Multiple selection in Spinner android

Question

I am using a Spinner and ArrayAdapter to populate items in Spinner. I want to make the spinner multi-selectable i.e. user can select multiple items and should also be able to deselect to none. My code is as below,

spnAdapter = new ArrayAdapter<String>(this,android.R.layout.select_dialog_multichoice,items);
spnAdapter.setDropDownViewResource(android.R.layout.simple_list_item_multiple_choice);
spnProducts.setAdapter(spnAdapter);

Here the items is an array list. This code gives me a spinner with check boxes but it does not allow multiple selection. What should I do to make it multiple selection?

Thanks in advance.