@MixinAfter
public class DefaultSort {
    public Enum Order { ASCENDING, DESCENDING };

    @Parameter(required=true, defaultPrefix="literal")
    private String sort;

    @Parameter(defaultPrefix="literal", value="literal:ascending")
    private Order order;

    @InjectContainer
    private Grid grid;

    @SetupRender
    void setupRender() {
        GridSortModel sortModel = grid.getSortModel();
        if (sortModel.getSortConstraints().isEmpty()) {
            sortModel.updateSort(sort);
            if (order == Order.ASCENDING) {
               // updateSort a second time for ascending
               sortModel.updateSort(sort);
            }
        }
    }
}

<t:grid source="queues" inPlace="true" t:mixins="defaultsort" sort="occupancy" />