SeamFramework.orgCommunity Documentation
Suppose you have a situation where you want to provide a default implementation of a particular service and allow
the user to override it as needed. Although this may sound like a job for an alternative, they have some
restrictions that may make them undesirable in this situation. If you were to use an alternative it would require
an entry in every beans.xml
file in an application.
Developers consuming the extension will have to open up the any jar file which references the default bean, and
edit the beans.xml
file within, in order to override the service. This is where default beans come
in.
Default beans allow you to create a default bean with a specified type and set of qualifiers. If no other bean is installed that has the same type and qualifiers, then the default bean will be installed.
Let's take a real world example - a module that allows you to evaluate EL (something that Solder
provides!). If JSF is available we want to use the FunctionMapper
provided by the JSF implementation
to resolve functions, otherwise we just want to use a a default FunctionMapper
implementation that
does nothing. We can achieve this as follows:
@DefaultBean(FunctionMapper.class)
@Mapper
class FunctionMapperImpl extends FunctionMapper {
@Override
public Method resolveFunction(String prefix, String localName) {
return null;
}
}
And in the JSF module:
class FunctionMapperProvider {
@Produces
@Mapper
FunctionMapper produceFunctionMapper() {
return FacesContext.getCurrentInstance().getELContext().getFunctionMapper();
}
}
If FunctionMapperProvider
is present then it will be used by default, otherwise the default
FunctionMapperImpl
is used.
A producer method or producer field may be defined to be a default producer by placing the
@DefaultBean
annotation on the producer. For example:
class CacheManager {
@DefaultBean(Cache.class)
Cache getCache() {
...
}
}
Any producer methods or producer fields declared on a default managed bean are automatically registered as default
producers, with Method.getGenericReturnType()
or Field.getGenericType()
determining the
type of the default producer. The default producer type can be overridden by specifying @DefaultBean
on the producer method or field.