Primitive or wrapper for hibernate primary keys

From an Hibernate point of view, it doesn't change anything as Hibernate uses the same Hibernate type to represent them.

However, as pointed out by Bytecode Ninja, you can't distinguish the default value of a primitive int 0 from a an assigned 0 while there is no possible ambiguity with a null (a null id always means a new entity), which is why I prefer to use a nullable wrapper type.

And this is the Hibernate recommendation. From the Reference Documentation:

4.1.2. Provide an identifier property (optional)

Cat has a property called id. This property maps to the primary key column of a database table. The property might have been called anything, and its type might have been any primitive type, any primitive "wrapper" type, java.lang.String or java.util.Date. If your legacy database table has composite keys, you can use a user-defined class with properties of these types (see the section on composite identifiers later in the chapter.)

The identifier property is strictly optional. You can leave them off and let Hibernate keep track of object identifiers internally. We do not recommend this, however.

In fact, some functionality is available only to classes that declare an identifier property:

• Transitive reattachment for detached objects (cascade update or cascade merge) - see Section 10.11, “Transitive persistence”
• Session.saveOrUpdate()
• Session.merge()

We recommend that you declare consistently-named identifier properties on persistent classes and that you use a nullable (i.e., non-primitive) type.

And I actually leverage this in my base class:

@MappedSuperclass
public class BaseEntity implements Serializable {
    private static final long serialVersionUID = 1L;
    private Long id;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Transient
    public boolean isNew() {
        return (this.id == null);
    }
}

I prefer using the wrapper types, because otherwise, when we use primitive types, the primary key id will be set to 0 by default, and 0 could be the value of an actual id column in the corresponding table of the entity, and this can sometimes lead to hard to pinpoint bugs.

But using a wrapper, the default value of id will be null and a primary key column cannot have a null value, thus you can't update an entity that its id value is not set with a valid value by your application's logic.

You can specify your primary key as a primitive or a wrapper. And since usually the avantages of an Integer aren't needed, it's best to keep it simple with an int.

More info at oracle.com.