构建multipart/form-data实现文件上传

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 = "
";
        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);
        }
    }