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:

The 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:

If your component extends UIComponentBase, it automatically implements only StateHolder. Because all components--directly or indirectly--extend UIComponentBase, they all implement StateHolder.

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 Javadoc 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 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 MapComponent class represents the component corresponding to the map tag: 

<bookstore:map id="worldMap" current="NAmericas" 
  immediate="true" 
  action="bookstore"
  actionListener="#{localeBean.chooseLocaleFromMap}">

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

<bookstore:area id="NAmerica" value="#{NA}"
  onmouseover="/template/world_namer.jpg" 
  onmouseout="/template/world.jpg" 
  targetImage="mapImage" />

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

The rendering of the map and input tags is performed by MapRenderer, but MapComponent delegates this rendering to MapRenderer.

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

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.

Because MapComponent delegates its rendering, it overrides the getFamily method: 

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 (page 466) explains how to define the component family in the renderer configuration. Registering a Custom Component (page 469) 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.

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: 

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");
} 
public void encodeEnd(FacesContext context) 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 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 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--to, for example, update a bean's values--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 Converter class 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 MapRenderer instance retrieves the local value of the hidden input field and sets the current attribute to this value by using its decode method. The setCurrent method of MapComponent queues the event by calling queueEvent, passing in the AreaSelectedEvent instance generated by MapComponent.

Here is the decode method of MapRenderer: 

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 a method expressions. It is recommended that you also enable your component attributes to accept expressions because this is what page authors expect, and it gives page authors much more flexibility when authoring their pages.

Creating the Component Tag Handler describes how MapTag, the tag handler for the map tag, sets the component's values when processing the tag. It does this by providing the following:

To retrieve the expression objects that setProperties stored, the component class must implement a method for each property that accesses the appropriate expression object, extracts the value from it and returns the value.

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.

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, if MapComponent extended UIComponentBase instead of UICommand, it would need to 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

Because component classes implement StateHolder, they must 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 must 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 the method from MapComponent: 

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 MapComponent.

A component that implements StateHolder must 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 the 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.

When you implement these methods in your component class, be sure 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.

To specify where state is saved for a particular web application, you need to set the javax.faces.STATE_SAVING_METHOD context parameter to either client or server in your application's deployment descriptor. See Saving and Restoring State for more information on specifying where state is saved in the deployment desciptor.