hibernate如何使用no-arg构造函数初始化final字段？

Question

So, I have this class which I am trying to use with Hibernate:

@Entity
@Access(AccessType.FIELD)
public class BasicUser {

    @Id 
    private final int userId;
    private final String userName;

    BasicUser(int userId, String userName) {
        super();
        this.userId = userId;
        this.userName = userName;
    }


    BasicUser() {
        super();
        this.userId=0;
        this.userName=null;
    }
    //getters

}

I tried pulling userId and userName from the database and make a BasicUser object and it worked fine. But, I am not getting how did it work? Hibernate required a no-arg constructor which I provided. Now since the fields are final, they had to be initialized in the constructor, so for sake of it I initialized them as shown in the code, expecting some error while running the code. But Hibernate formed the object with the field values as they were in the database. How is this happening? I need to understand because, there are a few objects in my application which are immutable. So, should I be doing the same way for them also? I saw a lot of posts telling to provide a no-arg constructor and Access type as field for immutable objects. But, when I provide no-arg constructor for a final field, the field has to be initialized. So, I am not getting whats going on here? Please help. Thanks!

Answer 1

Hibernate uses reflection (or some related low-level trickery) to set private instance fields. That approach can also by-pass final (ie reassign new values after the constructor returns).