的Android的AsyncTask：如何處理返回類型

Question

我上一個Android應用程序執行的HTTP POST請求工作，我跟着教程導致的android.os.NetworkOnMainThreadException

的原代碼是像這個。

public class JSONParser { 

static InputStream is = null; 
static JSONObject jObj = null; 
static String json = ""; 

// constructor 
public JSONParser() { 

} 

public JSONObject getJSONFromUrl(String url, List<NameValuePair> params) { 

    // Making HTTP request 
    try { 
     // defaultHttpClient 
     DefaultHttpClient httpClient = new DefaultHttpClient(); 
     HttpPost httpPost = new HttpPost(url); 
     httpPost.setEntity(new UrlEncodedFormEntity(params)); 

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

    } catch (UnsupportedEncodingException e) { 
     e.printStackTrace(); 
    } catch (ClientProtocolException e) { 
     e.printStackTrace(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 

    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(); 
     json = sb.toString(); 
     Log.e("JSON", json); 
    } catch (Exception e) { 
     Log.e("Buffer Error", "Error converting result " + e.toString()); 
    } 

    // try parse the string to a JSON object 
    try { 
     jObj = new JSONObject(json);    
    } catch (JSONException e) { 
     Log.e("JSON Parser", "Error parsing data " + e.toString()); 
    } 

    // return JSON String 
    return jObj; 

} 
} 

而這個類被調用這一行。

JSONObject json = jsonParser.getJSONFromUrl(loginURL, params); 

將此更改爲AsyncTask類後，代碼如下所示。

class JSONParser extends AsyncTask<String, Void, JSONObject>{ 

static InputStream is = null; 
static JSONObject jObj = null; 
static String json = ""; 

// variables passed in: 
String url; 
List<NameValuePair> params; 

// constructor 
public JSONParser(String url, List<NameValuePair> params) { 
    this.url = url; 
    this.params = params; 
} 

@Override 
protected JSONObject doInBackground(String... args) { 
    // Making HTTP request 
     try { 
      // defaultHttpClient 
      DefaultHttpClient httpClient = new DefaultHttpClient(); 
      HttpPost httpPost = new HttpPost(url); 
      httpPost.setEntity(new UrlEncodedFormEntity(params)); 

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


     } catch (UnsupportedEncodingException e) { 
      e.printStackTrace(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 


     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(); 
      json = sb.toString(); 
      Log.e("JSON", json); 
     } catch (Exception e) { 
      Log.e("Buffer Error", "Error converting result " + e.toString()); 
     } 

     // try parse the string to a JSON object 
     try { 
      jObj = new JSONObject(json);    
     } catch (JSONException e) { 
      Log.e("JSON Parser", "Error parsing data " + e.toString()); 
     } 

     // return JSON String 
     return jObj; 
} 

@Override 
protected void onPostExecute(JSONObject jObj) { 
    return; 
}  
} 

我的問題是，如何從這個新的AsyncTask類返回JSONObject？我可以看到，jObj在doInBackground()返回，但我不知道在那裏它被送回。

我需要修改什麼或如何調用我的新JSONParser類，以便返回JSONObject？

Answer 1

我可以看到jObj被返回doInBackground（），但我不是 確定它返回的位置。

doinBackground（）的結果作爲onPostExecute（）中的參數接收。你在doinBackground（）中返回一個json對象，它是onPostExecute（）的一個參數。

@Override 
protected void onPostExecute(JSONObject jObj) { 
return; 
} 

使用

new JSONParser().execute("url); 
class JSONParser extends AsyncTask<String, Void, JSONObject>{ 

    //string parameter to doInBackground() 
    //JSONObject - result returned in doInBackground() received as a param in onPostExecute() 
} 

您也可以通過paramters您的AsyncTask

new JSONParser("url",params).execute(); 

在你的AsyncTask的構造函數;

String url; 
List<NameValuePair> params; 

// constructor 
public JSONParser(String url, List<NameValuePair> params) { 
this.url = url; 
this.params = params; 
} 

Answer 2

從doInBackground方法

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

return jObj; 

}

你回你的JsonObject到

@Override 
protected void onPostExecute(JSONObject jObj) { 

    // Here you get your return JsonObject 
} 

Answer 3

異步任務有3個attribures

Params, the type of the parameters sent to the task upon execution. 

Progress, the type of the progress units published during the background computation. 

Result, the type of the result of the background computation. 

你需要明白的一點是，要創建異步任務類的物體雖然調用new JSONParser(loginURL, params);

解決的辦法是，在你的異步類中創建一個公共結果變量和類對象上調用​​然後從該對象訪問該公共對象。

Answer 4

看看這段代碼，它可能會讓你瞭解如何處理JSON對象的解析。我現在只是發佈onPostExecute函數，因爲你似乎已經正確地計算了所有其他的函數。

至於您對doInBackground中的數據對象返回的位置有疑問，它會自動發送到onPostExecute，您可以在其中進一步解析它。

  @Override 
    protected void onPostExecute(JSONObject result) 
    { 
     try 
     { 
      JSONObject data = result.getJSONObject("data"); 
         // whatever your JSON tag may be, in this case its data. 

      if (data.isNull("data")) 
      { 
         // action to handle null JSON object. 
      }    
      else 
      { 
       JSONArray jarray = data.getJSONArray("data"); 
       int len=jarray.length(); 
       for (int i = 0; i < jarray.length(); i++) 
       { 
       JSONObject obj = (JSONObject) jarray.get(i); 

      String instanceName = obj.getString("instanceName"); 
         //extract data by whatever tag names you require, in this case instanceName.  
      } 
     } 
} 
     catch (JSONException je) 
     { 
      je.printStackTrace(); 
      Log.d(TAG, "Error: " + je.getMessage()); 
     }  
    } 
}