Spring Collection Merging

It is a way of creating a base collection (list, set, map or properties) and modifying this base collection in other beans, best explained using an example –

Consider a Person class with a field holding a list of addresses:

public class Person {
 private List<Address> addresses;
..
}

Assume that all Person instances have a common set of addresses, which can be specified using an abstract person bean this way:

<bean name='basePerson' class='Person' abstract='true'>
 <property name='addresses'>
  <list>
   <bean class='Address' p:street='Street1' p:state='State1' p:zip='001'></bean>
   <bean class='Address' p:street='Street2' p:state='State2' p:zip='002'></bean>
   <bean class='Address' p:street='Street3' p:state='State3' p:zip='003'></bean>
  </list>
 </property>
</bean>

Now, Person bean instances with addresses in addition to the addresses from the basePerson can be specified using Collection Merging feature this way – note the ‘merge=true’ attribute of list:

<bean name='person1' class='Person' parent='basePerson'>
 <property name='addresses'>
  <list merge='true'>
   <bean class='Address' p:street='Street4' p:state='State4' p:zip='004'></bean>
  </list>
 </property>
</bean>

Collection merging also works with <set/>, <map/> and <props/>

With Spring 3.1 and higher, a simpler option though could be simply to use @Configuraion and @Bean options, as then the merging can be directly handled with Java code, for eg, an equivalent @Configuration for the above xml bean configuration:

@Configuration
public class CollectionConfig{
 
 private List<Address> baseAddresses(){
  return Lists.newArrayList(new Address('Street1', 'State1', '001'), new Address('Street2', 'State2', '002'), new Address('Street3', 'State3', '003'));
 }
 
 
 @Bean
 public Person person1(){
  Person person1 = new Person();
  person1.setAddresses(baseAddresses());
  person1.getAddresses().add(new Address('Street4', 'State4', '004'));
  return person1;
 }
}

Reference: Spring Collection Merging from our JCG partner Biju Kunjummen at the all and sundry blog.