Architecture

The Java persistence API is built around the following components:

• Persistence provider: A persistence provider is a JPA implementation provided by a vendor. As with most Java APIs, JPA is a standard API, where a set of different actual implementations are available from different vendors. The persistence provider means the vendor of the actual implementation we are using.
• Entities: An entity is a class that represents a domain object in our enterprise application. From a JPA perspective, an entity is represented with a table in the database, and an instance of this entity represents a record in that table.

• Entity managers: An entity manager is an object that represents a connection to a database, and contains all methods for the different operations that can be performed with the database, such as inserting a new record, retrieving a single record, performing advanced queries, and so on.
• Entity transaction: An entity transaction represents a database transaction that can be either committed or rolled back according to the application state. Any update operations (insert, update, delete) should be performed within the boundaries of an entity transaction.
• Query: As the name suggests, a query is an object that is used to perform a database query! In addition to the regular SQL, JPA provides a custom query language (called JPQL) that can be used to perform queries exactly like SQL, but with object-oriented concepts.
• Persistence unit: A persistence unit is a group of entities involved in a persistence context. A persistence unit is specified using a configuration file (persistence.xml), describing connection information, entities involved, and other useful configurations.