T - the result type@FunctionalInterface public interface TransactionCallback<T>
TransactionTemplate's
 execute method, often as anonymous class within a method implementation.
 Typically used to assemble various calls to transaction-unaware data access
 services into a higher-level service method with transaction demarcation. As an
 alternative, consider the use of declarative transaction demarcation (e.g. through
 Spring's Transactional annotation).
TransactionTemplate, 
CallbackPreferringPlatformTransactionManager| Modifier and Type | Method and Description | 
|---|---|
| T | doInTransaction(TransactionStatus status)Gets called by  TransactionTemplate.execute(org.springframework.transaction.support.TransactionCallback<T>)within a transactional context. | 
@Nullable T doInTransaction(TransactionStatus status)
TransactionTemplate.execute(org.springframework.transaction.support.TransactionCallback<T>) within a transactional context.
 Does not need to care about transactions itself, although it can retrieve and
 influence the status of the current transaction via the given status object,
 e.g. setting rollback-only.
 Allows for returning a result object created within the transaction, i.e. a domain object or a collection of domain objects. A RuntimeException thrown by the callback is treated as application exception that enforces a rollback. Any such exception will be propagated to the caller of the template, unless there is a problem rolling back, in which case a TransactionException will be thrown.
status - associated transaction statusnullTransactionTemplate.execute(org.springframework.transaction.support.TransactionCallback<T>), 
CallbackPreferringPlatformTransactionManager.execute(org.springframework.transaction.TransactionDefinition, org.springframework.transaction.support.TransactionCallback<T>)