[hibernate-dev] Lambda usage to run a code block in an isolated transaction

[Sanne Grinovero]

Today porting some benchmark code to Hibernate ORM 5.2 I had several
difficulties around the fact that the code now needs to be different
depending on transactions being container managed or not.

My goal was to have a single benchmark test which I could compile once
and run in either JavaSE or CMT; with some help from Steve I figured
the necessary incantations out but ... it looks very unpractical.

One way is to use an isolation delegate, which looks like this:

final SessionImplementor session = (SessionImplementor) s;
session.getTransactionCoordinator().createIsolationDelegate().delegateWork(
   new WorkExecutorVisitable() {
      @ Override
      public Object accept(WorkExecutor executor, Connection
connection) throws SQLException {
         /// Some work with PreparedStatement on Connection..
      }
}, true );

This worked fine for some raw SQL used for the benchmark
initialization, but in another case I'd prefer to use the Session API
rather than dealing with PreparedStatements and native connections;
it looks like we don't have an equivalent "run code in isolation" for
the Session ?

It would be great if I could just pass a lambda to a Session and have
this executed on a "child Session" in the scope of a "child
Transaction", or just start and commit a transaction if there isn't
one.

s.executeInIsolation( session -> session.save(...) );

So I'd expect that details like how to begin the transaction, how it
should be committed (or rolled back in case of exceptions), how to
lookup a TransactionManager, and especially how to not leak resources
should be handled for the user.

Obviously the inner Session instance is a different one than the
outer, so any data returned by this block should be considered
detached; maybe this limitation would be clearer if the method was
hosted on SessionFactory or StatelessSession instead?
Although it wouldn't necessarily have the limitations of a
StalessSession, and it would be nice to have the inner transaction
behave as a nested one when there's already one in the host Session.

Looking forward for comments and improvement ideas :)

Thanks,
Sanne