您需要使用addChildEventListener而不是addValueEventListener.

尝试:

queryRef.addChildEventListener(new ChildEventListener() {
    public void onChildAdded(DataSnapshot dataSnapshot, String previousKey) {
        test data = dataSnapshot.getValue(test.class);

                    System.out.println(data.getUrl());
    }
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
    }
    public void onChildRemoved(DataSnapshot dataSnapshot) {
    }
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
    }
    public void onCancelled(FirebaseError firebaseError) { }
});