Spring collection merge - spring

Question

I have two lists generated with a FactoryBean and I would like to initialize a bean with the merged version of this two. Is there a way doing this?
<bean id="listA" class="XFactoryBean" >
...
</bean>
<bean id="listB" class="YFactoryBean">
...
</bean>
<bean >
<property name="AwithB" ...>
</bean>
There is a solution that works with static lists (http://vikdor.blogspot.hu/2012/10/using-collection-merging-in-spring-to.html), but does not work with those generated lists.

Answer 1

Java #Configuration FTW:
#Configuration
public class Config {
#Resource
private List listA;
#Resource
private List listB;
#Bean
public List AwithB() {
List mergedList = new ArrayList(listA);
listB.addAll(listB);
return mergedList;
}
}
Much less boilerplate.

Answer 2

There is a sublte bug with the ListMergerFactoryBean solution above.
Config:
<util:list id="listA" value-type="java.lang.String">
<value>fooA</value>
<value>barA</value>
</util:list>
<util:list id="listB" value-type="java.lang.String">
<value>fooB</value>
<value>barB</value>
</util:list>
<util:list id="listC" value-type="java.lang.String">
<value>fooC</value>
<value>barC</value>
</util:list>
<bean id="AwithB" class="com.util.ListMergerFactoryBean">
<property name="listOfLists">
<list>
<ref bean="listA" />
<ref bean="listB" />
</list>
</property>
</bean>
<bean id="AwithC" class="com.util.ListMergerFactoryBean">
<property name="listOfLists">
<list>
<ref bean="listA" />
<ref bean="listC" />
</list>
</property>
</bean>
With this config you might be surprised when bean AwithB has the expected content of AwithC. ListA is a singleton and the getObject method alters it, therefore alters it for all users of ListMergerFactoryBean, even though the bean factory is not a singleton. The fix is to not re-use the first list in the listOfLists:
#Override
public List getObject() throws Exception {
List mergedList = new ArrayList();
for (List list : listOfLists) {
mergedList.addAll(list);
}
return mergedList;
}

Answer 3

After checking the SpEL-related solutions (how to extend a list in spring config) and extending the ListFactoryBean (http://ericlefevre.net/wordpress/2008/04/02/merging-lists-in-a-spring-configuration-file/) I came up with the following solution:
config:
<bean id="AwithB" class="com.util.ListMergerFactoryBean">
<property name="listOfLists">
<list>
<ref bean="listA" />
<ref bean="listB" />
</list>
</property>
</bean>
java:
public class ListMergerFactoryBean implements FactoryBean<List> {
private List<List> listOfLists;
#Override
public List getObject() throws Exception {
List mergedList = new ArrayList();
for (List list : listOfLists) {
mergedList.addAll(list);
}
return mergedList;
}
#Override
public Class<?> getObjectType() {
return (new ArrayList()).getClass();
}
#Override
public boolean isSingleton() {
return false;
}
public void setListOfLists(List<List> listOfLists) {
this.listOfLists = listOfLists;
}
}
UPDATE: I have eliminated a bug using Aron Bartle's solution. Thanks!