关于HttpURLConection的POST请求乱码

我在用HttpURLConection发送POST请求时，并未包含中文，但自己测试时收到的请求前面总是包含几个乱码，求高手解决！代码如下：

public class HttpUtil {
/**
 * 发送http POST请求
 * @param httpUrl  请求URL
 * @param httpContext 请求报文主体内容
 * @return
 * @throws IOException
 */
    public static String sendPost(String httpUrl,String httpContext) throws IOException{
        URL url = new URL(httpUrl);
        HttpURLConnection connection = (HttpURLConnection)url.openConnection();
        connection.setRequestMethod("POST");
        // 设置是否向httpUrlConnection输出，因为这个是post请求，参数要放在http正文内，因此需要设为true,默认情况下是false;   
        connection.setDoOutput(true);   
        connection.setRequestProperty("Content-Type","application/json; encoding=utf-8");
        connection.setRequestProperty("Connection", "keep-alive");


        OutputStream out = connection.getOutputStream();
        // 现在通过输出流对象构建对象输出流对象，以实现输出可序列化的对象。   
         ObjectOutputStream objOut = new ObjectOutputStream(out);   
         objOut.writeObject(httpContext);  // 向对象输出流写出数据，这些数据将存到内存缓冲区中
         out.close();
         objOut.close(); // 关闭对象输出流

         InputStream in = connection.getInputStream();
         InputStreamReader inr = new InputStreamReader(in);
         BufferedReader reader = new BufferedReader(inr);
         StringBuffer result = new StringBuffer();
         String temp;
         while((temp = reader.readLine()) != null){
             result.append(temp);
         }
         in.close();
         return result.toString();
    }