JPA

JPA, the Java Persistence API, is a Java specification for object-relational mapping and data persistence. It defines a standard way to manage relational data using Java objects, including how entities map to database tables, how queries are written, and how transactions are coordinated. JPA separates the API from the underlying ORM implementation, allowing developers to switch providers without changing application code.

Core concepts include entity classes annotated with @Entity, primary keys annotated with @Id and optionally @GeneratedValue, and mappings to tables via @Table and columns via @Column. Relationships are expressed with annotations such as @OneToMany, @ManyToOne, @OneToOne, and @ManyToMany. The primary runtime API is the EntityManager, which offers operations such as persist, find, merge, remove, and createQuery. Queries can be written in JPQL (a database-agnostic object query language), via the Criteria API for type-safe construction, or as native SQL. JPA manages a persistence context that tracks entity lifecycles, including states like new, managed, detached, and removed, and supports transactional boundaries.

JPA is implemented by persistence providers such as Hibernate, EclipseLink, and OpenJPA, with Hibernate being the most widely used. The API is part of Java EE prior to Jakarta EE and can be used in Java SE through a provider. In Jakarta EE, the specification uses the jakarta.persistence namespace. Providers handle mapping, caching, lazy loading, cascading, and, in many cases, schema generation, enabling portability across relational databases while reducing boilerplate data-access code.