What is Persistence Context?

I am new to the Java world and JPA. I was studying JPA and came across many new terms like Entity, persistence. While reading, I could not understand the exact definition for Persistence Context.

Can anyone explain it in simple laymen terms? What is it to do with the data used in the @Entity?

For example, I find this definition too complicated to understand: > A persistence context is a set of entities such that for any persistent identity there is a unique entity instance.

A persistence context handles a set of entities which hold data to be persisted in some persistence store (e.g. a database). In particular, the context is aware of the different states an entity can have (e.g. managed, detached) in relation to both the context and the underlying persistence store.

Although Hibernate-related (a JPA provider), I think these links are useful:

http://docs.jboss.org/hibernate/core/4.0/devguide/en-US/html/ch03.html

http://docs.jboss.org/hibernate/entitymanager/3.5/reference/en/html/architecture.html

In Java EE, a persistence context is normally accessed via an EntityManager.

http://docs.oracle.com/javaee/6/api/javax/persistence/EntityManager.html

The various states an entity can have and the transitions between these are described below:

http://docs.jboss.org/hibernate/entitymanager/3.6/reference/en/html/objectstate.html

https://vladmihalcea.com/wp-content/uploads/2014/07/jpaentitystates.png

1. Entities are managed by javax.persistence.EntityManager instance using persistence context.
2. Each EntityManager instance is associated with a persistence context.
3. Within the persistence context, the entity instances and their lifecycle are managed.
4. Persistence context defines a scope under which particular entity instances are created, persisted, and removed.
5. A persistence context is like a cache which contains a set of persistent entities , So once the transaction is finished, all persistent objects are detached from the EntityManager's persistence context and are no longer managed.

Taken from this page:

Here's a quick cheat sheet of the JPA world:

• A Cache is a copy of data, copy meaning pulled from but living outside the database.

• Flushing a Cache is the act of putting modified data back into the database.

• A PersistenceContext is essentially a Cache. It also tends to have it's own non-shared database connection.

• An EntityManager represents a PersistenceContext (and therefore a Cache)

• An EntityManagerFactory creates an EntityManager (and therefore a PersistenceContext/Cache)

A persistent context represents the entities which hold data and are qualified to be persisted in some persistent storage like a database. Once we commit a transaction under a session which has these entities attached with, Hibernate flushes the persistent context and changes(insert/save, update or delete) on them are persisted in the persistent storage.

Both the org.hibernate.Session API and javax.persistence.EntityManager API represent a context for dealing with persistent data.

This concept is called a persistence context. Persistent data has a state in relation to both a persistence context and the underlying database.

Persistence Context is an environment or cache where entity instances(which are capable of holding data and thereby having the ability to be persisted in a database) are managed by Entity Manager.It syncs the entity with database.All objects having @Entity annotation are capable of being persisted. @Entity is nothing but a class which helps us create objects in order to communicate with the database.And the way the objects communicate is using methods.And who supplies those methods is the Entity Manager.

"A set of persist-able (entity) instances managed by an entity manager instance at a given time" is called persistence context.

JPA @Entity annotation indicates a persist-able entity.

Refer JPA Definition here

In layman terms we can say that Persistence Context is an environment where entities are managed, i.e it syncs "Entity" with the database.

While @pritam kumar gives a good overview the 5th point is not true.

> Persistence Context can be either Transaction Scoped-- the Persistence > Context 'lives' for the length of the transaction, or Extended-- the > Persistence Context spans multiple transactions.

https://blogs.oracle.com/carolmcdonald/entry/jpa_caching

JPA's EntityManager and Hibernate's Session offer an extended Persistence Context.