Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Creating Custom Component Classes

As explained in When to Use a Custom Component, a component class defines the state and behavior of a UI component. The state information includes the component’s type, identifier, and local value. The behavior defined by the component class includes the following:

- Decoding (converting the request parameter to the component’s local value)

- Encoding (converting the local value into the corresponding markup)

- Saving the state of the component

- Updating the bean value with the local value

- Processing validation on the local value

- Queueing events

The `javax.faces.component.UIComponentBase` class defines the default behavior of a component class. All the classes representing the standard components extend from `UIComponentBase`. These classes add their own behavior definitions, as your custom component class will do.

Your custom component class must either extend `UIComponentBase` directly or extend a class representing one of the standard components. These classes are located in the `javax.faces.component` package, and their names begin with `UI`.

If your custom component serves the same purpose as a standard component, you should extend that standard component rather than directly extend `UIComponentBase`. For example, suppose you want to create an editable menu component. It makes sense to have this component extend `UISelectOne` rather than `UIComponentBase` because you can reuse the behavior already defined in `UISelectOne`. The only new functionality you need to define is to make the menu editable.

Whether you decide to have your component extend `UIComponentBase` or a standard component, you might also want your component to implement one or more of these behavioral interfaces defined in the `javax.faces.component` package:

- `ActionSource`: Indicates that the component can fire a `javax.faces.event.ActionEvent`

- `ActionSource2`: Extends `ActionSource` and allows component properties referencing methods that handle action events to use method expressions as defined by the EL

- `EditableValueHolder`: Extends `ValueHolder` and specifies additional features for editable components, such as validation and emitting value-change events

- `NamingContainer`: Mandates that each component rooted at this component has a unique ID

- `StateHolder`: Denotes that a component has state that must be saved between requests

- `ValueHolder`: Indicates that the component maintains a local value as well as the option of accessing data in the model tier

If your component extends `UIComponentBase`, it automatically implements only `StateHolder`. Because all components directly or indirectly extend `UIComponentBase`, they all implement `StateHolder`. Any component that implements `StateHolder` also implements the `StateHelper` interface, which extends `StateHolder` and defines a `Map`-like contract that makes it easy for components to save and restore a partial view state.

If your component extends one of the other standard components, it might also implement other behavioral interfaces in addition to `StateHolder`. If your component extends `UICommand`, it automatically implements `ActionSource2`. If your component extends `UIOutput` or one of the component classes that extend `UIOutput`, it automatically implements `ValueHolder`. If your component extends `UIInput`, it automatically implements `EditableValueHolder` and `ValueHolder`. See the JavaServer Faces API documentation to find out what the other component classes implement.

You can also make your component explicitly implement a behavioral interface that it doesn’t already by virtue of extending a particular standard component. For example, if you have a component that extends `UIInput` and you want it to fire action events, you must make it explicitly implement `ActionSource2` because a `UIInput` component doesn’t automatically implement this interface.

The Duke’s Bookstore image map example has two component classes: `AreaComponent` and `MapComponent`. The `MapComponent` class extends `UICommand` and therefore implements `ActionSource2`, which means it can fire action events when a user clicks on the map. The `AreaComponent` class extends the standard component `UIOutput`. The `@FacesComponent` annotation registers the components with the JavaServer Faces implementation:

    @FacesComponent("DemoMap")
    public class MapComponent extends UICommand {...}

    @FacesComponent("DemoArea")
    public class AreaComponent extends UIOutput {...}

The `MapComponent` class represents the component corresponding to the `bookstore:map` tag:

    <bookstore:map id="bookMap"
                   current="map1"
                   immediate="true"
                   action="bookstore">
        ...
    </bookstore:map>

The `AreaComponent` class represents the component corresponding to the `bookstore:area` tag:

    <bookstore:area id="map1" value="#{Book201}"
                    onmouseover="resources/images/book_201.jpg"
                    onmouseout="resources/images/book_all.jpg"
                    targetImage="mapImage"/>

`MapComponent` has one or more `AreaComponent` instances as children. Its behavior consists of the following actions:

- Retrieving the value of the currently selected area

- Defining the properties corresponding to the component’s values

- Generating an event when the user clicks on the image map

- Queuing the event

- Saving its state

- Rendering the HTML `map` tag and the HTML `input` tag

`MapComponent` delegates the rendering of the HTML `map` and `input` tags to the `MapRenderer` class.

