Java Entity Manager: Named Query and Persistence

Study Notes

JPA allows mapping entities to databases and querying them using different criteria.
JPA's power lies in offering an object-oriented way to query entities and their relationships without using underlying database foreign keys or columns.
The central piece of the API is the javax.persistence.EntityManager, which manages entities, reads from and writes to a database, and allows simple CRUD operations and complex queries using JPQL.

An EntityManager can be obtained using an EntityManagerFactory, which is created using Persistence.createEntityManagerFactory("chapter04PU").
The EntityManager interface can be used to manipulate entities, hiding JDBC calls to the database and INSERT or SELECT SQL statements.

JPQL queries manipulate objects and attributes, not tables and columns.
JPQL statements can be executed with dynamic queries (created dynamically at runtime) or static queries (defined statically at compile time).
Static queries, also known as named queries, are defined using annotations (@NamedQuery) or XML metadata.

Entities may include Bean Validation constraints and be automatically validated through JPA.
JPA runtime will throw a ConstraintViolation exception if the entity does not meet the validation constraints.

JPA is an abstraction above JDBC that makes it possible to be independent of SQL.
The main components of JPA are:
- ORM, which maps objects to data stored in a relational database.
- An entity manager API to perform database-related operations.
- JPQL, which allows retrieving data with an object-oriented query language.
- Transactions and locking mechanisms provided by the Java Transaction API (JTA).
- Callbacks and listeners to hook business logic into the life cycle of a persistent object.

The persistence unit indicates to the entity manager the type of database to use and the connection parameters.
The persistence unit is defined in the persistence.xml file, which must be accessible in the class path.