAttribute(String name)
3. removeAttribute(String name)

• ServletContext对象的范围：所有用户所有请求的数据

  		// 文件ServletContextDemo3.class
          // ServletContext对象获取
          ServletContext servletContext = request.getServletContext();
          // 设置数据
          servletContext.setAttribute("nihao","hello");

  		// 文件ServletContextDemo4.class
          // ServletContext对象获取
          ServletContext servletContext = request.getServletContext();
          Object nihao = servletContext.getAttribute("nihao");
          System.out.println(nihao);

获取文件的真实（服务器）路径：

1. String getRealPath(String path)

           // ServletContext对象获取
           ServletContext servletContext = request.getServletContext();
           // ‘/’代表web文件夹,web目录下的a.txt
           String realPath = servletContext.getRealPath("/a.txt");
           // web目录下WEB-INF下的b.txt
           String realPath1 = servletContext.getRealPath("/WEB-INF/b.txt");
           // src目录下的c.txt
           String realPath2 = servletContext.getRealPath("/WEB-INF/classes/c.txt");
           System.out.println(realPath2);

练习

1. 文件下载

   需求：
   	页面显示连接
   	点击超链接后弹出下载提示框
   	点击图片完成下载

   @WebServlet("/downloadservlet")
   public class DownLoadServlet extends HttpServlet {
       protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           // 1. 获取请求参数，获取文件名称
           String filename = request.getParameter("filename");
           // 2、使用字节输入流加载文件进内存
           // 2.1、找到文件的服务器路径
           ServletContext servletContext = this.getServletContext();
           String realPath = servletContext.getRealPath("/WEB-INF/classes/"+filename);
           System.out.println(realPath);
           // 2.2、用字节流关联
           FileInputStream fileInputStream = new FileInputStream(realPath);
           // 3、设置response的响应头
           // 3.1、设置响应头类型
           String mimeType = servletContext.getMimeType(filename);
           response.setHeader("content-type",mimeType);
           // 3.2、设置响应头打开方式
           response.setHeader("content-disposition","attachment;filename="+filename);
           // 4、将输入流的数据写出到输出流中
           ServletOutputStream outputStream = response.getOutputStream();
           byte[] buff=new byte[1024*8];
           int len=0;
           while ((len=fileInputStream.read(buff))!=-1){
               outputStream.write(buff,0,len);
           }
           fileInputStream.close();
       }
   
       protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           this.doPost(request,response);
       }
   }

   <a href="/day24_war_exploded/downloadservlet?filename=cat.jpg">下载图片</a>