Create a new layout called spinner_item.xml:
<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:gravity="center_vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
inside your activity MainActivity.java initialize your spinner view:
Spinner mySpinner= (Spinner) findViewById(R.id.mySpinner);
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this, R.array.items_array, R.layout.spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
mySpinner.setAdapter(adapter);
Finally use the following method to validate your spinner:
boolean validateSpinner(Spinner spinner, String error){
        View selectedView = spinner.getSelectedView();
        if (selectedView != null && selectedView instanceof TextView) {
            TextView selectedTextView = (TextView) selectedView;
            if (selectedTextView.getText().equals("")) {
                selectedTextView.setError(error);
                Toast.makeText(this, error, Toast.LENGTH_LONG).show();
                return false;
            }
        }
        return true;
    }