`AreaComponent` is bound to a bean that stores the shape and coordinates of the region of the image map. You will see how all this data is accessed through the value expression in Creating the Renderer Class. The behavior of `AreaComponent` consists of the following:

- Retrieving the shape and coordinate data from the bean

- Setting the value of the hidden tag to the `id` of this component

- Rendering the `area` tag, including the JavaScript for the `onmouseover`, `onmouseout`, and `onclick` functions

Although these tasks are actually performed by `AreaRenderer`, `AreaComponent` must delegate the tasks to `AreaRenderer`. See Delegating Rendering to a Renderer for more information.

The rest of this section describes the tasks that `MapComponent` performs as well as the encoding and decoding that it delegates to `MapRenderer`. Handling Events for Custom Components details how `MapComponent` handles events.

### Specifying the Component Family

If your custom component class delegates rendering, it needs to override the `getFamily` method of `UIComponent` to return the identifier of a component family, which is used to refer to a component or set of components that can be rendered by a renderer or set of renderers. The component family is used along with the renderer type to look up renderers that can render the component:

    public String getFamily() {
        return ("Map");
    }

The component family identifier, `Map`, must match that defined by the `component-family` elements included in the component and renderer configurations in the application configuration resource file. Registering a Custom Renderer with a Render Kit explains how to define the component family in the renderer configuration. Registering a Custom Component explains how to define the component family in the component configuration.

### Performing Encoding

During the Render Response phase, the JavaServer Faces implementation processes the encoding methods of all components and their associated renderers in the view. The encoding methods convert the current local value of the component into the corresponding markup that represents it in the response.

The `UIComponentBase` class defines a set of methods for rendering markup: `encodeBegin`, `encodeChildren`, and `encodeEnd`. If the component has child components, you might need to use more than one of these methods to render the component; otherwise, all rendering should be done in `encodeEnd`. Alternatively, you can use the `encodeALL` method, which encompasses all the methods.

Because `MapComponent` is a parent component of `AreaComponent`, the `area` tags must be rendered after the beginning `map` tag and before the ending `map` tag. To accomplish this, the `MapRenderer` class renders the beginning `map` tag in `encodeBegin` and the rest of the `map` tag in `encodeEnd`.

The JavaServer Faces implementation automatically invokes the `encodeEnd` method of `AreaComponent`'s renderer after it invokes `MapRenderer`'s `encodeBegin` method and before it invokes `MapRenderer`'s `encodeEnd` method. If a component needs to perform the rendering for its children, it does this in the `encodeChildren` method.

Here are the `encodeBegin` and `encodeEnd` methods of `MapRenderer`:

    @Override
    public void encodeBegin(FacesContext context, UIComponent component)
            throws IOException {
        if ((context == null)|| (component == null)) {
            throw new NullPointerException();
        }
        MapComponent map = (MapComponent) component;
        ResponseWriter writer = context.getResponseWriter();
        writer.startElement("map", map);
        writer.writeAttribute("name", map.getId(), "id");
    }

    @Override
    public void encodeEnd(FacesContext context, UIComponent component)
            throws IOException {
        if ((context == null) || (component == null)){
            throw new NullPointerException();
        }
        MapComponent map = (MapComponent) component;
        ResponseWriter writer = context.getResponseWriter();
        writer.startElement("input", map);
        writer.writeAttribute("type", "hidden", null);
        writer.writeAttribute("name", getName(context,map), "clientId");
        writer.endElement("input");
        writer.endElement("map");
    }

Notice that `encodeBegin` renders only the beginning `map` tag. The `encodeEnd` method renders the `input` tag and the ending `map` tag.

