Html:

<form wicket:id="form">
    <input type="submit" value="submit"/>
</form>

Java code:

Form<Void> form = new Form<Void>("form"){
    @Override
    protected void onSubmit() {
            System.out.println("Form submitted.");
    }
};
add(form);

Method onSubmit is called whenever a form has been submitted and it can be overridden to perform custom actions. Please note that a Wicket form can be submitted using a standard HTML submit button which is not mapped to any component (i.e. it does not have a wicket:id attribute). In the next chapter we will continue to explore Wicket forms and we will see how to submit forms using special components which implement interface org.apache.wicket.markup.html.form.IFormSubmitter.

11.4.1. Form and models

A form should contain some input fields (like text fields, check boxes, radio buttons, drop-down lists, text areas, etc.) to interact with users. Wicket provides an abstraction for all these kinds of elements with component org.apache.wicket.markup.html.form.FormComponent:

The purpose of FormComponent is to store the corresponding user input into its model when the form is submitted. The form is responsible for mapping input values to the corresponding components, avoiding us the burden of manually synchronizing models with input fields and vice versa.

11.4.2. Login form

As first example of interaction between the form and its models, we will build a classic login form which asks for username and password (project LoginForm).

This form needs two text fields, one of which must be a password field. We should also use a label to display the result of login process. For the sake of simplicity, the login logic is all inside onSubmit and is quite trivial.

The following is a possible implementation of our form:

public class LoginForm extends Form {
        private TextField usernameField;
        private PasswordTextField passwordField;
        private Label loginStatus;
        public LoginForm(String id) {
                super(id);
                usernameField = new TextField("username", Model.of(""));
                passwordField = new PasswordTextField("password", Model.of(""));
                loginStatus = new Label("loginStatus", Model.of(""));
                add(usernameField);
                add(passwordField);
                add(loginStatus);
        }
        public final void onSubmit() {
                String username = (String)usernameField.getDefaultModelObject();
                String password = (String)passwordField.getDefaultModelObject();
                if(username.equals("test") && password.equals("test"))
                        loginStatus.setDefaultModelObject("Congratulations!");
                else
                        loginStatus.setDefaultModelObject("Wrong username or password!");
        }
}

Inside form’s constructor we build the three components used in the form and we assign them a model containing an empty string:

usernameField = new TextField("username", Model.of(""));
passwordField = new PasswordTextField("password", Model.of(""));
loginStatus = new Label("loginStatus", Model.of(""));

If we don’t provide a model to a form component, we will get the following exception on form submission:

java.lang.IllegalStateException: Attempt to set model object on null model of component:

Component TextField corresponds to the standard text field, without any particular behavior or restriction on the allowed values. We must bind this component to the tag with the attribute type set to “text” PasswordTextField is a subtype of TextFiled and it must be used with an tag with the attribute type set to “password”. For security reasons component PasswordTextField cleans its value at each request, so it will be always empty after the form has been rendered. By default PasswordTextField fields are required, meaning that if we left them empty, the form won’t be submitted (i.e. onSubmit won’t be called). Class FormComponent provides method setRequired(boolean required) to change this behavior. Inside onSubmit, to get/set model objects we have used shortcut methods setDefaultModelObject and getDefaultModelObject. Both methods are defined in class Component (see class diagram from illustration 9.1).

The following are the possible markup and code for the login page:

Html:

<html>
        <head>
                  <title>Login page</title>
        </head>
        <body>
                <form id="loginForm" method="get" wicket:id="loginForm">
                          <fieldset>
                            <legend style="color: #F90">Login</legend>
                                    <p wicket:id="loginStatus"></p>
                                    <span>Username: </span><input wicket:id="username" type="text" id="username" /><br/>
                                    <span>Password: </span><input wicket:id="password" type="password" id="password" />
                                    <p>
                                            <input type="submit" name="Login" value="Login"/>
                                    </p>
                         </fieldset>
                </form>
        </body>
</html>

Java code:

public class HomePage extends WebPage {
   public HomePage(final PageParameters parameters) {
                super(parameters);
            add(new LoginForm("loginForm"));
    }
}

The example shows how Wicket form components can be used to store user input inside their model. However we can dramatically improve the form code using CompoundPropertyModel and its ability to access the properties of its model object. The revisited code is the following (the LoginFormRevisited project):

public class LoginForm extends Form{
                private String username;
                private String password;
                private String loginStatus;
                public LoginForm(String id) {
                        super(id);
                        setDefaultModel(new CompoundPropertyModel(this));
                        add(new TextField("username"));
                        add(new PasswordTextField("password"));
                        add(new Label("loginStatus"));
                }
                public final void onSubmit() {
                        if(username.equals("test") && password.equals("test"))
                                loginStatus = "Congratulations!";
                        else
                                loginStatus = "Wrong username or password !";
                }
        }

In this version the form itself is used as model object for its CompoundPropertyModel. This allows children components to have direct access to form fields and use them as backing objects, without explicitly creating a model for themselves.

