read data from firebase synchronously in java

import java.util.concurrent.Semaphore;

		// create a java.util.concurrent.Semaphore with 0 initial permits
		final Semaphore semaphore = new Semaphore(0);

		// attach a value listener to a Firebase reference
		ref.addValueEventListener(new ValueEventListener() {
		    // onDataChange will execute when the current value loaded and whenever it changes
		    @Override
		    public void onDataChange(DataSnapshot dataSnapshot) {
		        // TODO: do whatever you need to do with the dataSnapshot

		        semaphore.release();
		    }

		    @Override
		    public void onCancelled(FirebaseError firebaseError) {

		    }
		});

		// wait until the onDataChange callback has released the semaphore
		semaphore.acquire();

		// send our response message
		ref.push().setValue("response message");