爲什麼在Android中使用AsyncTask發出HTTP請求時會出現空指針異常？

Question

我試圖從FB獲取記分牌，爲了實現這一點，我向API發出HTTP請求。它必須在一個單獨的線程中完成的，這是我的任務異步類：

public class AsyncBuildScoreboard extends AsyncTask<Void, Void,String> { 
ProgressBar pb; 

@Override 
protected void onPreExecute() { 
    super.onPreExecute(); 

} 

@Override 
protected String doInBackground(Void... voids) { 

    String token = Session.getActiveSession().getAccessToken();  
    try{    
     HttpClient client = new DefaultHttpClient();     
     HttpGet get = new HttpGet("https://graph.facebook.com/"+GameStatic.app_id+"?fields=scores&access_token=" + token); 
     HttpResponse resp = client.execute(get);      
     HttpEntity responseEntity = resp.getEntity(); 
     String response = EntityUtils.toString(responseEntity);  
     return response; 

    } 
    catch (IOException e) 
    { 

    } 
    return ""; 

} 


protected void onPostExecute(String response) { 
    try{  
     JSONObject res = new JSONObject(response);       
     JSONObject scores = res.getJSONObject("scores"); 
     JSONArray data = scores.getJSONArray("data");      
     int len = data.length();       
     String[] values = new String[len]; 
     for(int i=0;i<len;i++) 
     { 
      JSONObject obj = data.getJSONObject(i);       
      JSONObject user = obj.getJSONObject("user"); 
      String name = user.getString("name"); 
      String score = obj.getString("score");   
      values[i] = name + " " + score; 
      GameStatic.scoreboard.add(values[i]); 
     } 
    }  
    catch (JSONException e) 
    { 

    } 



} 

} 

GameStatic是一個外部變量來存儲我從線程獲得。然而，這樣做時：發生

AsyncBuildScoreboard board = new AsyncBuildScoreboard(); 
    board.execute();   
    final ListView listview = (ListView) findViewById(R.id.scorelist);    
    final StableArrayAdapter adapter = new StableArrayAdapter(this, 
       android.R.layout.simple_list_item_1, GameStatic.scoreboard);   
    listview.setAdapter(adapter); 

一個空指針異常，這意味着，GameStatic.scoreboard沒有填充有我想要的條目。

我做錯了什麼，有什麼想法？

我有義務，因爲我真的很準時按下...

public class AsyncBuildScoreboard extends AsyncTask<Void, Void, Void> 
{ 
public ListView list; 
public Context ctx; 
public ArrayList<String> scoreboard = new ArrayList <String>(); 


public AsyncBuildScoreboard(ListView list, Context context) 
{ 
    this.list = list; 
    this.ctx = context; 
} 

@Override 
protected void onPreExecute() { 
    super.onPreExecute(); 

} 

@Override 
protected Void doInBackground(Void... voids) 
{ 
    Void nothing = null; 
    String token = Session.getActiveSession().getAccessToken();  
    try{    
     HttpClient client = new DefaultHttpClient();     
     HttpGet get = new HttpGet("https://graph.facebook.com/"+GameStatic.app_id+"?fields=scores&access_token=" + token); 
     HttpResponse resp = client.execute(get);      
     HttpEntity responseEntity = resp.getEntity(); 
     String response = EntityUtils.toString(responseEntity); 


    } 
    catch (IOException e) 
    { 

    } 
    return nothing;  
} 


protected void onPostExecute(String response) { 
    try{  
     JSONObject res = new JSONObject(response);       
     JSONObject scores = res.getJSONObject("scores"); 
     JSONArray data = scores.getJSONArray("data");      
     int len = data.length();       
     String[] values = new String[len]; 
     for(int i=0;i<len;i++) 
     { 
      JSONObject obj = data.getJSONObject(i);       
      JSONObject user = obj.getJSONObject("user"); 
      String name = user.getString("name"); 
      String score = obj.getString("score");   
      values[i] = name + " " + score; 
      scoreboard.add(values[i]); 
     } 
    }  
    catch (JSONException e) 
    { 

    } 
    final ArrayAdapter adapter = new ArrayAdapter(ctx, 
      android.R.layout.simple_list_item_1, scoreboard);  
    list.setAdapter(adapter); 


} 



} 

Answer 1

，你將需要使用onPostExecute用於顯示的分數時doInBackground執行完整的，而不只是後AsyncTask.execute()因爲doInBackground傳遞GameStatic.scoreboard總是在單獨的線程中執行。

可以使用AsyncBuildScoreboard類的構造函數從活動傳遞的ListView實例和語境爲：

ListView listview; 
Context context; 
public AsyncBuildScoreboard(ListView listview,Context context){ 
this.context=context; 
this.listview=listview; 
} 
.... 
protected void onPostExecute(String response) { 
    //...your code here.. 

    StableArrayAdapter adapter = new StableArrayAdapter(context, 
      android.R.layout.simple_list_item_1, GameStatic.scoreboard);   
    listview.setAdapter(adapter); 
} 

，並改變你的活動代碼，作爲傳遞上下文：

ListView listview = (ListView) findViewById(R.id.scorelist); 
AsyncBuildScoreboard board = new AsyncBuildScoreboard (listview, 
                Your_Activity.this); 
board.execute(); 

，或者你也可以創建回調方法使用接口，當UI線程在doInBackground執行完成時觸發。請參閱以下文章以獲取更多詳細信息：

android asynctask sending callbacks to ui