how can start and stop handler service in android -

i develop app use services. want start , stop service when phone in sleep mode. using following code not work.

my status checker services code

public void onstart(intent intent, int startd) {  log.v("check","services running"); log.v("spinner value","services running");   handler.removecallbacks(sendnetworkstatus);     handler.postdelayed(sendnetworkstatus, 1000);      datamanager = (connectivitymanager)getsystemservice(connectivity_service);      wifimanager = (wifimanager) getsystemservice(context.wifi_service);       boolean screenon = intent.getbooleanextra("screen_state", false);       if (!screenon) {           // code           log.v("screen","screen on>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");          /* if(!ismyservicerunning())           {               startservice(intent);             }*/           } else {           // code         // stopservice(intent);           log.v("screen","screen off>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");       } }    @override public ibinder onbind(intent intent) {     // todo auto-generated method stub     return null; }  private runnable sendnetworkstatus = new runnable() {     public void run() {         sendnetworkinfo();         handler.postdelayed(this, 5000);          checkstrength(wifistatus,mobilestatus);          messageshow(response);          log.v("check","ping url again , again");     } }; /*@override public boolean onunbind(intent intent) {     handler.removecallbacks(sendnetworkstatus);     handler = null;     return super.onunbind(intent); }*/ private void sendnetworkinfo() {     connectivitymanager connection = (connectivitymanager) this.getsystemservice(context.connectivity_service);     networkinfo         activenetwork       = connection.getactivenetworkinfo();      if (null != activenetwork) {         if(activenetwork.gettype() == connectivitymanager.type_wifi)         {//wifistatus = "wifi status: up";           wifistatus= getwifisignalstrengthindbm();          telephonymanager.listen(phonestatelistener,                           phonestatelistener.listen_signal_strengths);          }         else{             wifistatus=0;         }           if(activenetwork.gettype() == connectivitymanager.type_mobile)             {//mobilestatus = "mobile status: up";              message="mobile";             // filter.addaction("mobile");             telephonymanager.listen(phonestatelistener,                   phonestatelistener.listen_signal_strengths);              }          else{              //mobilestatus=0;              telephonymanager.listen(phonestatelistener,                               phonestatelistener.listen_signal_strengths);          }          try {                 response = network.ping(url);             } catch (ioexception e) {                 // todo auto-generated catch block                 e.printstacktrace();             } catch (interruptedexception e) {                 // todo auto-generated catch block                 e.printstacktrace();             }      }      else{          mobilestatus = 0;         wifistatus = 0;         response="-1";     }      intent.putextra("status", response);     sendbroadcast(intent); }  public int getwifisignalstrengthindbm() {         wifiinfo wifiinfo = wifimanager.getconnectioninfo();          return wifiinfo.getrssi();     }  public void checkstrength(int wifi,int mobile)     {          if(wifi!=0 && mobile!=0)         {             if(wifi<mobile){               response="-2";                wifimanager.setwifienabled(false);               internetcontrol.enableinternet(getapplicationcontext());                            }         }         else if(wifi==0&&mobile<=-111)         {              response="-3";             message="signal strength weak";         }     }   @override public void ondestroy() {     handler.removecallbacks(sendnetworkstatus);      thread.currentthread().interrupt();     unregisterreceiver(mreceiver);     toast.maketext(getbasecontext(), "service destroy", toast.length_long).show();     super.ondestroy(); } 

and log cat error

09-04 12:46:33.249: e/androidruntime(4177): fatal exception: main 09-04 12:46:33.249: e/androidruntime(4177): java.lang.runtimeexception: unable start service com.example.data_connection_monitor.statuschecker@415923f8 null: java.lang.nullpointerexception 09-04 12:46:33.249: e/androidruntime(4177):     @ android.app.activitythread.handleserviceargs(activitythread.java:2390) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.app.activitythread.access$1900(activitythread.java:128) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.app.activitythread$h.handlemessage(activitythread.java:1224) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.os.handler.dispatchmessage(handler.java:99) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.os.looper.loop(looper.java:137) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.app.activitythread.main(activitythread.java:4517) 09-04 12:46:33.249: e/androidruntime(4177):     @ java.lang.reflect.method.invokenative(native method) 09-04 12:46:33.249: e/androidruntime(4177):     @ java.lang.reflect.method.invoke(method.java:511) 09-04 12:46:33.249: e/androidruntime(4177):     @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:993) 09-04 12:46:33.249: e/androidruntime(4177):     @ com.android.internal.os.zygoteinit.main(zygoteinit.java:760) 09-04 12:46:33.249: e/androidruntime(4177):     @ dalvik.system.nativestart.main(native method) 09-04 12:46:33.249: e/androidruntime(4177): caused by: java.lang.nullpointerexception 09-04 12:46:33.249: e/androidruntime(4177):     @ com.example.data_connection_monitor.statuschecker.onstart(statuschecker.java:77) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.app.service.onstartcommand(service.java:438) 09-04 12:46:33.249: e/androidruntime(4177):     @ android.app.activitythread.handleserviceargs(activitythread.java:2373) 09-04 12:46:33.249: e/androidruntime(4177):     ... 10 more 

please me. new in android. in advance!!!!!!!!