不确定为什么我从httpurlconnection得到一个空的输出流

import java.io.*;
import java.net.*;

public class HttpURLConnectionExample {
    public static void main(String[] args) {
        try {
            // 创建URL对象
            URL url = new URL("http://example.com/api");

            // 打开连接
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

            // 设置请求方法为POST
            connection.setRequestMethod("POST");

            // 允许输出数据
            connection.setDoOutput(true);

            // 获取输出流
            OutputStream outputStream = connection.getOutputStream();

            // 写入数据
            String data = "Hello, World!";
            outputStream.write(data.getBytes());

            // 关闭输出流
            outputStream.close();

            // 获取响应代码
            int responseCode = connection.getResponseCode();

            // 处理响应
            if (responseCode == HttpURLConnection.HTTP_OK) {
                // 读取响应数据
                InputStream inputStream = connection.getInputStream();
                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
                String line;
                StringBuilder response = new StringBuilder();
                while ((line = reader.readLine()) != null) {
                    response.append(line);
                }
                reader.close();

                // 输出响应数据
                System.out.println(response.toString());
            } else {
                System.out.println("请求失败，响应代码：" + responseCode);
            }

            // 关闭连接
            connection.disconnect();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}