The encoding methods accept a `UIComponent` argument and a `javax.faces.context.FacesContext` argument. The `FacesContext` instance contains all the information associated with the current request. The `UIComponent` argument is the component that needs to be rendered.

The rest of the method renders the markup to the `javax.faces.context.ResponseWriter` instance, which writes out the markup to the current response. This basically involves passing the HTML tag names and attribute names to the `ResponseWriter` instance as strings, retrieving the values of the component attributes, and passing these values to the `ResponseWriter` instance.

The `startElement` method takes a `String` (the name of the tag) and the component to which the tag corresponds (in this case, `map`). (Passing this information to the `ResponseWriter` instance helps design-time tools know which portions of the generated markup are related to which components.)

After calling `startElement`, you can call `writeAttribute` to render the tag’s attributes. The `writeAttribute` method takes the name of the attribute, its value, and the name of a property or attribute of the containing component corresponding to the attribute. The last parameter can be null, and it won’t be rendered.

The `name` attribute value of the `map` tag is retrieved using the `getId` method of `UIComponent`, which returns the component’s unique identifier. The `name` attribute value of the `input` tag is retrieved using the `getName(FacesContext, UIComponent)` method of `MapRenderer`.

If you want your component to perform its own rendering but delegate to a renderer if there is one, include the following lines in the encoding method to check whether there is a renderer associated with this component:

    if (getRendererType() != null) {
        super.encodeEnd(context);
        return;
    }

If there is a renderer available, this method invokes the superclass’s `encodeEnd` method, which does the work of finding the renderer. The `MapComponent` class delegates all rendering to `MapRenderer`, so it does not need to check for available renderers.

In some custom component classes that extend standard components, you might need to implement other methods in addition to `encodeEnd`. For example, if you need to retrieve the component’s value from the request parameters, you must also implement the `decode` method.

### Performing Decoding

During the Apply Request Values phase, the JavaServer Faces implementation processes the `decode` methods of all components in the tree. The `decode` method extracts a component’s local value from incoming request parameters and uses a `javax.faces.convert.Converter` implementation to convert the value to a type that is acceptable to the component class.

A custom component class or its renderer must implement the `decode` method only if it must retrieve the local value or if it needs to queue events. The component queues the event by calling `queueEvent`.

Here is the `decode` method of `MapRenderer`:

    @Override
    public void decode(FacesContext context, UIComponent component) {
        if ((context == null) || (component == null)) {
            throw new NullPointerException();
        }
        MapComponent map = (MapComponent) component;
        String key = getName(context, map);
        String value = (String) context.getExternalContext().
                getRequestParameterMap().get(key);
        if (value != null)
            map.setCurrent(value);
        }
    }

The `decode` method first gets the name of the hidden `input` field by calling `getName(FacesContext, UIComponent)`. It then uses that name as the key to the request parameter map to retrieve the current value of the `input` field. This value represents the currently selected area. Finally, it sets the value of the `MapComponent` class’s `current` attribute to the value of the `input` field.

### Enabling Component Properties to Accept Expressions

Nearly all the attributes of the standard JavaServer Faces tags can accept expressions, whether they are value expressions or method expressions. It is recommended that you also enable your component attributes to accept expressions because it gives you much more flexibility when you write Facelets pages.

To enable the attributes to accept expressions, the component class must implement getter and setter methods for the component properties. These methods can use the facilities offered by the `StateHelper` interface to store and retrieve not only the values for these properties but also the state of the components across multiple requests.

