问HttpURLConnection.getInputStream在HTTP中的奇怪行为

EN

    ...
    HttpURLConnection connection = null;
    InputStream input = null;
    BufferedWriter output = null;
    BufferedReader reader = null;
    String result = null;
    try {
        connection = (HttpURLConnection) new URL(url).openConnection();
        connection.setRequestMethod("POST");
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "application/json");
        //Send data
        output = new BufferedWriter(new OutputStreamWriter(connection.getOutputStream()));
        output.write(data);
        output.flush();
        //Read response
        int responseCode = connection.getResponseCode();
        if (responseCode >= 400) {
            input = connection.getErrorStream();
        } else {
            input = connection.getInputStream();
        }
        reader = new BufferedReader(new InputStreamReader(input));
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line).append('\n');
        }
        result = "Status " + responseCode + ".\n" + sb.toString();
        ...