When to name a converter in JSF - jsf

Question

I've been working with converters in my PrimeFaces SelectOneMenu objects. They work fine if I only tell which class the converter is referring to:
#FacesConverter(forClass = DescriptorVO.class)
public class DescriptorVOConverter implements Converter { ... }
This way, I don't have to explicitly tell the JSF component which converter should be used when it is populated with objects of class DescriptorVO.
However, I made a page that used a p:SelectManyCheckbox and I couldn't for the life of me know why it wasn't calling my converter. Then I gave it a name, like so:
#FacesConverter(forClass = RoleVO.class, value = "roleVOConverter")
public class RoleVOConverter implements Converter { ... }
and passed it as one of the component's properties
<p:selectManyCheckbox id="cbx_roles" required="true" converter="roleVOConverter"
requiredMessage="At least one role must be selected."
value="#{userView.selectedRoles}" layout="responsive">
<f:selectItems value="#{userView.roles}" var="role"
itemLabel="#{role.title}" itemValue="#{role}" />
</p:selectManyCheckbox>
and voi la, it started calling the converter correctly. This raised a question to me regarding when I should name my converters (through the value attribute) and when telling them which class the converter should be used with (with the forClass attribute) is enough. I never had to namy any converters when working with PrimeFaces, only for this particular SelectManyCheckbox component. Do different components have different necessities regarding converters or did I just get the concept of converters wrong?

Answer 1

That can happen when the value of the UISelectMany component references a generic java.util.Collection type like List<Role> instead of an array like Role[]. This is specified in javadoc of UISelectMany (emphasis mine):
Obtain the Converter using the following algorithm:
If the component has an attached Converter, use it.
If not, look for a ValueExpression for value (if any). The ValueExpression must point to something that is:
An array of primitives (such as int[]). Look up the registered by-class Converter for this primitive type.
An array of objects (such as Integer[] or String[]). Look up the registered by-class Converter for the underlying element type.
A java.util.Collection. Do not convert the values.
If for any reason a Converter cannot be found, assume the type to be a String array.
The reason is, the generic type <Role> is lost during runtime and not directly determinable. It works when you're using MyFaces instead of Mojarra as it will in case of a java.util.Collection inspect the actual type by manually iterating over <f:selectItem(s)> and determine the Converter based on first non-null value.
I created spec issue 1422 on this to get this in JSF spec and improve Mojarra too.
See also:
UISelectMany on a List<T> causes java.lang.ClassCastException: java.lang.String cannot be cast to T
Why does JSF put String values in a Map<..., Integer>? And how to work around it?
Use enum in h:selectManyCheckbox