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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();
}

}
Retrieved from "http://shrubbery.mynetgear.net/wiki/Select_lists_with_Seam"
posted on 2009-04-09 10:54 seal 阅读(639) 评论(0)  编辑  收藏 所属分类: Seam

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