为什么session.save（object）在休眠状态下不会自动设置ID？

Question

I am using hibernate as a JPA provider for my application and now I have a requirement that I need the id of recently stored request object, but when I do a request.getId it prints null .

System.out.println("before save : request id's = "+request.getId());   
session.save(request);    
System.out.println("after save : request id's = "+request.getId());

I followed this question .

Output of this code:

before save : request id's = null

after save : reqeust id's = null

public class Request implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

Answer 1

Since the id is generated by the database it will only be there after the entity is actually written. Since hibernate tries to delay the actual write so it might safe some database-hits, session.save() will not necessarily trigger the id-generation.

Try adding session.flush() after the save and see if the id is populated then.