Spring Transactions With Supports Propagation
From javadoc:
Note: For transaction managers with transaction synchronization, PROPAGATION_SUPPORTS is slightly different from no transaction at all, as it defines a transaction scope that synchronization will apply for. As a consequence, the same resources (JDBC Connection, Hibernate Session, etc) will be shared for the entire specified scope. Note that this depends on the actual synchronization configuration of the transaction manager.
So, it means that, for example, multiple invocations of Hibernate's SessionFactory.getCurrentSession() inside methodWithSupportsTx() would return the same session.
A required transaction will create a new transaction if none exists. Therefore a new transaction would be made when you call serviceBean.methodWithSupportsTx(). If your method is truly transactional you will see an error from spring if no transaction exists.