1.普通文件下載

使用超鏈接。缺點：1）暴露文件的路徑 2）擴展型和安全性不好

2.使用servlet程序下載（推薦）

 //1.得到響應的下載內容
        String fileName = "abc.txt";
        InputStream in = this.getServletContext().getResourceAsStream("/upload/"+fileName);
        //對文件名進行url編碼設置
        fileName = URLEncoder.encode(fileName,"utf-8");
        //判斷不同的瀏覽器
        String userAgent = req.getHeader("user-agent");
        String filefix = null;
        System.out.println(userAgent);
        if (userAgent.contains("Trident")){
            //IE
            filefix = "filename="+fileName;
        }else if (userAgent.contains("Firefox")) {
            //Fire
            filefix = "filename*=" + fileName;
        }else{
            filefix = "filename="+fileName;
        }
        //告訴瀏覽器去用以下的方式去打開
        resp.setHeader("Content-Disposition","attachment;"+filefix);

        //2.把本地文件發送給瀏覽器
        OutputStream out = resp.getOutputStream();
        byte[] buf = new byte[1024];
        int len = 0;
        while ((len=in.read(buf))!=-1){
            out.write(buf,0,len);
        }
        //關閉流
        out.close();
        in.close();
    }