java - Google app engine standard + Firebase database -


i have google app engine + firebase database backend architecture. writing servlet, should values database, make calculations , build response values. problem ondatachange() method called asynchronously. @ first i'd introduce code , continue:

//here initialize listener called when ondatachange()  //method finished make code synchronous.     responsereadylistener = new responsereadylistener() {             @override             public void onresponseready() {                 responseready[0] = true;                 synchronized (myservlet.this) {                     myservlet.this.notify();                 }             }         };         ref.addlistenerforsinglevalueevent(new valueeventlistener() {             @override             public void ondatachange(datasnapshot datasnapshot) {                     //notify main thread response can sent                     responsereadylistener.onresponseready();                 }             }              @override             public void oncancelled(databaseerror databaseerror) {              }         });         //the loop stops main thread until ondatachange() finishes         while (!responseready[0]) {             try {                 synchronized (this) {                     if (!responseready[0]) {                        this.wait();                     }                 }             } catch (interruptedexception e) {                 e.printstacktrace();             }         }

now question, have read 1 servlet instance created respond http requests. why cant use synchronized(this) stop synchronized threads of client responses server gets (i need stop 1 main thread, main thread of 1 request). how rid of method asynchronicity properly?

i've made research on own , found class countdownlatch. other classes executorservice not suitable when cannot runnable instance (when not api creates thread) case. here example of usage: 

final countdownlatch synchronizer = new countdownlatch(n/*this number represents nubmer of threads should finish before continuation*/);      ref.addlistenerforsinglevalueevent(new valueeventlistener() {         @override         public void ondatachange(datasnapshot datasnapshot) {                 //another thread, countdown() says countdownlatch                  //one thread has finished work (n = n - 1).                 synchronizer.countdown();             }         }     });      try {         //main thread. await() method stops thread until countdown()          //called n times, execution continues.         synchronizer.await();     } catch (interruptedexception e) {         e.printstacktrace();     } }

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