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EntityManager Introduction#

What Is The EntityManager?#

  • EntityManager: The resource manager that maintains the active collection of Entity objects that are being used by the application. The EntityManager handles the database interaction and metadata for object-relational mappings (ORM) . An instance of an EntityManager represents a Persistence Context.

    • An application in a container can obtain the EntityManage through injection into the application or by looking it up in the Java component name-space.
    • If the application manages its persistence, the EntityManager is obtained from the EntityManagerFactory .

  • In JPA, the EntityManager interface is used to allow applications to manage and search for entities in the relational database.

    • The EntityManager is an API that manages the life-cycle of Entity instances. An EntityManager object manages a set of entities that are defined by a Persistence Unit. Each EntityManager instance is associated with a persistence context. A persistence context defines the scope under which particular entity instances are created, persisted, and removed through the APIs made available by EntityManager. In some ways, a persistence context is conceptually similar to a transaction context.
    • The EntityManager tracks all Entity objects within apersistence context for changes and updates that are made, and flushes these changes to the database. Once a persistence context is closed, all managed entity object instances become detached from the persistence context and its associated entity manager, and are no longer managed. Once an object is detached from a persistence context, it can no longer be managed by an entity manager, and any state changes to this object instance will not be synchronized with the database.
    • There's always a new Persistence Context if there's a new EntityManager.

  • You can view more here

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