Spring 3.2 Autowire generic types

Question

So I have a number of generics in Spring 3.2 and ideally my architecture would look something like this.

class GenericDao<T>{}

class GenericService<T, T_DAO extends GenericDao<T>>
{
    // FAILS
    @Autowired
    T_DAO;
}

@Component
class Foo{}

@Repository
class FooDao extends GenericDao<Foo>{}

@Service
FooService extends GenericService<Foo, FooDao>{}

Unfortunately with multiple implementations of the generics the autowiring throws an error about multiple matching bean definitions. I assume this is because @Autowired processes before type erasure. Every solution I've found or come up with looks ugly to me or just inexplicably refuses to work. What is the best way around this problem?

Answer 1

How about adding a constructor to the GenericService and move the autowiring to the extending class, e.g.

class GenericService<T, T_DAO extends GenericDao<T>> {
    private final T_DAO tDao;

    GenericService(T_DAO tDao) {
        this.tDao = tDao;
    }
}

@Service
FooService extends GenericService<Foo, FooDao> {

    @Autowired
    FooService(FooDao fooDao) {
        super(fooDao);
    }
}

---

Update:

As of Spring 4.0 RC1, it is possible to autowire based on generic type, which means that you can write a generic service like

class GenericService<T, T_DAO extends GenericDao<T>> {

    @Autowired
    private T_DAO tDao;
}

and create multiple different Spring beans of it like:

@Service
class FooService extends GenericService<Foo, FooDao> {
}

Answer 2

Here is a closest solution. The specialized DAOs are annotated at the business layer. As in the question from OP, the best effort would be having an annotated DAO in the EntityDAO generic template itself. Type erasure seems to be not allowing the specialized type information to get passed onto the spring factories [resulting in reporting matching beans from all the specialized DAOs]

The Generic Entity DAO template

public class EntityDAO<T> 
{
    @Autowired
    SessionFactory factory;

    public Session getCurrentSession()
    {
        return factory.getCurrentSession();
    }

    public void create(T record)
    {
        getCurrentSession().save(record);
    }

    public void update(T record)
    {
        getCurrentSession().update(record);
    }

    public void delete(T record)
    {
        getCurrentSession().delete(record);
    }

    public void persist(T record)
    {
        getCurrentSession().saveOrUpdate(record);
    }

    public T get(Class<T> clazz, Integer id)
    {
        return (T) getCurrentSession().get(clazz, id);
    }
}

The Generic Entity Based Business Layer Template

public abstract class EntityBusinessService<T>
implements Serializable
{
    public abstract EntityDAO<T> getDAO();

    //Rest of code.
}

An Example Specialized Entity DAO

@Transactional
@Repository
public class UserDAO
extends EntityDAO<User>
{
}

An Example Specialized Entity Business Class

@Transactional
@Service
@Scope("prototype")
public class UserBusinessService
extends EntityBusinessService<User>
{
    @Autowired
    UserDAO dao;

    @Override
    public EntityDAO<User> getDAO() 
    {
        return dao;
    }

    //Rest of code
}

Answer 3

You can remove the @autowire annotation and perform delayed “autowire” using @PostConstruct and ServiceLocatorFactoryBean.
Your GenericService will look similar to this

    public class GenericService<T, T_DAO extends GenericDao<T>>{

        @Autowired
        private DaoLocator daoLocatorFactoryBean;

        //No need to autowried, autowireDao() will do this for you 
        T_DAO dao;


        @SuppressWarnings("unchecked")
        @PostConstruct
        protected void autowireDao(){
        //Read the actual class at run time
        final Type type; 
        type = ((ParameterizedType) getClass().getGenericSuperclass())
                                              .getActualTypeArguments()[1]; 
        //figure out the class of the fully qualified class name
        //this way you can know the bean name to look for
        final String typeClass = type.toString();      
        String daoName = typeClass.substring(typeClass.lastIndexOf('.')+1
                                            ,typeClass.length());
        daoName = Character.toLowerCase(daoName.charAt(0)) + daoName.substring(1);
        this.dao = (T_DAO) daoLocatorFactoryBean.lookup(daoName);
       }

daoLocatorFactoryBean does the magic for you.
In order to use it you need to add an interface similar to the one below:

 public interface DaoLocator {
        public GenericDao<?> lookup(String serviceName);           
 }    

