POST 请求发送数据后，服务器运行一段时间出现空指针异常，问题出在哪里？

outputstreamwriter out = new outputstreamwriter(conn.getoutputstream(), "utf-8");
out.write(gson.tojson(orderinfo));

orderinfo orderinfo = new orderinfo();
// 填充 orderinfo 数据

string posturl = "api地址";
string postdata = converttojsonstring(orderinfo);
string result = httprequestutil.sendpost(posturl, postdata);

public static String sendPost(String url, String postData) {
    String data = null;
    try {
        URL dataUrl = new URL(url);
        HttpURLConnection con = (HttpURLConnection) dataUrl.openConnection();
        con.setRequestMethod("POST");
        con.setRequestProperty("Content-type", "application/json");
        con.setDoOutput(true);
        con.setDoInput(true);
        con.setUseCaches(false);

        OutputStream os = con.getOutputStream();
        DataOutputStream dos = new DataOutputStream(os);
        if (postData != null) {
            dos.write(postData.getBytes("UTF-8"));
        }
        dos.flush();
        dos.close();

        BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream(), "UTF-8"));
        StringBuffer buffer = new StringBuffer();
        String line;
        while ((line = in.readLine()) != null) {
            buffer.append(line);
        }
        data = buffer.toString();
        con.disconnect();
    } catch (Exception ex) {
        // 处理异常
    }
    return data;
}