Spring 4 generic class, get the parametrized type

Question

Hi have a Generic class in Spring, and I would like to get the generic T type class for an injected bean. I know the classic way in Java and read how Spring 4 implements Java Generics. Also, I tried to find a solution using ResolvableType but nothing works.

@Autowired
GenericDao<SpecificClass> specificdao;

public GenericDaoImpl <T> {

    private Class<T> type;

    public DaoImpl () { 
         this.type = ...? 
    }

    public T findById(Serializable id) {
        return (T) HibernateUtil.findById(type, id);
    }
}

Are there any way to avoid this?

@Autowired
@Qualifier
GenericDao<SpecificClass> specificdao;

@Repository("specificdao")
public SpecificDaoImpl extends GenericDao<SpecificClass> {
      public SpecificDaoImpl () {
          // assuming the constructor is implemented in GenericDao
          super(this.getClass())
      }
}

Thanks.

I may be wrong, but I doubt if this can be easily done (due to type erasure). BTW: from the code you've provided, it seems you're trying to re-implement the functionality provided by the Spring Data. Why not to use it instead? — G. Demecki, Nov 26 '15 at 14:19
One small remark: The result `this.getClass()` would be the same if called from the super class `GenericDaoImpl`, since `getClass()` returns the runtime type. So the class object doesn't need to be a parameter of the `GenericDaoImpl` constructor, `GenericDaoImpl` can call it itself. — Lii, Nov 27 '15 at 12:14
Also: You don't need the unchecked cast in `findById` since you have the class object. You can use `return this.type.cast(HibernateUtil.findById(type, id));`, which is a little more safe since it will actually check that the result is of the right type. — Lii, Nov 27 '15 at 13:30
[This answer](http://stackoverflow.com/a/18709327/452775) seems to contain info about almost the same thing. — Lii, Nov 27 '15 at 13:32

score 2 · Accepted Answer · edited Jul 07 '19 at 18:26

If I understand your question: what you want to achieve is very tricky.

You can use TypeTools and then do something like:

import net.jodah.typetools.TypeResolver;

public GenericDaoImpl <T> {

    private Class<T> type;

    public GenericDaoImpl () { 
         Class<?>[] typeArguments = TypeResolver.resolveRawArguments(GenericDaoImpl.class, getClass());
         this.type = (Class<T>) typeArguments[0];
    }

    public T findById(Serializable id) {
        return (T) HibernateUtil.findById(type, id);
    }
}

But still I doubt if it's a good idea. Because usually you don't want:

@Autowired
GenericDao<SpecificClass> specificDao;

But instead:

@Autowired
SpecificDao specificDao;

Why? Because each DAO has almost always completely different methods than other DAOs. The only generic methods, common to all, are probably: findById, findAll, save, count, delete etc.

So subclassing from a GenericDAO is the most obvious way, as it allows you to add any needed method inside a concrete DAO.

BTW: You said that you want re-implement Spring Data functionality on your own. But please notice that in Spring way, you still have to create concrete repositories.

Spring 4 generic class, get the parametrized type

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