hibernate从mysql返回null为自动生成的时间戳

Question

i have a table in mysql which has a data type of timestamp as one of the columns, which gets a default value of CURRENT_TIME upon insertion. and i have another timestamp column that has a default value of CURRENT_TIME upon update. i have these so that timestamp columns will get updated automatically on insertion and update (which works fine).

now i am using cxf, hibernate/jpa, mysql, jackson to build a web service. i am simply creating a new record and retrieving it right away as below code shows.

Session session = getSession();  // sessionFactory.getCurrentSession();
String accountId = (String)session.save(account);
Account newAccount = (Account)session.load(Account.class, accountId);
logger.info("created timestamp=" + newAccount.getCreatedTimestamp());

after above code is ran, i can see that new record is created in mysql with correct timestamps for createdTimestamp. however, logger.info() line above throws an exception because newAccount.getCreatedTimestamp() returns null. if i remove logger.info() line, i can see that newAccount object is populated with correct values except for createdTimestamp which is null.

what's more odd is that after above code is ran (which is a part of HTTP POST operation), i call a HTTP GET service which just fetches a record that i just inserted by doing

session.get(Account.class, accountId);

and it correctly shows timestamps!

i tried to sleep before session.load() or session.get() thinking that there might be a delay in inserting timestamp, but that didn't do much. is there something special about hibernate session management that does not retrieve columns that mysql generates? what am i missing here? please help.

Answer 1

Your actual save isn't being committed until the session is flushed. Hibernate doesn't actually commit anything to the database until the session is flushed or closed so that if an exception is thrown, a rollback doesn't actually have to touch the physical database, the changes are just not sent. However if Hibernate detects that a query is going to receive stale data, it will automatically flush before running that query.

For example, you add a record to the database and immediately call a SELECT COUNT(*) query. Hibernate will flush the session (committing the record in the process) and then perform the SELECT COUNT(*) query on the now clean session ensuring that you get correct data. Hibernate didn't do this in your case because it saw that you were requesting the same object that you were trying to insert (in the same session) so it just returned you that reference.

If you are letting hibernate manage its sessions (using a session factory or similar) I don't think that you have to explicitly close sessions. I know that I don't, but I'm using Hibernate with Spring, and using the @Transactional annotation which manages the actual Hibernate session. If you want an immediate insert, make your call to save() the last call in the method. Usually, once the method exits, a commit() will be called automatically.

All the load() will be doing is giving you the same instance of Account that you passed into session.save() . Either close or flush the session, then try the load() again, and your value should be set.