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form-data实现文件上传.md

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##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-Typemultipart/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。