android - get handler return null in LooperThread -

public class looperthread extends thread {  private handler handler = null;  public handler gethandler() {     return handler; }  @override public void run() {     looper.prepare();     handler = new handler();     looper.loop(); }  }  class helper {     private static looperthread databasethread = null;  static {     databasethread = new looperthread();     databasethread.start(); }  public void postrunable(runnable r) {     databasethread.gethandler().post(r);     databasethread.gethandler().sendmessage(new message()); } }  //ui thread.   class uiactivity extends activity {       private helper helper = new helper();       public void oncreate(bundle savedinstancestate) {          helper.postrunnable(new runnable() {               public void run() {                   //work asyn,like query db.               }          });      }  } 

sometimes call databasethread.gethandler().post(r); ,it return null,sometime not,why this?as usual,handler should initial static block.

you times getting null handler because calling databasethread.start(); in the static initializer ensures thread started @ point in future means thats creating race condition between handler getting created inside new thread , gethandler() being called in old one. having thread background looper common pattern in android there class this.

first rid of looperthread class , use sdk's handlerthread instead.

your helper class should

class helper {     private static final handlerthread databasethread;     private static final handler dbhandler;     static {         databasethread = new handlerthread("database thread");         databasethread.start();         // if have called handelerthread#start()         // handlerthread#getlooper() block until looper initialized , looping         dbhandler = new handler(databasethread.getlooper());     }      public void postrunable(runnable r) {         dbhandler.post(r);     } }