http://www.skill-guru.com/blog/2010/12/19/transactionalreadonlytrue-in-spring/
@Transactional(readOnly=true) in Spring
What does this mean ?
When a Multi Version Concurrency Control database (Oracle, Postgresql, MySQL + InnoDb) is used, a read only transaction can be translated to the non standard isolation level: READ_ONLY.
The READ_ONLY isolation level provides the same protection as the SERIALIZED isolation level (no dirty reads, no unrepeatable reads, no phantom reads) but doesn’t allow any updates. It also doesn’t cause any lock contention because no locking is required (the database is able to revert back to previous versions of the srecords ignoring all new changes).
Ms Sql 2005 also has a similar isolation level: SNAPSHOT.
If you specify readOnly as true, the flush mode will be set as FlushMode.NEVER in the current Hibernate Session preventing the session from committing the transaction.
Furthermore, setReadOnly(true) will be called on the JDBC Connection, which is also a hint to the underlying database. If your database supports it (most likely it does), this has basically the same effect as FlushMode.NEVER, but it’s stronger since you cannot even flush manually.
Now let’s see how transaction propagation works.
If you don’t explicitly set readOnly to true, you will have read/write transactions. Depending on the transaction attributes (like REQUIRES_NEW), sometimes your transaction is suspended at some point, a new one is started and eventually committed, and after that the first transaction is resumed.
If a method in a read/write transaction calls a method that requires a readOnly transaction, the first one should be suspended, because otherwise a flush/commit would happen at the end of the second method.
Conversely, if you call a method from within a readOnly transaction that requires read/write, again, the first one will be suspended, since it cannot be flushed/committed, and the second method needs that.
In the readOnly-to-readOnly, and the read/write-to-read/write cases the outer transaction doesn’t need to be suspended (unless you specify propagation otherwise, obviously).