公共静态方法中的异步任务返回 null

Question

所以我有一个异步任务，我将它放在一个公共静态方法中，以避免挂起 UI 线程（因为它执行网络调用），同时仍然能够从我的应用程序的几个部分调用它。 Async 任务成功运行并返回一个 JSONOBJECT（我已经记录了部分代码以确认这一点）。但是问题是因为我使用公共静态方法，它应该有一个返回类型（在可能的情况下应该返回 JSONOBJECT）但它总是返回 null ..我怎样才能重新编写我的代码才能返回我的异步任务获取的 JSONOBJECT。代码如下。

public class JSONmethod {
    static InputStream is = null;
    static String res = "";
    static JSONObject jArray = null;
public static JSONObject getJSONfromURL(final String url){


    new AsyncTask<String, Void, JSONObject>() {

        @Override
        protected JSONObject doInBackground(String... params) {

    try{
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(url);
            HttpResponse response = httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            is = entity.getContent();



    }
    catch(Exception e){
            Log.e("log_tag", "Error in http connection "+e.toString());
    }


    try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                    sb.append(line + "\n");
            }
            is.close();
            res=sb.toString();
    }
    catch(Exception e){
            Log.e("log_tag", "Error converting result "+e.toString());
    }
    Log.i("Result BG ", res);
    try{

        jArray = new JSONObject(res);            
    }catch(JSONException e){
            Log.e("log_tag", "Error parsing data "+e.toString());
    }
    return jArray; //at this point the JSONOBJECT has been fetched

     }



    }.execute();
    return jArray;  //Always null
 }

}

Answer 1

如果您的方法应该是异步的，那么它现在无法返回 JSON 结果...

你可以做的是为你的异步任务提供一个回调，当结果可用时将被调用：

    public class JSONmethod {
    static InputStream is = null;
    static String res = "";
    static JSONObject jArray = null;

    public static interface JSONCallback {
        public void onResult(JSONObject result);
    }

    public static void getJSONfromURL(final String url, final JSONCallback callback) {

        new AsyncTask<String, Void, JSONObject>() {

            @Override
            protected JSONObject doInBackground(String... params) {

                try {
                    HttpClient httpclient = new DefaultHttpClient();
                    HttpPost httppost = new HttpPost(url);
                    HttpResponse response = httpclient.execute(httppost);
                    HttpEntity entity = response.getEntity();
                    is = entity.getContent();

                } catch (Exception e) {
                    Log.e("log_tag",
                            "Error in http connection " + e.toString());
                }

                try {
                    BufferedReader reader = new BufferedReader(
                            new InputStreamReader(is, "iso-8859-1"), 8);
                    StringBuilder sb = new StringBuilder();
                    String line = null;
                    while ((line = reader.readLine()) != null) {
                        sb.append(line + "\n");
                    }
                    is.close();
                    res = sb.toString();
                } catch (Exception e) {
                    Log.e("log_tag",
                            "Error converting result " + e.toString());
                }
                Log.i("Result BG ", res);
                try {

                    jArray = new JSONObject(res);
                } catch (JSONException e) {
                    Log.e("log_tag", "Error parsing data " + e.toString());
                }

                callback.onResult(jArray);

                return jArray; // at this point the JSONOBJECT has been
                                // fetched

            }

        }.execute();
    }
}

你会这样使用它：

    JSONmethod.getJSONfromURL(url, new JSONCallback() {

        @Override
        public void onResult(JSONObject result) {
            // Do whatever you want with the result
        }
    });

Answer 2

异步任务的基础是，在调用方法作为返回值后，您不会立即得到它的结果。

您的调用（无论是否静态）只会执行创建任务的过程（不执行它！）并立即返回。系统可能不会立即执行真正的任务流程代码（可能队列中有一些先前的任务等）。一旦它完成 AsyncTask.doInBackground() 方法，并在调用 AsyncTask.onPostExecute() 之后，任务将完成。默认情况下，它不会调用您的任何方法。在 AsyncTask.onPostExecute() 方法上更新 UI 是您的责任。