Java 从JForm按钮上的click事件调用HTTP请求时无法运行HTTP请求，而从主类调用HTTP请求时工作正常

我有方法调用signin的web服务，当从主方法调用它时，它工作良好。。。但当从JFrame上按钮的事件处理程序调用它时，我得到：服务器返回了URL:Error的HTTP响应代码：500。 这发生在代码行：Reader in=new BufferedReadernew InputStreamReaderconn.getInputStream，UTF-8

方法是：

public boolean doLogin(String username, String password) throws MalformedURLException{
        try{
        URL url = new URL("http://localhost/postrec/index.php/api/Site/Login");
        Map<String,Object> params = new LinkedHashMap<>();
        params.put("username", username);
        params.put("password", password);

        StringBuilder postData = new StringBuilder();
        for (Map.Entry<String,Object> param : params.entrySet()) {
            if (postData.length() != 0) postData.append('&');
            postData.append(URLEncoder.encode(param.getKey(), "UTF-8"));
            postData.append('=');
            postData.append(URLEncoder.encode(String.valueOf(param.getValue()), "UTF-8"));
        }
        byte[] postDataBytes = postData.toString().getBytes("UTF-8");

        HttpURLConnection conn = (HttpURLConnection)url.openConnection();
        conn.setRequestMethod("POST");
        conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        conn.setRequestProperty("Content-Length", String.valueOf(postDataBytes.length));
        conn.setDoOutput(true);
        conn.getOutputStream().write(postDataBytes);

        Reader in = new BufferedReader(new InputStreamReader(conn.getInputStream(), "UTF-8"));
        String responseStr = "";
        for (int c; (c = in.read()) >= 0; responseStr  = responseStr + (char)c);

        JSONObject jsonObject = new JSONObject(responseStr);

        return (jsonObject.getString("code").equals("200"));
         }catch(JSONException | MalformedURLException | UnsupportedEncodingException e){
             throw new MalformedURLException(e.getMessage());
        }catch(IOException e){
            System.out.println(e.getMessage());

        }
        return false;
    }

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