//Create a person named 'John Smith'
Person person = new Person("John", "Smith");
//Create a person named 'Jill Smith'
Person spouse = new Person("Jill", "Smith");
//Set Jill as John's spouse
person.setSpouse(spouse);
setDefaultModel(new CompoundPropertyModel(person));
WebMarkupContainer spouseContainer = new WebMarkupContainer("spouse");
Label name;
spouseContainer.add(name = new Label("name"));
add(spouseContainer);

The value displayed by label “name” will be “John” and not the spouse’s name “Jill” as you may expect. In this example the label doesn’t own a model, so it must search up its container hierarchy for an inheritable model. However, its container (WebMarkup Container with id ‘spouse’) doesn’t own a model, hence the request for a model is forwarded to the parent container, which in this case is the page. In the end the label inherits CompoundPropertyModel from page but only its own id is used for the property expression. The containers in between are never taken into account for the final property expression.

11.5. Component DropDownChoice

Class org.apache.wicket.markup.html.form.DropDownChoice is the form component needed to display a list of possible options as a drop-down list where users can select one of the proposed options. This component must be used with tag: Html: <form wicket:id="form"> Select a fruit: <select wicket:id="fruits"></select> <div><input type="submit" value="submit"/></div> </form> Java code: List<String> fruits = Arrays.asList("apple", "strawberry", "watermelon"); form.add(new DropDownChoice<String>("fruits", new Model(), fruits)); Screenshot of generated page: In addition to the component id, in order to build a DropDownChoice we need to provide to its constructor two further parameters: a model containing the current selected item. This parameter is not required if we are going to inherit a CompoundPropertyModel for this component. a list of options to display which can be supplied as a model or as a regular java.util.List. In the example above the possible options are provided as a list of String objects. Now let’s take a look at the markup generated for them: <select name="fruits" wicket:id="fruits"> <option value="" selected="selected">Choose One</option> <option value="0">apple</option> <option value="1">strawberry</option> <option value="2">watermelon</option> </select> The first option is a placeholder item corresponding to a null model value. By default DropDownChoice cannot have a null value so users are forced to select a not-null option. If we want to change this behavior we can set the nullValid flag to true via the setNullValid method. Please note that the placeholder text (“Chose one”) can be localized, as we will see in chapter 15. The other options are identified by the attribute value. By default the value of this attribute is the index of the single option inside the provided list of choices, while the text displayed to the user is obtained by calling toString()on the choice object. This default behavior works fine as long as our options are simple objects like strings, but when we move to more complex objects we may need to implement a more sophisticated algorithm to generate the value to use as the option id and the one to display to user. Wicket has solved this problem with org.apache.wicket.markup.html.form.IChoiceRender interface. This interface defines method getDisplayValue(T object) that is called to generate the value to display for the given choice object, and method getIdValue(T object, int index) that is called to generate the option id. The built-in implementation of this interface is class org.apache.wicket.markup.html.form.ChoiceRenderer which renders the two values using property expressions. In the following code we want to show a list of Person objects using their full name as value to display and using their passport code as option id: Java code: List<Person> persons; //Initialize the list of persons here... ChoiceRenderer personRenderer = new ChoiceRenderer("fullName", "passportCode"); form.add(new DropDownChoice<String>("persons", new Model<Person>(), persons, personRenderer)); The choice renderer can be assigned to the DropDownChoice using one of its constructors that accepts this type of parameter (like we did in the example above) or after its creation invoking setChoiceRenderer method. 11.6. Model chainingModels that extend org.apache.wicket.model.ChainingModel can be used to build a chain of models. These models are able to recognize whether their model object is itself an implementation of IModel and if so, they will call getObject on the wrapped model and the returned value will be the actual model object. In this way we can combine the action of an arbitrary number of models, making exactly a chain of models. Chaining models allows to combine different data persistence strategies, similarly to what we do with chains of I/O streams. To see model chaining in action we will build a page that implements the List/Detail View pattern, where we have a drop-down list of Person objects and a form to display and edit the data of the current selected Person. The example page will look like this: What we want to do in this example is to chain the model of the DropDownChoice (which contains the selected Person) with the model of the Form. In this