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joi, 24 septembrie 2015

JSF and Composite design pattern - part III (JSF implementation)


In this part, we try to indentify a real use case of the composite design pattern inside JSF source code. Remember that the composite pattern  work with composite and leaf (primitive) objects that should be treated equally (uniformly), and a composite object is a container for leaf  (primitive) objects. Keeping this in mind, we can start diving in JSF source code in order to identify the base component object, the leaf objects and the composite objects.

As a JSF developer, you should know the importance of the view (UI view) in JSF lifecycle, and you should know that the view is build in the first phase, Restore View Phase, and in the last phase, Render Response phase. The view is basically the JSF unit of work and it contains JSF UI components. All JSF UI components have a super-component known as UIComponentBase (extends Component) which represents an abstract base component in composite pattern. Basically, the UIComponentBase provides a set of common methods for JSF UI components, such as methods for decoding (e.g. decode(),processDecodes()), encoding (e.g. encodeBegin(), encodeChildren(), encodeEnd()), subscribe to events (e.g. subscribeToEvent()), etc.

base component in composite pattern
// javax.faces.component.UIComponentBase
public abstract class UIComponentBase extends UIComponent {

 // methods for UI components

 public void decode(FacesContext context) {
  ...
 }

 public void encodeBegin(FacesContext context) throws IOException {
  ...
 }

 public void processDecodes(FacesContext context) {
  ...
 }
 ...
}

So, the JSF UI components extends and shapes the UIComponentBase contract. Components like, UIInput, UIOutput, UIPanel, UISelectMany, UISelectOne, UIForm, etc, extend the UIComponentBase and override the necessary methods. This means that we can easily intuit that these components are actually the leaf (primitive) objects in the composite pattern, and the base component (UIComponentBase) defines common methods for leaf objects (UI components)  and composite objects.

Leaf object in composite pattern
// javax.faces.component.UIOutput 
public class UIOutput extends UIComponentBase implements ValueHolder {
 ...
}

Leaf object in composite pattern
// javax.faces.component.UIPanel 
public class UIPanel extends UIComponentBase {
 ...
}

Further, we have to identify the composite object. Theoretically speaking, this object should "hold" together the leaf objects (UI components) and implements the operations in base component (UIComponentBase). Since the JSF UI components are "grouped" in a view, we can say that the view represents the composite object, which programmatically is represented by the UIViewRoot component:

composite objects in composite pattern
// javax.faces.component.UIViewRoot 
public class UIViewRoot extends UIComponentBase implements UniqueIdVendor {
 ...
}

So, we have identified the following JSF artifacts as the composite pattern implementation:
·         base component object - UIComponentBase
·         leaf (primitive) objects - UI components (e.g. UIOutputUIPanel, UIForm, etc)
·         composite object - UIViewRoot (JSF view)

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