构建multipart/form-data实现文件上传

通常文件上传都是通过form表单中的file控件，并将form中的content-type设置为multipart/form-data。现在我们通过java来构建这部分请求内容实现文件上传功能。

一、关于multipart/form-data

文件上传本质上是一个POST请求。只不过请求头以及请求内容遵循一定的规则（协议）

请求头（Request Headers）中需要设置 Content-Type 为 multipart/form-data; boundary=${boundary}。其中${boundary}分割线，需要在代码中替换，且尽量复杂，不易重复
请求正文（Request Body）需要使用在 Header中设置的 ${boundary}来分割当前正文中的FormItem，内容格式如下
```
--${boundary}

Content-Disposition: form-data; name="id"

testCodeUpload

--${boundary}

Content-Disposition: form-data; name="file";filename="xx.txt"

Content-Type: application/octet-stream

{{这里写入文件流}}

--${boundary}--
```
正文开始以前缀+${boundary}开始，以 前缀 +${boundary}+前缀结束。中间每个FormItem 以 前缀+${boundary}开始，以一个空白的换行结束。

二、代码实现

实例代码采用HttpURLConnection实现一个简单POST请求

建立http请求，设置基本参数

URL postUrl = new URL(url);

HttpURLConnection conn = (HttpURLConnection) postUrl.openConnection();

conn.setRequestMethod("POST");

conn.setDoInput(true);

conn.setDoOutput(true);

conn.setUseCaches(false);

conn.setRequestProperty("connection", "Keep-Alive");

conn.setRequestProperty("Charset", "UTF-8");

添加文件上传必须的请求信息，获取http请输出流

String boundary = "----" + UUID.randomUUID().toString();

conn.setRequestProperty("Content-Type",

"multipart/form-data; boundary=" + boundary);

OutputStream out = conn.getOutputStream();

StringBuilder sb = new StringBuilder();

一组FormItem

sb.append(boundaryPrefix);

sb.append(boundary);

sb.append(newLine);

sb.append("Content-Disposition: form-data; name=\"id\"");

sb.append(newLine);

sb.append(newLine);

sb.append("testCodeUpload");

sb.append(newLine);

文件写人

sb.append(boundaryPrefix);

sb.append(boundary);

sb.append(newLine);

sb.append("Content-Disposition: form-data; name=\"file\"; filename=\""

+ fileName + "\"");

sb.append("Content-Type: application/octet-stream");

sb.append(newLine);

sb.append(newLine);

out.write(sb.toString().getBytes());

File file = new File(file1);

FileInputStream in = new FileInputStream(file);

byte[] bufferOut = new byte[1024];

int bytes = 0;

while ((bytes = in.read(bufferOut)) != -1) {

out.write(bufferOut, 0, bytes);

}

out.write(newLine.getBytes());

in.close();

结束标志 前缀+boundary +前缀

byte[] end_data = (newLine + boundaryPrefix + boundary + boundaryPrefix + newLine)

.getBytes();

out.write(end_data);

out.flush();

out.close();

三、文件接收

文件接收端通过迭代每个FileItem获取不同的数据

FileItemFactory factory = new DiskFileItemFactory();

ServletFileUpload upload = new ServletFileUpload(factory);

upload.setHeaderEncoding("UTF-8");

try {

  items = upload.parseRequest(request);

} catch (FileUploadException ex) {

  ex.printStackTrace();

  out.println(ex.getMessage());

  return;

}

Iterator<FileItem> itr = items.iterator();

String id = "", fileName = "";

int chunks = 1, chunk = 0;

FileItem tempFileItem = null;

while (itr.hasNext()) {

FileItem item = (FileItem) itr.next();

if (item.getFieldName().equals("id")) {

  id = item.getString();

} else if (item.getFieldName().equals("name")) {

  fileName = new String(item.getString().getBytes("ISO-8859-1"), "UTF-8");

} else if (item.getFieldName().equals("file")) {

  tempFileItem = item;

}

四、总结

通过代码实现一遍文件上传，了解其运行机制，解开了以前在写文件上传代码中item.getFieldName().equals("name")等相关判断的疑惑。所以，对于已有的基础代码，还是多看，多写，多实践。

附完整代码

 @Test

     public void buildUploadStream() throws IOException {

         String url ="uploadurl";

         file1 = "D:\\test.xls";

         fileName = "test.xls";

         String newLine = "\r\n";

         String boundaryPrefix = "--";

         String boundary = "----" + UUID.randomUUID().toString();

         URL postUrl = new URL(url);

         HttpURLConnection conn = (HttpURLConnection) postUrl.openConnection();

         conn.setRequestMethod("POST");

         conn.setDoInput(true);

         conn.setDoOutput(true);

         conn.setUseCaches(false);

         conn.setRequestProperty("connection", "Keep-Alive");

         conn.setRequestProperty("Charset", "UTF-8");

         conn.setRequestProperty("Content-Type",

                 "multipart/form-data; boundary=" + boundary);

         OutputStream out = conn.getOutputStream();

         StringBuilder sb = new StringBuilder();

         sb.append(boundaryPrefix);

         sb.append(boundary);

         sb.append(newLine);

         sb.append("Content-Disposition: form-data; name=\"id\"");

         sb.append(newLine);

         sb.append(newLine);

         sb.append("testCodeUpload");

         sb.append(newLine);

         sb.append(boundaryPrefix);

         sb.append(boundary);

         sb.append(newLine);

         sb.append("Content-Disposition: form-data; name=\"name\"");

         sb.append(newLine);

         sb.append(newLine);

         sb.append(fileName);

         sb.append(newLine);

         sb.append(boundaryPrefix);

         sb.append(boundary);

         sb.append(newLine);

         sb.append("Content-Disposition: form-data; name=\"file\"; filename=\""

                 + fileName + "\"");

         sb.append("Content-Type: application/octet-stream");

         sb.append(newLine);

         sb.append(newLine);

         out.write(sb.toString().getBytes());

         File file = new File(file1);

         FileInputStream in = new FileInputStream(file);

         byte[] bufferOut = new byte[1024];

         int bytes = 0;

         while ((bytes = in.read(bufferOut)) != -1) {

             out.write(bufferOut, 0, bytes);

         }

         out.write(newLine.getBytes());

         in.close();

         byte[] end_data = (newLine + boundaryPrefix + boundary + boundaryPrefix + newLine)

                 .getBytes();

         out.write(end_data);

         out.flush();

         out.close();

         BufferedReader reader = new BufferedReader(new InputStreamReader(

                 conn.getInputStream()));

         String line = null;

         while ((line = reader.readLine()) != null) {

             System.out.println(line);

         }

     }

all code

巴特西

构建multipart/form-data实现文件上传

构建multipart/form-data实现文件上传

一、关于multipart/form-data

二、代码实现

三、文件接收

四、总结

最新文章

热门文章