way the Form will work with the selected Person as backing object. The DropDownChoice component can be configured to automatically update its model each time we change the selected item on the client side. All we have to do is to add a FormComponentUpdatingBehavior to it: The behavior will submit the components value every time JavaScript event “change” occurs, and its model will be consequently updated. To leverage this functionality, the form component doesn’t need to be inside a form. The following is the resulting markup of the example page: ... <body> List of persons <select wicket:id="persons"></select> <br/> <br/> <form wicket:id="form"> <div style="display: table;"> <div style="display: table-row;"> <div style="display: table-cell;">Name: </div> <div style="display: table-cell;"> <input type="text" wicket:id="name"/> </div> </div> <div style="display: table-row;"> <div style="display: table-cell;">Surname: </div> <div style="display: table-cell;"> <input type="text" wicket:id="surname"/> </div> </div> <div style="display: table-row;"> <div style="display: table-cell;">Address: </div> <div style="display: table-cell;"> <input type="text" wicket:id="address"/> </div> </div> <div style="display: table-row;"> <div style="display: table-cell;">Email: </div> <div style="display: table-cell;"> <input type="text" wicket:id="email"/> </div> </div> </div> <input type="submit" value="Save"/> </form> </body> The initialization code for DropDownChoice is the following: Model<Person> listModel = new Model<Person>(); ChoiceRenderer<Person> personRenderer = new ChoiceRenderer<Person>("fullName"); personsList = new DropDownChoice<Person>("persons", listModel, loadPersons(), personRenderer); personsList.add(new FormComponentUpdatingBehavior()); As choice renderer we have used the basic implementation provided with the org.apache.wicket .markup.html.form.ChoiceRenderer class that we have seen in the previous paragraph. loadPersons() is just an utility method which generates a list of Person instances. The model for DropDownChoice is a simple instance of the Model class. Here is the whole code of the page (except for the loadPersons() method): public class PersonListDetails extends WebPage { private Form<Void> form; private DropDownChoice<Person> personsList; public PersonListDetails(){ Model<Person> listModel = new Model<Person>(); ChoiceRenderer<Person> personRender = new ChoiceRenderer<Person>("fullName"); personsList = new DropDownChoice<Person>("persons", listModel, loadPersons(), personRender); personsList.add(new FormComponentUpdatingBehavior()); add(personsList); form = new Form<>("form", new CompoundPropertyModel<Person>(listModel)); form.add(new TextField("name")); form.add(new TextField("surname")); form.add(new TextField("address")); form.add(new TextField("email")); add(form); } //loadPersons() //... } The two models work together as a pipeline where the output of method getObject of Model is the model object of CompoundPropertyModel. As we have seen, model chaining allows us to combine the actions of two or more models without creating new custom implementations. 11.7. Detachable modelsIn chapter 6 we have seen how Wicket uses serialization to store page instances. When an object is serialized, all its referenced objects are recursively serialized. For a page this means that all its children components, their related models as well as the model objects inside them will be serialized. For model objects this could be a serious issue for (at least) two main reasons: The model object could be a very large instance, hence serialization would become very expensive in terms of time and memory. We simply may not be able to use a serializable object as model object. In paragraphs 1.4 and 9.2 we stated that Wicket allows us to use a POJO as backing object, but POJOs are ordinary objects with no prespecified interface, annotation or superclass, hence they are not required to implement the standard Serializable interface. To cope with these problems IModel extends another interface called IDetachable. This interface provides a method called detach() which is invoked by Wicket at the end of web request processing when data model is no more needed but before serialization occurs. Overriding this method we can clean any reference to data object keeping just the information needed to retrieve it later (for example the id of the table row where our data are stored). In this way we can avoid the serialization of the object wrapped into the model overcoming both the problem with non-serializable objects and the one with large data objects. Since IModel inherits from IDetachable, every model of Wicket is “detachable”, although not all of them implement a detaching policy (like the Model class). Usually detaching operations are strictly dependent on the persistence technology adopted for model objects (like a relational db, a NoSQL db, a queue, etc), so it’s not unusual to write a custom detachable model suited for the persistence technology chosen for a given project. To ease this task Wicket provides abstract model LoadableDetachableModel. This class internally holds a transient reference to a model object which is initialized the first time getObject()is called to precess a request. The concrete data loading is delegated to abstract method