Question

I am trying to understand how Wicket LoadableDetachable model is working.
What i understand from the Wicket documentation is in a normal scenario , when the request completes the processing wicket will auto serialize the all components with associated model values. This one will consumes more memory . If we use the LoadableDetachable model at the time of serialization the model values will not get serialized. Is this right? . So it will detach the model object automatically . So For the next request the model value will be reload again automatically? See my below code.

public class ProductListPanel extends Column<Product> {

    @SpringBean
    private ProductService productService;
    private List productList;


    public ProductListPanel(String id) {
        super(id);
        class ProductModel extends LoadableDetachableModel {

            @Override
            public void detach() {
                // TODO Auto-generated method stub
                productList = null;
                System.out.print("Called Detach Object\n");
            }


            @Override
            protected Object load() {
                // TODO Auto-generated method stub
                productList = productService.findAll();
                System.out.print("Called Get Object\n");
                return productList;
            }

        }
        System.out.print("Before creating also calling\n");
        final ProductModel productModel = new ProductModel();

        ListView view = new ListView("list", productModel) {

            protected void populateItem(ListItem item) {
                System.out.print("\nBefore start also calling\n");
                System.out.print("Before this one is callling \n");
                Product result = (Product) item.getModelObject();
                item.add(new Label("code", result.getCode()));
                item.add(new Label("name", result.getName()));
                final Link deleteLink = new Link("remove", item.getModel()) {

                    @Override
                    public void onClick() {
                        Product product = (Product) getModelObject();
                        productService.delete(product);

                    }

                };
                item.add(deleteLink);

            }
        };

        add(view);
        Link addProductLink = new Link("addProduct") {

            @Override
            public void onClick() {
                // TODO Auto-generated method stub
                setResponsePage(new AddProduct());
            }

        };
        add(addProductLink);
        productModel.detach();
    }
}

In the above code i am listing all the products from the DB. and i am having Remove link for each product , when we are clicking that link i am removing the product from the DB. After clicking the Remove link the page is not getting refreshed means it still showing the deleted product . if i add this line productModel.detach(); then it is working properly . My question here is why i have to call productModel.detach(); manually? LoadableDetachableModel suppose do automatically right?

Please help me

Answer 1

LoadableDetachableModel caches the retrieved object for the duration it is attached. When you load the data from the database, all items are there. Then you execute a query to delete a record, but don’t update the list. So either you have to change your onClick handler to also update the retrieved list, or just detach the model.

There is no magic in LoadableDetachableModel. Take a look at the code below:

List<Person> people = dao.getListOfPeopleAttendingParty();

// assume that the number of people attending the party is 4
assert people.size() == 4;

Person guest = people.get(0);
dao.delete(guest);

// what is the number of people attending the party now?
assert people.size() == ?;

What do you think that people.size() is now?

The execution of LoadableDetachableModel is not any different than the code snippet above:

List<Person> people = peopleModel.getObject(); // is called by ListView

// assume that the number of people attending the party is 4
assert people.size() == 4;

Person guest = people.get(0);
dao.delete(guest);

// what is the number of people attending the party now?
assert people.size() == ?;

So: no magic involved, just plain Java and ordinary logic.

PS. The answer to both questions is 4 even though the database no longer holds the guest