我有一个从广播接收器开始的asynctask。
代码：
public static class SrvPositioning extends Service {
// An alarm for rising in special times to fire the
// pendingIntentPositioning
private AlarmManager alarmManagerPositioning;
// A PendingIntent for calling a receiver in special times
public PendingIntent pendingIntentPositioning;
Location location;
//GPSTracker gps;
@Override
public void onCreate() {
super.onCreate();
alarmManagerPositioning = (AlarmManager)
getSystemService(Context.ALARM_SERVICE);
Intent intentToFire = new Intent(
ReceiverPositioningAlarm.ACTION_REFRESH_SCHEDULE_ALARM);
intentToFire.putExtra(ReceiverPositioningAlarm.COMMAND,
ReceiverPositioningAlarm.SENDER_SRV_POSITIONING);
pendingIntentPositioning = PendingIntent.getBroadcast(this, 0,
intentToFire, 0);
};
@Override
public void onStart(Intent intent, int startId) {
try {
long interval = 3000;//3 secs
System.out.println("Service called");
int alarmType = AlarmManager.ELAPSED_REALTIME_WAKEUP;
long timetoRefresh = SystemClock.elapsedRealtime();
alarmManagerPositioning.setInexactRepeating(alarmType,
timetoRefresh, interval, pendingIntentPositioning);
//Toast.makeText(this, "SrvPositioning fired", Toast.LENGTH_SHORT).show();
} catch (NumberFormatException e) {
Toast.makeText(this,
"error running service: " + e.getMessage(),
Toast.LENGTH_SHORT).show();
} catch (Exception e) {
Toast.makeText(this,
"error running service: " + e.getMessage(),
Toast.LENGTH_SHORT).show();
}
}
@Override
public void onDestroy() {
this.alarmManagerPositioning.cancel(pendingIntentPositioning);
// ReceiverPositioningAlarm.stopLocationListener();
}
@Override
public IBinder onBind(Intent intent) {
// TODO Auto-generated method stub
return null;
}
}
public static class ReceiverPositioningAlarm extends BroadcastReceiver {
public static final String COMMAND = "SENDER";
public static final int SENDER_ACT_DOCUMENT = 0;
public static final int SENDER_SRV_POSITIONING = 1;
public static final String ACTION_REFRESH_SCHEDULE_ALARM =
"ACTION_REFRESH_SCHEDULE_ALARM";
//private static final Context _context=null;
String jsonStr="";
@Override
public void onReceive(final Context context, Intent intent) {
//Toast.makeText(context, "new request received by receiver",
//Toast.LENGTH_SHORT).show();
// create class object
//SessionManager s=new SessionManager(context);
//userid=s.getUserId();
System.out.println("RecieverPositioning Alarm");
//_context = context;
new ChatScreen().new GetPeriodicChat(recieverid).execute();
}
}
服务和广播接收器都是ChatScreen类的内部类。
我在ChatScreen.java的onCreate（）中初始化了一个共享首选项，如：
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.chatscreen2);
preftimestamp= ChatScreen.this.getSharedPreferences(Constants.ACCESS_TIMESTAMP, Context.MODE_PRIVATE);
现在，当我尝试在asynctask GetPeriodicChat（recieverid）（从广播接收器启动的那个）的doInBackground（）中访问此共享首选项时，我遇到了NPE。
以下是asynctask的代码：
public class GetPeriodicChat extends AsyncTask<String,String,String>{
String r_id="";
public GetPeriodicChat(String r_id) {
// TODO Auto-generated constructor stub
System.out.println("get periodic chat called");
chatlisthashmapperiodically = new ArrayList<HashMap<String, String>>();
preftimestamp= ChatScreen.this.getSharedPreferences(Constants.ACCESS_TIMESTAMP, Context.MODE_PRIVATE);
this.r_id=r_id;
}
protected void onPreExecute() {
