The Java EE 5 Tutorial

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().
    if (value != null)

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.