Understanding javax.el.ValueExpression purpose

Question

I'm trying to understand the working principles of UIInput component and EL-expressions in JSF. Here is the code fragment of UIInput component source code:

public void updateModel(FacesContext context) {

        if (context == null) {
            throw new NullPointerException();
        }

        if (!isValid() || !isLocalValueSet()) {
            return;
        }
        ValueExpression ve = getValueExpression("value");
        if (ve != null) {
            Throwable caught = null;
            FacesMessage message = null;
            try {
                ve.setValue(context.getELContext(), getLocalValue()); //1
                setValue(null);
                setLocalValueSet(false);
            }
            //catch clause

Now, I have a simple bean and the input component binded to its property:

<h:inputText value="#{myBean.hello}" />

@ManagedBean
@SessionScoped
public class MyBean{
    private String hello;
    //getter,setter
}

As far as I understand, the updateModel method is called by the inherited from the UIComponentBase class processUpdates(FacesContext) method which is a standard callback for the Update Model Values phase. So, after setting breakpoint at //1 and performing step next the flow's stopped at the corresponding bean property setter method.

My question is about of purpose of javax.el.ValueExpression. Is it resposible for all interatctions (incapsulates) between the component class and the bean's property in order for getting/setting values to bean's properties?

Answer 1

EL is like a "path" and is needed to navigate the bean. The UIInput is the MVC "view" to the MVC "model" represented by your bean. EL links the two together. It only encapsulates the traversal itself. It identifies the subject of the interaction, but the interactions are defined by the component class. Compare h:outputText to h:inputText: both basically take a value="#{some.el}" attribute; the fact that one allows the user to modify it is a fact about the component, not the path to the bean property itself.

Some components define attributes that should be populated by references to methods rather than bean properties, but again, the EL is just the traversal—invoking those methods is something the component does. And in that case you would be dealing with a MethodExpression instead of a ValueExpression.

The JSF lifecycle dictates that things happen in a certain overall order, but the component author still has ample opportunity to be creative (or wreak havoc) by doing things their own way.