一、关于multipart/form-data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。
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。
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 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
1 @Test 2 public void buildUploadStream() throws IOException { 3 String url ="uploadurl"; 4 file1 = "D:\test.xls"; 5 fileName = "test.xls"; 6 7 String newline = "rn"; 8 String boundaryPrefix = "--"; 9 String boundary = "----" + UUID.randomUUID().toString();10 11 URL postUrl = new URL(url);12 HttpURLConnection conn = (HttpURLConnection) postUrl.openConnection();13 14 conn.setRequestMethod("POST");15 conn.setDoInput(true);16 conn.setDoOutput(true);17 conn.setUseCaches(false);18 19 conn.setRequestProperty("connection", "Keep-Alive");20 conn.setRequestProperty("Charset", "UTF-8");21 conn.setRequestProperty("Content-Type",22 "multipart/form-data; boundary=" + boundary);23 24 OutputStream out = conn.getOutputStream();25 26 27 StringBuilder sb = new StringBuilder();28 29 sb.append(boundaryPrefix);30 sb.append(boundary);31 sb.append(newline);32 33 sb.append("Content-Disposition: form-data; name="id"");34 sb.append(newline);35 sb.append(newline);36 sb.append("testCodeUpload");37 sb.append(newline);38 39 sb.append(boundaryPrefix);40 sb.append(boundary);41 sb.append(newline);42 43 sb.append("Content-Disposition: form-data; name="name"");44 sb.append(newline);45 sb.append(newline);46 sb.append(fileName);47 sb.append(newline);48 49 sb.append(boundaryPrefix);50 sb.append(boundary);51 sb.append(newline);52 53 sb.append("Content-Disposition: form-data; name="file"; filename=""54 + fileName + """);55 sb.append("Content-Type: application/octet-stream");56 sb.append(newline);57 sb.append(newline);58 59 out.write(sb.toString().getBytes());60 61 File file = new File(file1);62 FileInputStream in = new FileInputStream(file);63 byte[] bufferOut = new byte[1024];64 int bytes = 0;65 while ((bytes = in.read(bufferOut)) != -1) {66 out.write(bufferOut, 0, bytes);67 }68 out.write(newline.getBytes());69 in.close();70 byte[] end_data = (newline + boundaryPrefix + boundary + boundaryPrefix + newline)71 .getBytes();72 out.write(end_data);73 out.flush();74 out.close();75 76 BufferedReader reader = new BufferedReader(new InputStreamReader(77 conn.getInputStream()));78 String line = null;79 while ((line = reader.readLine()) != null) {80 System.out.println(line);81 }82 }all code
原文出处：https://www.cnblogs.com/yfrs/p/uploadbyjavacode.html
