README.md

Persistence context

What is persistence context?

In document of EntityManager, there is a definition of persistence context:

An EntityManager instance is associated with a persistence context. A persistence context is a set of entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed. The EntityManager API is used to create and remove persistent entity instances, to find entities by their primary key, and to query over entities.

In other words, the persistence context is a set of unique entities and is responsible for managing the lifecycle of entities.

In Spring Framework

Commonly (other type will be discussed soon), each EntityManager has its own persistence context associated with a transaction. Consequently, each transaction creates a new EntityManager with an independent persistence context. When the transaction concludes, both the EntityManager and its associated context are terminated.

In a word, different transaction has different EntityManager and persistence context. but other type of context can across multiple transactions.

You can inject EntityManager with @PersistenceContext annotation.

@PersistenceContext
private EntityManager entityManager;

Persistence context type

There are two type of persistence context in JPA:

• Transaction-scoped persistence context
• Extended persistence context

Transaction-scoped persistence context

Transaction-scoped persistence context is bounded to a transaction. As soon as the transaction finishes, the entities present in the persistence context will be flushed into persistent storage. This also implies that, just like when the transaction finishes, there are no entities remaining in the persistence context.

In a transaction,EntityManager checks first whether the entity is in persistence context. If not, it will be loaded from storage.

Extended persistence context

An extended persistence context can span across multiple transactions. We can persist the entity without the transaction but cannot flush it without a transaction.

In the stateless session bean, the extended persistence context in one component is completely unaware of any persistence context of another component.

extendedContext.saveWithoutTransaction(member);

assertAll(
        () ->
                assertThat(extendedContext.find(member.getId()))
                        .isNotNull(), // exists in context so does not query sql
        () ->
                assertThat(transactionContext.find(member.getId()))
                        .isNull()); // but other context does not know so query sql

Manipulating entities whether in transaction or not

In the transaction-scoped context, processes need to be executed within a transaction. Therefore, it flushes the context to storage when the transaction automatically commits using AOP.

package jakarta.persistence;

public interface EntityManager {

    /**
     * ...
     *
     * @throws TransactionRequiredException if there is no transaction when
     *         invoked on a container-managed entity manager of that is of type 
     *         <code>PersistenceContextType.TRANSACTION</code>
     */
    public void persist(Object entity);


    // Does not require a transaction    
    public <T> T find(Class<T> entityClass, Object primaryKey);

On the other hand, in an extended persistence context, processes can be executed without a transaction, but it does not flush the context to storage. It only exists in the persistence context.

References

• JPA/Hibernate Persistence Context - Baeldung