What is Persistence Context?

Question

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.

Curzon

Answer 1

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

Hitandmiss

hmmm, it makes sense now. Do you have a similar "Simple" definition which tells the difference between container-managed & application-managed Entity Managers? – Curzon

container vs application just basically tells where Entity Manager is created - outside in a container, or inside, in application. – Dunnage

Answer 2

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.

Mastectomy

I found that EclipseLink does not detach entities after a transaction is finished... – Immesh

The container managed persistence context's cache will only remain for the duration of the transaction. Entities read in a transaction will become detached after the completion of the transaction and will require to be merged to be edited in subsequent transactions. EclipseLink. @RayHulha – Mastectomy

@pritamkumar, you explained well the concept of a persistence context. I'd just add that there's also the javax.persistence.PersistenceContext annotation, which is used to inject an EntityManager object and establish the scope of the injected object (e.g., a transaction). – Schell

so why isnt this an implementation detail and why do I have to manually specify my EntityManager to use a PersistenceContext ? – Lampas

Answer 3

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)

Unrequited

can one PersisnteceContext have any EntityManager instances? And can one Entity Manager have any PersistenceContext? PersisntenceContext is only one for all application? – Needs

Answer 4

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.

Parachute

Answer 5

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.

Pskov

Answer 6

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

Govan

Answer 7

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.

Dairymaid

How is this answer different from the existing answers? – Maloney

Answer 8

"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

Eventful

Answer 9

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.

Pooi

Hi user2771889, You are right I didn't mention the extended scope of persistence context. 5th point was just related to transaction scoped persistence context. – Mastectomy

Answer 10

The persistence context is the first-level cache where all the entities are fetched from the database or saved to the database.

Codie