php - How to listen to change in internet connection in my Android app? -

i created android app uses web service send , retrieve json data.

when make request while device online works fine when device goes offline app stuck , prints null pointer exception error.

is there way listen internet connection?

before request web service call method

private boolean isonline() {     connectivitymanager cm = (connectivitymanager) getsystemservice(context.connectivity_service);     networkinfo ninfo = cm.getactivenetworkinfo();      if (ninfo != null && ninfo.isconnectedorconnecting())         return true;     else         return false; }  if(isonline()){  // request service   // make sure u have surrounded calling web-service try , catch try{      // here make request when connection go offline app    catch   error , ignore process  }catch (exception e) { } 

add permission

 <uses-permission android:name="android.permission.access_network_state" />  <uses-permission android:name="android.permission.access_wifi_state" />