Because `MapComponent` extends `UICommand`, the `UICommand` class already does the work of getting the `ValueExpression` and `MethodExpression` instances associated with each of the attributes that it supports. Similarly, the `UIOutput` class that `AreaComponent` extends already obtains the `ValueExpression` instances for its supported attributes. For both components, the simple getter and setter methods store and retrieve the key values and state for the attributes, as shown in this code fragment from `AreaComponent`:

    enum PropertyKeys {
        alt, coords, shape, targetImage;
    }
    public String getAlt() {
        return (String) getStateHelper().eval(PropertyKeys.alt, null);
    }
    public void setAlt(String alt) {
        getStateHelper().put(PropertyKeys.alt, alt);
    }
    ...

However, if you have a custom component class that extends `UIComponentBase`, you will need to implement the methods that get the `ValueExpression` and `MethodExpression` instances associated with those attributes that are enabled to accept expressions. For example, you could include a method that gets the `ValueExpression` instance for the `immediate` attribute:

    public boolean isImmediate() {
        if (this.immediateSet) {
            return (this.immediate);
        }
        ValueExpression ve = getValueExpression("immediate");
        if (ve != null) {
            Boolean value = (Boolean) ve.getValue(
                getFacesContext().getELContext());
            return (value.booleanValue());
        } else {
            return (this.immediate);
        }
    }

The properties corresponding to the component attributes that accept method expressions must accept and return a `MethodExpression` object. For example, if `MapComponent` extended `UIComponentBase` instead of `UICommand`, it would need to provide an `action` property that returns and accepts a `MethodExpression` object:

    public MethodExpression getAction() {
        return (this.action);
    }
    public void setAction(MethodExpression action) {
        this.action = action;
    }

### Saving and Restoring State

As described in Enabling Component Properties to Accept Expressions, use of the `StateHelper` interface facilities allows you to save the component’s state at the same time you set and retrieve property values. The `StateHelper` implementation allows partial state saving; it saves only the changes in the state since the initial request, not the entire state, because the full state can be restored during the Restore View phase.

Component classes that implement `StateHolder` may prefer to implement the `saveState(FacesContext)` and `restoreState(FacesContext, Object)` methods to help the JavaServer Faces implementation save and restore the state of components across multiple requests.

To save a set of values, you can implement the `saveState(FacesContext)` method. This method is called during the Render Response phase, during which the state of the response is saved for processing on subsequent requests. Here is a hypothetical method from `MapComponent`, which has only one attribute, `current`:

    @Override
    public Object saveState(FacesContext context) {
        Object values[] = new Object[2];
        values[0] = super.saveState(context);
        values[1] = current;
        return (values);
    }

This method initializes an array, which will hold the saved state. It next saves all of the state associated with the component.

A component that implements `StateHolder` may also provide an implementation for `restoreState(FacesContext, Object)`, which restores the state of the component to that saved with the `saveState(FacesContext)` method. The `restoreState(FacesContext, Object)` method is called during the Restore View phase, during which the JavaServer Faces implementation checks whether there is any state that was saved during the last Render Response phase and needs to be restored in preparation for the next postback.

Here is a hypothetical `restoreState(FacesContext, Object)` method from `MapComponent`:

    public void restoreState(FacesContext context, Object state) {
        Object values[] = (Object[]) state;
        super.restoreState(context, values[0]);
        current = (String) values[1];
    }

This method takes a `FacesContext` and an `Object` instance, representing the array that is holding the state for the component. This method sets the component’s properties to the values saved in the `Object` array.

Whether or not you implement these methods in your component class, you can use the `javax.faces.STATE_SAVING_METHOD` context parameter to specify in the deployment descriptor where you want the state to be saved: either `client` or `server`. If state is saved on the client, the state of the entire view is rendered to a hidden field on the page. By default, the state is saved on the server.

The web applications in the Duke’s Forest case study save their view state on the client.

Saving state on the client uses more bandwidth as well as more client resources, whereas saving it on the server uses more server resources. You may also want to save state on the client if you expect your users to disable cookies.

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