Spring - No EntityManager with actual transaction available for current thread - cannot reliably process 'persist' call

EntityManager and Transaction Fundamentals

Navigating through the JPA/software labyrinth requires profound understanding of EntityManager and transaction management.

The @Transactional annotation

The @Transactional annotation should encapsulate service layer methods, providing well-defined transaction boundaries. Refrain from decorating controller methods with @Transactional, as it's outside their job description.

EntityManager via @PersistenceContext

Dependency injection of the EntityManager into DAOs or repositories can be accomplished using @PersistenceContext. This allows Spring to oversee the EntityManager's lifecycle aligning with transactional contexts.

@PersistenceContext
private EntityManager entityManager;
// "Keep your friends close, but your EntityManager closer." -The DAO-father, probably. 
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Shared vs Stateful EntityManager

In majority of web applications, a shared EntityManager attached to the transaction lifecycle suits all needs, a setup achieved via PersistenceContextType.TRANSACTION. Stateful scenarios, conversely, that demand extended conversations in enterprise applications should consider PersistenceContextType.EXTENDED.

Tweaking the EntityManagerFactory and transactionManager

Ensure your entityManagerFactory and transactionManager are appropriately set up - a trip here may result in an army of problems, including the lack of an EntityManager coupled with the current thread.

<bean id="entityManagerFactory" ... />
<bean id="transactionManager" ... />
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State transitions with persist() and merge()

Understanding entity lifecycle and state transitions is vital. If persist() doesn't fit the bill, for instance with detached instances, merge() could be your lifeline. Also, ensure your JPA entities have accurate equals and hashCode implementations to avoid data discrepancies.

Working with custom repository methods

Take caution when working with custom repository methods, as they might bypass transaction management. Inspect their implementation to avoid any EntityManager misusage.

Uncovering the problem via diagnostics

If you stumble upon transaction-related exceptions, examine stack traces and error logs - they provide excellent clues. Consider writing unit tests with JUnit and @Transactional to test transaction boundaries and lifecycle issues.

Advanced Scenarios

AspectJ and proxies

In a setup where AspectJ is utilized for AOP, care must be taken to prevent potential conflicts with Spring proxies. Guarantee that AspectJ weaving configuration doesn't step on Spring's toes.

Multi-entity deletions

For bulk deletion operations, use entityManager.getCriteriaBuilder().createCriteriaDelete() followed by entityManager.createQuery(criteriaDelete).executeUpdate(). You’ll be deleting faster than Thanos with all infinity stones!

EntityManager Scopes

Familiarize with how EntityManager behaves across different beans and scopes. @Scope(proxyMode = ScopedProxyMode.INTERFACES) can assist in proper scoping and proxy behavior.