JavaScript文件上传和下载

前一段时间做附件的上传和下载功能。期间遇到不少问题，网上找的方法都不算完整。这里把前端和后端的实现都整理一下。

需要的东西：JQuery，Pako.js，base64.js。其中pako/base64是为了对上传的文件进行压缩而使用。如果前端有对文件进行压缩，那么后端也应该对应进行解压。

上传最简单的实现：前端选择一个文件-读取文件信息-通过ajax请求后端方法把文件信息传输并保存(当然，当文件太大，在客户端应该要分批读取并上传，在服务端也是依次分批保存)

• HTML只需要一个input

  <input type="file" id="filePicker" />

• JS实现

  $(document).ready(function () {
      $(document).on("change", "#filePicker", onFileUpload);
  });
  
  function onFileUpload(e) {
      if (!window.FileReader) { // 使用了HTML 5 的 FileReader API。目前大部分浏览器都能支持。
          alert("该浏览器不支持HTML5，请升级或者更换其它浏览器。");
          return;
      }
      var fileInfo = e.currentTarget.files[0];
      var fileName = fileInfo.name;
      var fileSize = fileInfo.size;
  
      var fileReader = new FileReader();
      //var blob = fileInfo.slice(0, fileSize);
      //fileReader.readAsArrayBuffer(blob);
      fileReader.readAsArrayBuffer(fileInfo);
      fileReader.onload = function (result) {
          var pakoString = getUploadingFileContentString(this.result);
          $.ajax({
              url: "http://localhost/Server/ashx/FileProcess.ashx", //用一般处理程序接收请求
              type: "POST",
              data: {
                  fileContent: pakoString,
                  fileName: fileName
              },
              success: function (data) {
                  if (data == "True") {
                      alert("上传成功！");
                  }
                  else {
                      alert("上传失败");
                  }
              },
              error: function (e) {
                  alert(e.responseText);
              }
          });
      }
  }
  
  function getUploadingFileContentString(readResult) {
      if (readResult == null) {
          return null;
      }
      var fileContentArr = new Uint8Array(readResult);
      var fileContentStr = "";
      for (var i = 0; i < fileContentArr.length; i++) {
          fileContentStr += String.fromCharCode(fileContentArr[i]);
      }
      //如果不压缩，直接转base64 string进行传输
      //window.btoa: 将ascii字符串或二进制数据转换成一个base64编码过的字符串,该方法不能直接作用于Unicode字符串.
      //var base64FileString = window.btoa(fileContentStr);
      //return base64FileString;
  
      //压缩
      var pakoBytes = pako.gzip(fileContentStr);
      var pakoString = fromByteArray(pakoBytes);
      return pakoString;
  }

• 服务端实现

  public class FileProcess : IHttpHandler
      {
  
          public void ProcessRequest(HttpContext context)
          {
              UploadFile(context);
          }
  
          private void UploadFile(HttpContext context)
          {
              var fileName = context.Request["fileName"];
              var compressedFileContent = context.Request["fileContent"];
              //如果前端没有对文件进行压缩，那么直接获取文件的字节数组即可
              //byte[] fileBytes = Convert.FromBase64String(compressedFileContent);
  
              //解压
              var fileContent = GZip.GZipDecompress(compressedFileContent);
              byte[] fileBytes = Utils.ConvertJSBytes2Str(fileContent);
              bool isSavedSuccess = Utils.SaveFile2Disk(fileBytes, fileName, isUploadPartly);
  
              context.Response.Write(isSavedSuccess);
          }
  }
  //其中GZipDecompress/ConvertJSBytes2Str/SaveFile2Disk实现如下
  public static string GZipDecompress(string zipString)
          {
              if (string.IsNullOrEmpty(zipString))
              {
                  return string.Empty;
              }
  
              byte[] zipBytes = Convert.FromBase64String(zipString);
              return System.Text.Encoding.UTF8.GetString(Decompress(zipBytes));
          }
  
  private static byte[] Decompress(byte[] zipData)
          {
              MemoryStream m = new MemoryStream(zipData);
              GZipStream zipStream = new GZipStream(m, CompressionMode.Decompress);
              MemoryStream outStream = new MemoryStream();
              byte[] buffer = new byte[1024];
              while (true)
              {
                  var readCount = zipStream.Read(buffer, 0, buffer.Length);
                  if (readCount <= 0)
                  {
                      break;
                  }
                  outStream.Write(buffer, 0, readCount);
              }
              zipStream.Close();
              return outStream.ToArray();
          }
  
