Spring Data JPA

[ythy]

Hibernate is an implementation of the JPA spec, which itself is part of the Java EE spec. ORM is the approach of mapping objects to relational tables

In a typical Spring application, many important objects are JavaBeans: data access templates, data access objects, transaction managers, business services that use the data access objects and transaction managers, web view resolvers, web controllers that use the business services, and so on.

[ythy]

Spring Data JPA is not a JPA provider. It is a library/framework that adds an extra layer of abstraction on the top of our JPA provider (like Hibernate). Hibernate is a JPA implementation, while Spring Data JPA is a JPA Data Access Abstraction. Spring Data offers a solution to GenericDao custom implementations. It can also generate JPA queries on your behalf through method name conventions. With Spring Data, you may use Hibernate, Eclipse Link, or any other JPA provider. Spring Data JPA is not an implementation or JPA provider, it's just an abstraction used to significantly reduce the amount of boilerplate code required to implement data access layers for various persistence stores. Hibernate provides a reference implementation of the Java Persistence API that makes it a great choice as an ORM tool with benefits of loose coupling. Remember, Spring Data JPA always requires the JPA provider such as Hibernate or Eclipse Link.

[ythy]

Spring Boot configures Hibernate as the default JPA provider, so it’s no longer necessary to define the entityManagerFactory bean unless we want to customize it.

[ythy]

EntityManager是JPA的一个概念

[ythy]

An entity manager factory should be considered as an immutable configuration holder, it is defined to point to a single datasource and to map a defined set of entities. This is the entry point to create and manage EntityManagers. The Persistence class is bootstrap class to create an entity manager factory.

// Use persistence.xml configuration
EntityManagerFactory emf = Persistence.createEntityManagerFactory("manager1")
EntityManager em = emf.createEntityManager(); // Retrieve an application managed entity manager
// Work with the EM
emf.close(); //close at application end

An entity manager factory is typically create at application initialization time and closed at application end. It's creation is an expensive process. For those who are familiar with Hibernate, an entity manager factory is very much like a session factory. Actually, an entity manager factory is a wrapper on top of a session factory. Calls to the entityManagerFactory are thread safe.

Thanks to the EntityManagerFactory, you can retrieve an extended entity manager. The extended entity manager keep the same persistence context for the lifetime of the entity manager: in other words, the entities are still managed between two transactions (unless you call entityManager.clear() in between). You can see an entity manager as a small wrapper on top of an Hibernate session.

[ythy]

EntityManagerFactory

An entity manager factory provides entity manager instances, all instances are configured to connect to the same database, to use the same default settings as defined by the particular implementation, etc. You can prepare several entity manager factories to access several data stores. This interface is similar to the SessionFactory in native Hibernate.

EntityManager

The EntityManager API is used to access a database in a particular unit of work. It is used to create and remove persistent entity instances, to find entities by their primary key identity, and to query over all entities. This interface is similar to the Session in Hibernate.

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 is managed by a particular entity manager. The scope of this context can either be the transaction, or an extended unit of work.

Persistence unit

The set of entity types that can be managed by a given entity manager is defined by a persistence unit. A persistence unit defines the set of all classes that are related or grouped by the application, and which must be collocated in their mapping to a single data store.