java - 尽管之前在 AsyncTask 类中定义，但 JSON 对象为空？

Question

我试图通过收集 JSON 格式的信息来连接到 MySQL 数据库。我的代码收到这些信息。我创建了一个扩展 AsyncTask 的类，在我的 onPostExecute 类中，我想将完整的 JSONArray 保存到不同的 JSONArray 在 AsyncTask 类之外。这样做我不会收到错误消息，但是一旦我想使用此对象，我的代码就会崩溃，并且我会收到 java.lang.NullPointerException 并且我的新 JSON 数组为空。谁能帮帮我？

class task extends AsyncTask<String, String, Void>
{    
   InputStream is = null ;

   protected Void doInBackground(String... params) {
        String url_select = "http://pois.lima-city.de/test1.php";

        HttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(url_select);

        ArrayList<NameValuePair> param = new ArrayList<NameValuePair>();

        try {
            httpPost.setEntity(new UrlEncodedFormEntity(param));

            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();

            //read content
            is =  httpEntity.getContent();

        } catch (Exception e) {

            Log.e("log_tag", "Error in http connection "+e.toString());
            //Toast.makeText(MainActivity.this, "Please Try Again", Toast.LENGTH_LONG).show();
        }
        try {
            BufferedReader br = new BufferedReader(new InputStreamReader(is));
            StringBuilder sb = new StringBuilder();
            String line = "";
            while((line=br.readLine())!=null)
            {
                sb.append(line+"\n");
            }
            is.close();
            result=sb.toString();

        } catch (Exception e) {
            // TODO: handle exception
            Log.e("log_tag", "Error converting result "+e.toString());
        }

        return null;

    }

    protected void onPostExecute(Void v) {
        try {


            LatLng coordinates;
            String between = "";
            JSONArray JarrayP = new JSONArray(result);
            myJSONArray = JarrayP;
          } catch (Exception e) {
            // TODO: handle exception
            Log.e("log_tag", "Error parsing data "+e.toString());
        }
    }

PS:它在另一个类中。

Answer 1

您当前在 doInBackground(String...) 中返回 Void。这就是您在 onPostExecute 中得到 null 的原因。将您的类(class)更改为:

public class YourTask extends AsyncTask<String,Void,String>{
 ....

然后在 doInBackground() 中:

....
return result;