Mastering Spring @Resource: Inject Maps, Lists, Arrays, and Single Beans

Environment: Spring 6.1.2

1. Prepare Environment

Define an interface DAO and three implementations: WorkerDAO, StudentDAO, and TeacherDAO.

public interface DAO {}
public class WorkerDAO implements DAO {}
public class StudentDAO implements DAO {}
public class TeacherDAO implements DAO {}

PersonService

injects the DAO implementations.

public class PersonService {
    @Resource
    private Type daos;
    public String toString() {
        return "PersonService [daos=" + daos + "]";
    }
}

The main method registers beans, refreshes the context, and prints the PersonService instance.

public static void main(String[] args) {
    try (AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext()) {
        context.registerBean("worker", WorkerDAO.class);
        context.registerBean("student", StudentDAO.class);
        context.registerBean("teacher", TeacherDAO.class);
        context.registerBean(PersonService.class);
        context.refresh();
        System.out.println(context.getBean(PersonService.class));
    }
}

2. Type Injection

2.1 Map injection

@Resource
private Map<String, DAO> daos;

Output shows the map keyed by bean names.

PersonService [daos={worker=com.pack.WorkerDAO@..., student=com.pack.StudentDAO@..., teacher=com.pack.TeacherDAO@...}]

2.2 List injection

@Resource
private List<DAO> daos;

Output shows the list of DAO instances.

PersonService [daos=[com.pack.TeacherDAO@..., com.pack.StudentDAO@..., com.pack.WorkerDAO@...]]

2.3 Array injection

@Resource
private DAO[] daos;

Output is similar to the list injection.

PersonService [daos=[com.pack.TeacherDAO@..., com.pack.StudentDAO@..., com.pack.WorkerDAO@...]]

2.4 Single bean injection

@Resource
private DAO daos;

When multiple DAO beans exist, Spring throws NoUniqueBeanDefinitionException.

NoUniqueBeanDefinitionException: No qualifying bean of type 'com.pack.DAO' available: expected single matching bean but found 3: worker,student,teacher

Resolution methods:

@Priority – assign priority values; lower number wins.

@Priority(-1)
public class WorkerDAO implements DAO {}
@Priority(-2)
public class StudentDAO implements DAO {}
@Priority(-3)
public class TeacherDAO implements DAO {}

After setting priorities, TeacherDAO is injected.

PersonService [daos=com.pack.TeacherDAO@...]

@Primary – mark the preferred bean.

@Primary
public class WorkerDAO implements DAO {}

Now WorkerDAO is injected.

PersonService [daos=com.pack.WorkerDAO@...]

Other ways to obtain a specific bean manually:

Using

BeanFactory

public class PersonService implements BeanFactoryAware {
    private DAO daos;
    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.daos = beanFactory.getBean("student", StudentDAO.class);
    }
}

Using

ApplicationContext

public class PersonService implements ApplicationContextAware {
    private DAO daos;
    @Override
    public void setApplicationContext(ApplicationContext context) throws BeansException {
        this.daos = context.getBean("student", StudentDAO.class);
    }
}

Using @Qualifier to specify bean name

@Resource
@Qualifier("student")
private DAO daos;

Custom qualifier annotation example:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.FIELD})
public @interface PackInject {}

Register the custom qualifier with Spring:

try (AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext()) {
    CustomAutowireConfigurer configurer = new CustomAutowireConfigurer();
    configurer.setCustomQualifierTypes(Set.of(PackInject.class));
    context.registerBean(CustomAutowireConfigurer.class, () -> configurer);
}

Apply @PackInject to a DAO implementation and to the injection point:

@PackInject
public class StudentDAO implements DAO {}
@Resource
@PackInject
private DAO daos;

All these techniques provide precise control over which bean is injected in a Spring application.