super.onPreExecute();
try{
//showLoading(R.string.loading);
}catch(Exception e){
e.printStackTrace();
}
}
@Override
protected String doInBackground(String... params) {
// TODO Auto-generated method stub
urlgetallchatmessage=Constants.getmessagetimestamp;
nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs.add(new BasicNameValuePair("sender_id",userid));
nameValuePairs.add(new BasicNameValuePair("receiver_id",r_id));//rec
String last_timestamp =preftimestamp.getString(Constants.ACCESS_TIMESTAMP,null);
System.out.println("Last timestamp in periodically-->"+last_timestamp);
nameValuePairs.add(new BasicNameValuePair("last_timestamp",last_timestamp));
ServiceHandle sh = new ServiceHandle();
String jsonResponse = sh.makeServiceCall(urlgetallchatmessage, ServiceHandle.POST,nameValuePairs);
// after getting JSON string from the URL
System.out.println(""+ jsonResponse );
try {
if ( jsonResponse != null)
{
JSONObject c = new JSONObject( jsonResponse );
//JSONObject c = jsonObj;
successallchat=Integer.parseInt(c.getString("success"));
if(successallchat==1)
{
if(c.has(TAG_GETHISTORY_ARRAY)){
chatlistperiodically = c.getJSONArray(TAG_GETHISTORY_ARRAY);
// looping through All Contacts
for (int i = 0; i <chatlistperiodically.length(); i++) {
JSONObject j_chat = chatlistperiodically.getJSONObject(i);
String timestamp= j_chat.getString(TAG_TIMESTAMP);
String chatmessage= j_chat.getString(TAG_MESSAGE);
String sender_id= j_chat.getString(TAG_SENDER_ID);
// creating new HashMap
HashMap<String, String> map = new HashMap<String, String>();
// adding each child node to HashMap key => value
map.put(TAG_TIMESTAMP,timestamp);
map.put(TAG_MESSAGE,chatmessage);
map.put(TAG_SENDER_ID,sender_id);
SharedPreferences.Editor editor = preftimestamp.edit();
// System.out.println(projectid+"--->In Projectactivity");
editor.putString(Constants.ACCESS_TIMESTAMP,timestamp);
editor.commit();
//map.put(TAG_PDESC,pdetails);
chatlisthashmapperiodically.add(map);
}
}
}
}
else
{
runOnUiThread(new Runnable() {
@Override
public void run() {
// TODO Auto-generated method stub
Toast.makeText(getBaseContext(), "Server not responding!", Toast.LENGTH_SHORT).show();
}
});
}
} catch (Exception e)
{
e.printStackTrace();
}
return null;
}//doInbacckground
@SuppressLint("NewApi")
protected void onPostExecute(String file_url) {
//removeLoading();
if(successallchat==1)
{
}
}//postexecute
}//Asynctask
我在asynctask的构造函数中遇到行preftimestamp= ChatScreen.this.getSharedPreferences(Constants.ACCESS_TIMESTAMP, Context.MODE_PRIVATE);上的NPE。可能是什么问题？如果我没有在构造函数上定义asynctask，那么我也遇到了NPE.Please帮助。< / p>
答案 0 :(得分：1)
将AsyncTask更新为：
public class GetPeriodicChat extends AsyncTask<String,String,String>{
String r_id="";
public GetPeriodicChat(String r_id, Context context) {
...
// ************ Instead of ChatScreen.this pass Context object in constuctor and use it here.
preftimestamp= context.getSharedPreferences(Constants.ACCESS_TIMESTAMP, Context.MODE_PRIVATE);
...
}
}//Asynctask
用法：
new ChatScreen().new GetPeriodicChat(recieverid, context).execute();
将活动上下文作为参数传递给AsyncTask。除非您在活动中定义静态Context变量，否则您将无法访问活动之外的Activity上下文，这不是最佳做法。
答案 1 :(得分：0)
您不应在广播接收器中执行任何长时间操作，最好从接收器调用服务并在句柄意图上定义异步任务。 如果它在主线程上运行，你永远不应该在其中执行长时间运行的操作（在考虑阻塞接收器和被杀死的候选者之前，系统允许超时10秒。）