Custom page controller for DXA JAVA. Also see https://tridion.stackexchange.com/a/18256/88

MyPageController.class

@Controller
public class MyPageController {

    // Inject the PageController, because we cannot override and change the RequestMapping
    @Autowired
    private PageController pageController;

    // This mapping is more specific than the DXA PageController mapping
    @RequestMapping(
            value = {"/**"},
            produces = {"text/html", "*/*"},
            method = {RequestMethod.GET, RequestMethod.POST}
    )
    public String handleGetPage(HttpServletRequest request, HttpServletResponse response) throws Exception {
        String view = pageController.handleGetPage(request, response);

        PageModel pageModel = (PageModel) request.getAttribute("pageModel");
        pageModel = this.enrichModel(pageModel, request);
        request.setAttribute("pageModel", pageModel);

        return view;
    }

    // enrich the page model, with the DXA enrichModel() pattern 
    protected PageModel enrichModel(PageModel pageModel, HttpServletRequest httpServletRequest) throws Exception {

        //do your thing here
        pageModel.setTitle("Hello, World!");

        return pageModel;
    }
}