通过servlet实现文件的下载，实例代码为下载一张jpg的图片

public class Demo extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //通过路径得到一个输入流
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/img.jpg");
        FileInputStream fis = new FileInputStream(path);
        //创建字节输出流
        ServletOutputStream sos = response.getOutputStream();
        
        //得到要下载的文件名
        String filename = path.substring(path.lastIndexOf("\\")+1);
        
        //设置文件名的编码
        filename = URLEncoder.encode(filename, "UTF-8");//将不安全的文件名改为UTF-8格式
        
        //告知客户端要下载文件
        response.setHeader("content-disposition", "attachment;filename="+filename);
        response.setHeader("content-type", "image/jpeg");
        
        //执行输出操作
        int len = 1;
        byte[] b = new byte[1024];
        while((len=fis.read(b))!=-1){
            sos.write(b,0,len);
        }

    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

}