What is Persistence Context?

问题

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.

回答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

http://gerrydevstory.com/wp-content/uploads/2012/05/jpa-state-transtition.png

回答2:

Entities are managed by javax.persistence.EntityManager instance using persistence context.
Each EntityManager instance is associated with a persistence context.
Within the persistence context, the entity instances and their lifecycle are managed.
Persistence context defines a scope under which particular entity instances are created, persisted, and removed.
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.

回答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)

回答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.

回答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.

回答6:

"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

回答7:

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.