我有一个应用程序，它从数据库中检索数据并使用CustomListView显示它。当应用程序要显示ListView时，它会使用此日志崩溃：
03-28 13:22:16.963: E/AndroidRuntime(853): java.lang.NullPointerException
03-28 13:22:16.963: E/AndroidRuntime(853): at com.gabriele.tesina.Leggi_Pizzaiolo$LoadAllProducts$1.run(Leggi_Pizzaiolo.java:162)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.app.Activity.runOnUiThread(Activity.java:4644)
03-28 13:22:16.963: E/AndroidRuntime(853): at com.gabriele.tesina.Leggi_Pizzaiolo$LoadAllProducts.onPostExecute(Leggi_Pizzaiolo.java:156)
03-28 13:22:16.963: E/AndroidRuntime(853): at com.gabriele.tesina.Leggi_Pizzaiolo$LoadAllProducts.onPostExecute(Leggi_Pizzaiolo.java:1)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.os.AsyncTask.finish(AsyncTask.java:631)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.os.AsyncTask.access$600(AsyncTask.java:177)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.os.Handler.dispatchMessage(Handler.java:99)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.os.Looper.loop(Looper.java:137)
03-28 13:22:16.963: E/AndroidRuntime(853): at android.app.ActivityThread.main(ActivityThread.java:5039)
03-28 13:22:16.963: E/AndroidRuntime(853): at java.lang.reflect.Method.invokeNative(Native Method)
03-28 13:22:16.963: E/AndroidRuntime(853): at java.lang.reflect.Method.invoke(Method.java:511)
03-28 13:22:16.963: E/AndroidRuntime(853): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
03-28 13:22:16.963: E/AndroidRuntime(853): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
03-28 13:22:16.963: E/AndroidRuntime(853): at dalvik.system.NativeStart.main(Native Method)
这是我遇到问题的活动：
public class Leggi_Pizzaiolo extends Activity
{
// Progress Dialog
private ProgressDialog pDialog;
public List list = new LinkedList();
// Creating JSON Parser object
JSONParser jParser = new JSONParser();
ArrayList<HashMap<String, String>> productsList;
// url to get all products list
private static String url_all_products = "http://10.0.2.2/tesina/Leggi_Pizzaiolo.php";
// JSON Node names
private static final String TAG_SUCCESS = "Esito";
private static final String TAG_PRODUCTS = "comande";
private static final String TAG_PID = "ID";
private static final String TAG_NAME = "Nome";
private static final String TAG_TABLE = "Tavolo";
public ListView lv;
// products JSONArray
JSONArray products = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.ordini_cuoco);
// Hashmap for ListView
productsList = new ArrayList<HashMap<String, String>>();
// Loading products in Background Thread
new LoadAllProducts().execute();
// Get listview
lv = (ListView)findViewById(R.id.listView1);
}
/**
* Background Async Task to Load all product by making HTTP Request
* */
class LoadAllProducts extends AsyncTask<String, String, String>
{
/**
* Before starting background thread Show Progress Dialog
* */
@Override
protected void onPreExecute() {
super.onPreExecute();
pDialog = new ProgressDialog(Leggi_Pizzaiolo.this);
pDialog.setMessage("Loading products. Please wait...");
pDialog.setIndeterminate(false);
pDialog.setCancelable(false);
pDialog.show();
}
/**
* getting All products from url
* */
protected String doInBackground(String... args) {
// Building Parameters
List<NameValuePair> params = new ArrayList<NameValuePair>();
// getting JSON string from URL
JSONObject json = jParser.makeHttpRequest(url_all_products, "GET", params);
// Check your log cat for JSON reponse
Log.d("All Products: ", json.toString());
try {
// Checking for SUCCESS TAG
int success = json.getInt(TAG_SUCCESS);
if (success == 1) {
// products found
// Getting Array of Products
products = json.getJSONArray(TAG_PRODUCTS);
// looping through All Products
for (int i = 0; i < products.length(); i++) {
JSONObject c = products.getJSONObject(i);
// Storing each json item in variable
String id = c.getString(TAG_PID);
String name = c.getString(TAG_NAME);
int Tavolo= c.getInt(TAG_TABLE);
list.add(new Comanda(name, id, Tavolo));
}
} else {
// no products found
// Launch Add New product Activity
Intent i = new Intent(getApplicationContext(),
Listino.class);
// Closing all previous activities
i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(i);
}
} catch (JSONException e) {
e.printStackTrace();
}
return null;
}
/**
* After completing background task Dismiss the progress dialog
* **/
protected void onPostExecute(String file_url)
{
// dismiss the dialog after getting all products
pDialog.dismiss();
// updating listview
runOnUiThread(new Runnable()
{
public void run()
{
Context context = getApplicationContext();
ComandaCursorAdapter adapter = new ComandaCursorAdapter(context, R.layout.comanda_cuoco, list);
lv.setAdapter(adapter);
}
});
}
}
}
CustomAdapter Comanda：
public class ComandaCursorAdapter extends ArrayAdapter<Comanda>
{
public ComandaCursorAdapter(Context context, int comandaCuoco, List list) {
super(context, comandaCuoco, list);
// TODO Auto-generated constructor stub
}
@Override
public View getView(int position, View convertView, ViewGroup parent)
{
LayoutInflater inflater = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
convertView = inflater.inflate(R.layout.comanda_cuoco, null);
TextView Nome = (TextView)convertView.findViewById(R.id.Comanda);
TextView Tavolo = (TextView)convertView.findViewById(R.id.Tavolo);
TextView Codice = (TextView)convertView.findViewById(R.id.Codice);
Comanda c = getItem(position);
Nome.setText(c.getNome());
Tavolo.setText(c.getTavolo());
Codice.setText(c.getCodice());
return convertView;
}
}
我已经在Google上搜索过，如果你需要展示视觉对象，必须使用UI线程，但我不知道出了什么问题。
答案 0 :(得分：1)
首先，不需要显式调用runOnUiThread()，因为onPostExecute（）已经与UI线程同步。
然后我认为你在这一行得到 NPE ：
Context context = getApplicationContext();
将上下文分配给 NULL 。由于您的AsyncTask是Activity类的内部类，因此您无需调用getApplicationContext（）。你不应该在其他情况下调用它有更好的方法。 Activity从Context扩展，因此您只需使用返回Context的 ActivityName.this 。
所以用这一行改变你的：
new ComandaCursorAdapter(Leggi_Pizzaiolo.this, R.layout.comanda_cuoco, list);
现在它应该有效。让我知道。
按照您的建议完成，NPE现在位于lv.setAdapter（适配器）
现在，您的ListView很可能已分配到 NULL 。我认为你应该在执行AsyncTask之前初始化它：
lv = (ListView)findViewById(R.id.listView1);
new LoadAllProducts().execute();