使用HttpURLConnection进行multipart/form-data请求

    @WorkerThread
    private String postImage(String urlStr, String formDataKey, String formData, String fileKey, String fileContentType, File file) {
        String dash = "--";
        String boundary = generateBoundary();
        String newLine = "\r\n";
        HttpURLConnection connection = null;
        DataOutputStream outputStream = null;
        FileInputStream inputStream = null;
        try {
            URL url = new URL(urlStr);
            connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("POST");
            connection.setRequestProperty("Connection", "Keep-Alive");
            connection.setRequestProperty("User-Agent", "Android Multipart HTTP Client 1.0");
            connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
            connection.setDoOutput(true);
            connection.setDoInput(true);
            connection.setUseCaches(false);

            outputStream = new DataOutputStream(connection.getOutputStream());
            // form data
            String knowledgeRequest = dash + boundary + newLine +
                    "Content-Disposition: form-data; name=\"" + formDataKey + "\"" + newLine +
                    "Content-Type: text/plain; charset=utf-8" + newLine +
                    newLine +// important !
                    formData + newLine +
                    dash + boundary + newLine;
            outputStream.writeBytes(knowledgeRequest);
            // file data
            String fileHeader = "Content-Disposition: form-data; name=\"" + fileKey + "\"; filename=\"" + file.getName() + "\"" + newLine +
                    "Content-Type:" + fileContentType + newLine +
                    "Content-Transfer-Encoding: binary" + newLine +
                    newLine;// important !
            outputStream.writeBytes(fileHeader);
            byte[] buffer = new byte[1024];
            int count;
            inputStream = new FileInputStream(file);
            while (true) {
                count = inputStream.read(buffer);
                if (count == -1) {
                    break;
                }
                outputStream.write(buffer, 0, count);
            }
            outputStream.writeBytes(newLine);// important !
            outputStream.writeBytes(dash + boundary + dash + newLine);
            outputStream.flush();
            int statusCode = connection.getResponseCode();
            StringBuilder response = new StringBuilder();
            if (statusCode == 200) {
                BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String line;
                while ((line = reader.readLine()) != null) {
                    response.append(line);
                }
                return response.toString();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (connection != null) {
                    connection.disconnect();
                }
                if (outputStream != null) {
                    outputStream.close();
                }
                if (inputStream != null) {
                    inputStream.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    private String generateBoundary() {
        return "--------------------------" +
                UUID.randomUUID().toString().replace("-", "");
    }