Java-HttpURLConnection上传文件

public static String upFile(String url, File file) throws Throwable {
	String PREFIX = "--";  // 前缀符
	String BOUNDARY = "*"; // 边界符(任意字符串)
	String CRLF = "\r\n";  // 换行符

	HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();
	conn.setChunkedStreamingMode(4 * 1024);// 切换为分块流模式,取消内部缓冲区(默认模式是数据全部写入内存后才开始上传,对于大文件,明显内存不足导致崩溃)
	conn.setConnectTimeout(10 * 1000);
	conn.setDoOutput(true); // 允许输出

	// 1.Http请求行/头
	conn.setRequestMethod("POST");
	conn.setRequestProperty("Connection", "Keep-Alive");
	conn.setRequestProperty("Charset", "UTF-8");
	conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);

	// 2.Http请求体
	DataOutputStream out = new DataOutputStream(conn.getOutputStream());
	out.writeUTF(PREFIX + BOUNDARY + CRLF);
	out.writeUTF("Content-Disposition: form-data; name=\"file\"; filename=\"" + file.getName() + "\"" + CRLF);
	out.writeUTF("Content-Type: application/octet-stream" + CRLF);
	out.writeUTF(CRLF);
	InputStream fin = new FileInputStream(file);
	byte[] b = new byte[4 * 1024];
	int len;
	while ((len = fin.read(b)) != -1)
		out.write(b, 0, len); // 写入文件
	fin.close();
	out.writeUTF(CRLF);
	out.writeUTF(PREFIX + BOUNDARY + PREFIX + CRLF);
	out.flush();
	out.close();

	// 3.Http响应
	BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream(), "UTF-8"));
	StringBuilder result = new StringBuilder();
	String line;
	while ((line = in.readLine()) != null)
		result.append(line);
	in.close();
//  conn.disconnect(); // 断开TCP连接,后续HTTP请求不能重用该连接
	return result.toString();
}