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 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:

• A method for each attribute that takes either a ValueExpression or MethodExpression object depending on what kind of expression the attribute accepts.

• A setProperties method that stores the ValueExpression or MethodExpression object for each component property so that the component class can retrieve the expression object later.

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;
}