You need to add the following snippet to your applicationContext.xml

  <bean id="daoLocatorFactoryBean" 
      class="org.springframework.beans.factory.config.ServiceLocatorFactoryBean">
      <property name="serviceLocatorInterface"
              value="org.haim.springframwork.stackoverflow.DaoLocator" />
    </bean>

This is a nice trick and it will save you little boilerplate classes.
B.T.W I do not see this boilerplate code as a big issue and the project I working for uses matsev approach.

Answer 4

Why do you want a generic service ? Service classes are meant for specific units of work involving multple entities. You can just inject a repository straight into a controller.

Here is an example of generic repository with constructor argument, you could also make each method Generic instead and have no constructor argument. But each method call would require class as parameter:

public class DomainRepository<T> {

   @Resource(name = "sessionFactory")
   protected SessionFactory sessionFactory;

   public DomainRepository(Class genericType) {
        this.genericType = genericType;
   }

   @Transactional(readOnly = true)
   public T get(final long id) {
       return (T) sessionFactory.getCurrentSession().get(genericType, id);
   }

Example of bean definition for the generic repository - you could have multple different beans, using different contstructor args.

<bean id="tagRepository" class="com.yourcompnay.data.DomainRepository">
        <constructor-arg value="com.yourcompnay.domain.Tag"/>
</bean>

Depdncy injection of bean using resource annotation

@Resource(name = "tagRepository")
private DomainRepository<Tag> tagRepository;

And this allows the Domainreposiroty to be subclassed for specific entities/methods, which woul dallow autowiring :

public class PersonRepository extends DomainRepository<Person> {
    public PersonRepository(){
        super(Person.class);
    }
    ...

Answer 5

For this question one needs to understand about what autowire is. In common terms we can say that through autowire we create a object instance/bean at the time of deployment of the web app. So now going with the question if you are declaring autowiring in multiple places with the same name. Then this error comes. Autowiring can be done in multiple ways so if you are using multiple type of autowiring technique, then also one could get this error.

Answer 6

You should use autowiring in classes which extends these generics

Answer 7

Complete Generic Solution using Spring 4:

---

Domain Class

@Component
class Foo{
}

@Component
class Bar{
}

---

DAO Layer

interface GenericDao<T>{
//list of methods
}

class GenericDaoImpl<T> implements GenericDao<T>{
 @Autowired
 SessionFactory factory;

 private Class<T> domainClass; // Get Class Type of <T>

 public Session getCurrentSession(){
    return factory.getCurrentSession();
 }

 public DaoImpl() {
    this.domainClass = (Class<T>) GenericTypeResolver.resolveTypeArgument(getClass(), DaoImpl.class);
 }
 //implementation of methods
}

interface FooDao extends GenericDao<Foo>{
//Define extra methods if required
}

interface BarDao extends GenericDao<Bar>{
//Define extra methods if required
}

@Repository
class FooDao extends GenericDaoImpl<Foo> implements FooDao{
 //implementation of extra methods
}

@Repository
class BarDao extends GenericDaoImpl<Bar> implements BarDao{
 //implementation of extra methods
}

---

Service Layer

interface GenericService<T>{
//List of methods
}

class GenericServiceImpl<T> implements GenericService<T>{
 @Autowire
 protected GenericDao<T> dao; //used to access DAO layer
}

class FooService extends GenericService<Foo>{
//Add extra methods of required
}

class BarService extends GenericService<Bar>{
//Add extra methods of required
}

@Service
class FooServiceImpl extends GenericServiceImpl<Foo> implements GenericService<Foo>{
//implementation of extra methods
}

@Service
class BarServiceImpl extends GenericServiceImpl<Bar> implements GenericService<Bar>{
//implementation of extra methods
}