CODEWITHSUNDEEP
javagenerics
AlexSC
AlexSC

Reputation: 1933

How to build a generic entity repository in Java

I have a generic repository class that looks like this:

public class HibernateRepository<E extends Object, EId>
       implements EntityRepository<E, EId> {

    // Many things that are working fine    

    @Override
    public E getById(EId id) {
        return entityManager.getReference(E.class, id); // <<< Error here!
    }
}

The method getById() is supposed to return a certain entity instance given its Id. Something very trivial in many other situations, but not in this one, since the compiler returns:

Illegal class literal for the type parameter E

Notice that I don't want to pass the desired class to the method, since I already did that in the class definition.

The usage for this class could be:

public class MyClassRepositoryHibernate
       extends HibernateRepository<MyClass, Long> {
}

Now this new repository works on MyClass instances and the Id is typed with Long.

Is that possible in Java?

Upvotes: 0

Views: 2614

Answers (2)

atamanroman
atamanroman

Reputation: 11818

The type will be erased after compilation and thus is not available. This is called Type Erasure. You have to pass the Class object somewhere.

Upvotes: 1

tddmonkey
tddmonkey

Reputation: 21184

You will have to pass the actual class type to the constructor of your HibernateRepository and use that throughout your methods. The JVM has no knowledge of what "E" is at runtime hence you need to provide the concrete class type.

You can ensure you don't instantiate the object incorrectly by using the Generic parameter type in the constructor, like this:

public class HibernateRepository<E extends Object, EId>
   implements EntityRepository<E, EId> {
    private Class<E> clazz;

    public HibernateRepository(Class<E> clazz) {
        this.clazz = clazz;
    }

    @Override
    public E getById(EId id) {
        return entityManager.getReference(clazz, id); // <<< Error here!
    }
}

A pain, but a necessity due to type erasure.

Upvotes: 2

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