Knight of the round table

Selecting from a list of entities

See http://docs.jboss.org/seam/latest/reference/en/html/controls.html

• Use <s:selectItems> to produce a list of labeled select items from a list of entities.
• Use <s:convertEntity> to map back and forth between the select items and the actual entity values. This is what allows you to map the value of the <h:selectOneMenu> directly to the property of the referencing entity (e.g. a property that is a many-to-one).

<h:selectOneMenu value="#{person.continent}" required="true">                        (1)



<s:selectItems value="#{continents.resultList}" var="continent"                  (2)



label="#{continent.name}" noSelectionLabel="Please Select..."/>



<s:convertEntity />                                                              (3)



</h:selectOneMenu>

1. person is an entity that has been outjected into the conversation. It has a 'continent' property which is many-to-one association with another entity.
2. continents is a Seam application framework 'query' object. This 'query' object should probably use a Seam-managed EntityManager because we want have the Hibernate session-in-view behavior so we don't get lazy initialization exceptions when rendering the labels, etc.
3. s:convertEntity will convert the Continent entities into values for the HTML select, and vice versa.

Tips

1. To avoid LazyInitializationExceptions and/or writing extra code in your EJB/Controller bean to initialize objects, use session in view.
2. For required fields, put required="true" on the selectOneMenu and override javax.faces.component.UIInput.REQUIRED in messages.properties (see Standard Faces Error Messages).

Select from an enum

This works just like selecting an entity, but <s:convertEnum/> is used instead.

XHTML:

<h:selectOneMenu id="marketStatus" value="#{person.status}"         (1)



required="true">



<s:selectItems value="#{enumLists.statusArray}" var="status"    (2)



label="#{status}"



noSelectionLabel="Select a status..."/>



<s:convertEnum/>



</h:selectOneMenu>

EnumLists.java:

@Name("enumLists")



@Scope(ScopeType.STATELESS)



public class EnumLists



{



public Status[] getStatusArray()



{



return Status.values();



}



}

1. person is an entity that has been outjected into the conversation. It has a 'status' property which is an enum.
2. We need to expose the values of the enum as a list or an array, so we make a stateless POJO component with getters that returns arrays for various enums called enumLists.

Multi-select from an enum

Here we use a selectManyCheckbox.

<h:selectManyCheckbox id="roles" 



layout="pageDirection" value="#{person.roles}"



required="true">



<s:selectItems value="#{enumLists.roleArray}" var="role"



label="#{role}"/>



<s:convertEnum/>



</h:selectManyCheckbox>

Unfortunately, Seam's convertEnum can't handle multi selects yet. This example will yeild a strange exception:

java.lang.IllegalArgumentException: java.util.List is not an enum type

Luckily, it's very easy to create custom converter tags with Facelets. Here is the converter class that handles both ordinary enums and multi-selects:

package eg;







import javax.faces.component.*;



import javax.faces.context.*;



import javax.faces.convert.*;



import javax.faces.el.ValueBinding;



import java.util.List;



import java.util.Collection;







/**



* Converter for enum multi-selects.



* <br>User: Joshua Davis



* Date: May 16, 2007



* Time: 7:25:58 AM



*/



public class EnumListConverter implements Converter



{



@SuppressWarnings({"unchecked"})



public Object getAsObject(FacesContext context,



UIComponent comp,



String value)



throws ConverterException



{



ValueBinding binding = comp.getValueBinding("value");



Class enumType = binding.getType(context);



if (enumType.isEnum())  // Single enum?



return Enum.valueOf(enumType, value);



else    // List of enums.



{



// Find the s:selectItems so we can get the enum.



List children = comp.getChildren();



for (Object child : children)



{



if (child instanceof UIComponent)



{



UIComponent c = (UIComponent) child;



ValueBinding b = c.getValueBinding("value");



Class t = b.getType(context);



// Array of enums: use the component type.



if (t.isArray() && t.getComponentType().isEnum())



{



t = t.getComponentType();



return Enum.valueOf(t,value);



}



else



{



Object v = b.getValue(context);



// Collection of enum values, get the type of the first element.



if (v instanceof Collection)



{



t = ((Collection) v).iterator().next().getClass();



return Enum.valueOf(t,value);



}



}



}



}



throw new ConverterException("Unable to find selectItems with enum values!");



}



}







public String getAsString(FacesContext context,



UIComponent component,



Object object)



throws ConverterException



{



if (object == null) {



return null;



}







return ((Enum) object).name();



}







}