Creating Custom Component Classes - The J2EE 1.4 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 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:

ActionSource: Indicates that the component can fire an ActionEvent
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 have 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 UICommand, it automatically implements ActionSource and StateHolder. If your component extends UIOutput or one of the component classes that extend UIOutput, it automatically implements StateHolder and ValueHolder. If your component extends UIInput, it automatically implements EditableValueHolder, StateHolder, and ValueHolder. If your component extends UIComponentBase, it automatically implements only StateHolder. See the JavaServer Faces API Javadoc to find out what the other component classes implement.

If you want your custom component to exhibit the behavior of one of these interfaces, it must either explicitly implement the interface or extend a standard component class that implements the interface. For example, if you have a component that extends UIInput and you want it to fire action events, it must also implement ActionSource.

The image map example has two component classes: AreaComponent and MapComponent. The MapComponent class extends UICommand and therefore implements ActionSource, 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

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 map tag and the input tag

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

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 details how MapRenderer performs encoding and decoding, how it defines properties for the component's local values, and how it saves the state of MapComponent. 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 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.

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 converts 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 Value-Binding of Component Properties

Creating the Component Tag Handler describes how MapTag sets the component's values when processing the tag. For those component attributes that take value-binding expressions that point to a backing bean property, MapTag uses a ValueBinding instance to evaluate the expression.

To get the value of a component attribute that accepts a value-binding expression pointing to a backing bean property, the component class must get the ValueBinding instance associated with the attribute. Because MapComponent extends UICommand, the UICommand class already does the work of getting the ValueBinding instance associated with each of the attributes that it supports. However, if you have a custom component class that extends UIComponentBase, you will need to get the ValueBinding instance associated with those attributes that are value-binding enabled. For example, if MapComponent extended UIComponentBase instead of UICommand, it would need to include a method that gets the ValueBinding instance for the immediate attribute:

public boolean isImmediate() {
  if (this.immediateSet) {
      return (this.immediate);
  }
  ValueBinding vb = getValueBinding("immediate");
  if (vb != null) {
    Boolean value = (Boolean) vb.getValue(getFacesContext());
    return (value.booleanValue());
  } else {
      return (this.immediate);
  }
}  

The properties corresponding to the component attribute that accepts a method-binding expression pointing to a backing bean method must accept and return a MethodBinding object. For example, if MapComponent extended UIComponentBase instead of UICommand, it would need to provide an action property that returns and accepts a MethodBinding object:

public MethodBinding getAction() {
  return (this.action);
}
public void setAction(MethodBinding 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 launch deploytool, select the web application from the tree, and set its javax.faces.STATE_SAVING_METHOD context parameter to either client or server. See Specifying Where State Is Saved for more information on specifying where state is saved using deploytool.

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