  public static byte[] ConvertJSBytes2Str(string fileContent)
          {
              if (string.IsNullOrEmpty(fileContent))
              {
                  return null;
              }
              // JS中，String.fromCharCode接受Unicode字符，并转成字符串
              byte[] fileBytes = System.Text.Encoding.Unicode.GetBytes(fileContent);
              byte[] adjustedFileBytes = new byte[fileBytes.Length / 2];
              var index = 0;
              for (var i = 0; i < fileBytes.Length; i = i + 2)
              {
                  adjustedFileBytes[index] = fileBytes[i];
                  index++;
              }
  
              return adjustedFileBytes;
          }
  
  public static bool SaveFile2Disk(byte[] fileContent, string fileName, bool isSavedPartly = false)
          {
              if (fileContent == null || fileContent.Length == 0)
              {
                  throw new ArgumentNullException("文件内容不能为空!");
              }
              if (string.IsNullOrEmpty(fileName))
              {
                  throw new ArgumentNullException("文件名不能为空!");
              }
              var targetFloder = HttpContext.Current.Server.MapPath("~/UploadFileFloder/");
              var fullPath = Path.Combine(targetFloder, fileName);
              DirectoryInfo di = new DirectoryInfo(targetFloder);
              if (di.Exists == false)
              {
                  di.Create();
              }
              FileStream fileStream;
              try
              {
                  if (isSavedPartly)
                  {
                      fileStream = new FileStream(fullPath, FileMode.Append, FileAccess.Write, FileShare.Read, 1024);
                  }
                  else
                  {
                      fileStream = new FileStream(fullPath, FileMode.Create, FileAccess.ReadWrite);
                  }
              }
              catch (Exception ex)
              {
                  //throw ex;
                  //write log
                  return false;
              }
              try
              {
                  fileStream.Write(fileContent, 0, fileContent.Length);
              }
              catch (IOException ex)
              {
                  //write log
                  return false;
              }
              finally
              {
                  fileStream.Flush();
                  fileStream.Close();
                  fileStream.Dispose();
              }
              return true;
          }

  这样，一个简单的完整的前端到后端的文件上传保存就完成了

下载文件。(这里简化程序，服务端指定的文件夹里面已经有待下载的file1.txt)

• HTML

  <input type="button" id="downloadFileBtn" value="下载"/>

• JS

  $(document).on("click", "#downloadFileBtn", downloadFile);
  
  function downloadFile()
  {
      var fileName = "file1.txt";
      $.ajax({
          url: "http://localhost/Server/ashx/FileProcess.ashx",
          type: "POST",
          success: function (data) {
              var fileContent = window.atob(data);
              saveFile(fileName, fileContent);
          },
          error: function (e) {
              alert(e.responseText);
          }
      });
  }
  function saveFile(fileName, fileContent) {
          var byteArr = new Array(fileContent.length);
          for (var i = 0; i < fileContent.length; i++) {
              byteArr[i] = fileContent.charCodeAt(i);
          }
  
          var blob = new Blob([new Uint8Array(byteArr)], { type: "application/octet-stream" });
          var url = window.URL.createObjectURL(blob);
  
          var a = document.createElement("a");
          if ("download" in a) { // 支持download属性
              document.body.appendChild(a);
              a.style = "display:none";            a.href = url;
              //download属性IE不支持。。。
              a.download = fileName;
              a.click(); // 触发下载
              //revokeObjectURL会导致firefox不能下载。。。
              //window.URL.revokeObjectURL(url);
              document.body.removeChild(a);
          }
          else { //IE 10+
              if (typeof navigator !== "undefined" && navigator.msSaveOrOpenBlob) {
                  return navigator.msSaveOrOpenBlob(blob, name);
              }
          }
  }

• 服务端实现

  //ProcessRequest中调用Download
  
  public void Download(HttpContext context)

          {
              var filePath = Path.Combine(HttpContext.Current.Server.MapPath("~/UploadFileFloder/"), "file1.txt");
              byte[] fileContentBytes = Utils.GetByteArrayByFilePath(filePath);
              context.Response.Write(Convert.ToBase64String(fileContentBytes));
          }
  
  //其中，GetByteArrayByFilePath实现如下
  public static byte[] GetByteArrayByFilePath(string fileFullPath)
          {
              if (string.IsNullOrEmpty(fileFullPath))
              {
                  return null;
              }
  
              FileStream fs = null;
              byte[] fileContent = null;
              try
              {
                  FileInfo fi = new FileInfo(fileFullPath);
                  fileContent = new byte[fi.Length];
                  //fs = new FileStream(fileFullPath, FileMode.Open);
                  fs = File.OpenRead(fileFullPath);
                  fs.Read(fileContent, 0, fileContent.Length);
              }
              catch (Exception ex)
              {
                  return null;
              }
              finally
              {
                  if (fs != null)
                  {
                      fs.Close();
                  }
              }
              return fileContent;
          }

  这样，一个简单的完整的文件下载功能就实现了