Service返回连接对象的null值

Question

服务类：

    private IBinder mBinder ;
        @Override
        public IBinder onBind(Intent arg0) {
            return null;
        }

        @Override
        public void onCreate() {
        }
        @Override
        public void onDestroy() {
        }

        @Override
        public int onStartCommand(Intent intent, int flags, int startId) {
            Log.d("service","started");
            new Connect().execute("");
            return Service.START_STICKY;
    }


    private class Connect extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... params) {
            ConnectionConfiguration connConfig = new ConnectionConfiguration(
                    HOST, PORT, SERVICE);
            final XMPPConnection connection = new XMPPConnection(connConfig);
        Thread t=   new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        connection.connect();
                        // SASLAuthentication.supportSASLMechanism("PLAIN", 0);
                        connection.login(USERNAME, PASSWORD);
                        Log.i("NetWorkConnection",
                                "Logged in as " + connection.getUser());
                        System.out.println(connection);
                        setConnection(connection);
                    } catch (XMPPException ex) {
                        Log.e("NetWorkConnection", "Failed to log in as "
                                + USERNAME);
                        Log.e("NetWorkConnection", ex.toString());
                        setConnection(null);
                    }
                }});
        t.start();
            return null;       
        }
    }

    public void setConnection(XMPPConnection connection) {
        NetworkConnection.connection = connection;
    }

    public class MyBinder extends Binder 
    {
        NetworkConnection getService() {
            return NetworkConnection.this;
        }
    }
    public XMPPConnection getconnection()
    {
        if (connection != null) {
            Log.d("NetworkConnection","connection send");
            return connection;  
        }
        else
        {
            Log.d("NetworkConnection","connection null");
            return null;    
        }
    }

}

活动类：

private NetworkConnection service;
    XMPPConnection connection=NetworkConnection.connection;
    Button next;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
         setContentView(R.layout.sign_in);
         next=(Button)findViewById(R.id.bRetry);
         if(!isMyServiceRunning())  
            {
            Intent i=new Intent(this,NetworkConnection.class);
            startService(i);
            }

         next.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                System.out.println(connection);
                if(connection!=null)
                {
                    Intent i = new Intent (getApplicationContext(),UserIndex.class);
                    startActivity(i);
                }else{
                Intent i = new Intent(SignIn.this,SignIn.class);
                startActivity(i);
                }
            }
        });

    }

     @Override
      protected void onResume() {
        bindService(new Intent(this, NetworkConnection.class), mConnection,
            Context.BIND_AUTO_CREATE);
        super.onResume();
      }

      @Override
      protected void onPause() {
        unbindService(mConnection);
        super.onPause();
      }

      private ServiceConnection mConnection = new ServiceConnection() {

            public void onServiceConnected(ComponentName className, IBinder binder) {
              service = ((NetworkConnection.MyBinder) binder).getService();
              Log.d("Service","Connected");
            System.out.println("service connection methoddd");
                connection=service.getconnection();
            }

            public void onServiceDisconnected(ComponentName className) {
                connection=null;
                service = null;
            }


          };
    private boolean isMyServiceRunning() {
        ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
        for (RunningServiceInfo service : manager.getRunningServices(Integer.MAX_VALUE)) {
            if (NetworkConnection.class.getName().equals(service.service.getClassName())) {
                return true;
            }
        }
        return false;
    }
}

我正在使用XMPP和asmack创建聊天应用程序，所以我使用了服务。但问题是，当我启动服务onServiceConnected方法永远不会执行。这就是为什么当我从服务中检索连接对象时，它总是给出null值。我对此并不太了解。请指导我。