[FIXED] Spring - creating bean from collection or map

Issue

Some time ago i read about creating beans in Spring and there is an exception in creating bean for collections and maps. I mean to create a bean for collection or map, it is needed to create a wrapper class with for example a list as a field of this class

public class CollectionBean {

  private List<String> someList;

}

and then it can be used in dependency injection. I wonder why it cannot be inject without wrapper class?

Solution

You can absolutely create beans that are collections:

public class CollectionsBean {

    @Autowired
    private List<String> nameList;

    public void printNameList() {
        System.out.println(nameList);
    }
}

@Configuration
public class CollectionConfig {

    @Bean
    public CollectionsBean getCollectionsBean() {
        return new CollectionsBean();
    }

    @Bean
    public List<String> nameList() {
        return Arrays.asList("John", "Adam", "Harry");
    }
}

What might be causing confusion is that Java drops the generic part of a type at compilation. So Spring can't look at an object at runtime and know whether it's a List<String> or a List<Integer> -- to Java and Spring, at runtime, it's just a List.

This means that type-based autowiring won't work, when there's more than one bean of type List.

One way to get around this is to create a wrapper class, as you have done. Another way is to use one of the many other ways of disambiguating autowiring -- for example by name.

Answered By - slim