解决方案：使用下面的代码（您可以查看注解行以查看指南）

String defaultValue = pref02.getString("fonte", "Normal");
String[] items = new String[]{"Normal", "10", "12", "14", "16", "18", "20", "24", "28", "30"};
ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_dropdown_item, items);
dropdown.setAdapter(adapter);
int spinnerPosition = adapter.getPosition(defaultValue);
dropdown.setSelection(spinnerPosition); //select the atual saved value.
// Add this block of code
dropdown.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {
    @Override
    public void onLayoutChange(View v, int left, int top, int right, int bottom, int oldLeft, int oldTop, int oldRight, int oldBottom) {
        // Remove listener to make sure it doesn't called later.
        dropdown.removeOnLayoutChangeListener(this);
        // Set your listener here
        dropdown.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parentView, View selectedItemView, int position, long id) {
                SharedPreferences pref2 = getApplicationContext().getSharedPreferences("pref02", MODE_PRIVATE);
                SharedPreferences.Editor editor = pref2.edit();
                String drop = dropdown.getSelectedItem().toString();
                editor.putString("fonte", drop);
                editor.apply();
                Toast.makeText(Config.this, "Saved :)", Toast.LENGTH_LONG).show();
            }
            @Override
            public void onNothingSelected(AdapterView<?> parentView) {
                // your code here
            }
        });
    }
});