T load(). The reference to a model object is automatically set to null at the end of the request by the detach() method. The following class diagram summarizes the methods defined inside LoadableDetachableModel. onDetach and onAttach can be overridden in order to obtain further control over the detaching procedure. Now as example of a possible use of LoadableDetachableModel, we will build a model designed to work with entities managed via JPA. To understand the following code a basic knowledge of JPA is required even if we won’t go into the detail of this standard. The following model is provided for example purposes only and is not intended to be used in production environment. Important aspects such as transaction management are not taken into account and you should rework the code before considering to use it. public class JpaLoadableModel<T> extends LoadableDetachableModel<T> { private EntityManagerFactory entityManagerFactory; private Class<T> entityClass; private Serializable identifier; private List<Object> constructorParams; public JpaLoadableModel(EntityManagerFactory entityManagerFactory, T entity) { super(); PersistenceUnitUtil util = entityManagerFactory.getPersistenceUnitUtil(); this.entityManagerFactory = entityManagerFactory; this.entityClass = (Class<T>) entity.getClass(); this.identifier = (Serializable) util.getIdentifier(entity); setObject(entity); } @Override protected T load() { T entity = null; if(identifier != null) { EntityManager entityManager = entityManagerFactory.createEntityManager(); entity = entityManager.find(entityClass, identifier); } return entity; } @Override protected void onDetach() { super.onDetach(); T entity = getObject(); PersistenceUnitUtil persistenceUtil = entityManagerFactory.getPersistenceUnitUtil(); if(entity == null) return; identifier = (Serializable) persistenceUtil.getIdentifier(entity); } } The constructor of the model takes as input two parameters: an implementation of the JPA interface javax.persistence.EntityManagerFactory to manage JPA entities and the entity that must be handled by this model. Inside its constructor the model saves the class of the entity and its id (which could be null if the entity has not been persisted yet). These two informations are required to retrieve the entity at a later time and are used by the load method. onDetach is responsible for updating the entity id before detachment occurs. The id can change the first time an entity is persisted (JPA generates a new id and assigns it to the entity). Please note that this model is not responsible for saving any changes occurred to the entity object before it is detached. If we don’t want to loose these changes we must explicitly persist the entity before the detaching phase occurs. Since the model of this example holds a reference to the EntityManager Factory, the implementation in use must be serializable. 11.8. Using more than one model in a componentSometimes our custom components may need to use more than a single model to work properly. In such a case we must manually detach the additional models used by our components. In order to do this we can overwrite the Component’s onDetach method that is called at the end of the current request. The following is the generic code of a component that uses two models: /** * * fooModel is used as main model while beeModel must be manually detached * */ public class ComponetTwoModels extends Component{ private IModel<Bee> beeModel; public ComponetTwoModels(String id, IModel<Foo> fooModel, IModel<Bee> beeModel) { super(id, fooModel); this.beeModel = beeModel; } @Override public void onDetach() { if(beeModel != null) beeModel.detach(); super.onDetach(); } } When we overwrite onDetach we must call the super class implementation of this method, usually as last line in our custom implementation. 11.9. Use models!Like many people new to Wicket, you may need a little time to fully understand the power and the advantages of using models. Taking your first steps with Wicket you may be tempted to pass raw objects to your components instead of using models: /** * * NOT TO DO: passing raw objects to components instead of using models! * */ public class CustomComponent extends Component{ private FooBean fooBean; public CustomComponent(String id, FooBean fooBean) { super(id); this.fooBean = fooBean; } //...some other ugly code :)... } That’s a bad practice and you must avoid it. By using models we do not only decouple our components from the data source, but we can also rely on them (if they are dynamic) to work with the most up-to-date version of our model object. If we decide to bypass models we lose all these advantages and we force model objects to be serialized. 11.10. SummaryModels are at the core of Wicket and they are the basic ingredient needed to taste the real power of the framework. In this chapter we have seen how to use models to bring data to our components without littering their code with technical details about their persistence strategy. We have also introduced Wicket forms as complementary topic. With forms and models we are able to bring our applications to life allowing them to interact with users. But what we have seen in this chapter about Wicket forms is just the tip of the iceberg. That’s why the next chapter is entirely dedicated to them.