How does EL empty operator work in JSF?
JsfElJsf Problem Overview
In JSF an component can be rendered or not using the EL empty operator
rendered="#{not empty myBean.myList}"
As I've understood the operator works both as null-check, but also check checks if the list is empty.
I want to do empty checks on some objects of my own custom class, which interface(s) or parts of interfaces do I need to implement? Which interface is the empty operator compatible with?
Jsf Solutions
Solution 1 - Jsf
From EL 2.2 specification (get the one below "Click here to download the spec for evaluation"):
> ##1.10 Empty Operator - empty A
> The empty
operator is a prefix operator that can be used to determine if a value is
null or empty.
> To evaluate empty A
> - If A
is null
, return true
> - Otherwise, if A
is the empty string, then return true
> - Otherwise, if A
is an empty array, then return true
> - Otherwise, if A
is an empty Map
, return true
> - Otherwise, if A
is an empty Collection
, return true
> - Otherwise return false
So, considering the interfaces, it works on Collection
and Map
only. In your case, I think Collection
is the best option. Or, if it's a Javabean-like object, then Map
. Either way, under the covers, the isEmpty()
method is used for the actual check. On interface methods which you can't or don't want to implement, you could throw UnsupportedOperationException
.
Solution 2 - Jsf
Using BalusC's suggestion of implementing Collection i can now hide my primefaces p:dataTable
using not empty operator on my dataModel
that extends javax.faces.model.ListDataModel
Code sample:
import java.io.Serializable;
import java.util.Collection;
import java.util.List;
import javax.faces.model.ListDataModel;
import org.primefaces.model.SelectableDataModel;
public class EntityDataModel extends ListDataModel<Entity> implements
Collection<Entity>, SelectableDataModel<Entity>, Serializable {
public EntityDataModel(List<Entity> data) { super(data); }
@Override
public Entity getRowData(String rowKey) {
// In a real app, a more efficient way like a query by rowKey should be
// implemented to deal with huge data
List<Entity> entitys = (List<Entity>) getWrappedData();
for (Entity entity : entitys) {
if (Integer.toString(entity.getId()).equals(rowKey)) return entity;
}
return null;
}
@Override
public Object getRowKey(Entity entity) {
return entity.getId();
}
@Override
public boolean isEmpty() {
List<Entity> entity = (List<Entity>) getWrappedData();
return (entity == null) || entity.isEmpty();
}
// ... other not implemented methods of Collection...
}