Understand the key concepts of the ORM/JPA framework based on language integrated queries in Java class libraries

Key Concepts of ORM/JPA Framework Based on Language Integrated Queries in Java Class Libraries ORM (Object Relational Mapping) is a programming technique used to map object models to data in relational databases. It enables developers to access and operate databases in an object-oriented manner, without the need to directly use SQL language. JPA (Java Persistence API) is an important ORM framework in Java class libraries, providing a standardized way to implement object relational mapping. JPA is part of the Java SE and Java EE platforms, so it can be used together with Java applications and enterprise applications. In order to better understand the key concepts of the JPA framework, let's introduce the following key concepts in sequence: Entity classes: In JPA, entity classes are Java classes mapped to database tables. Each entity class instance corresponds to a record in the database. Entity classes are typically identified using annotations (such as @ Entity) and mapped one-to-one with columns in database tables. The following is a simple example of an entity class: @Entity @Table(name = "customers") public class Customer { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; @Column(name = "name") private String name; // Getters and Setters } Entity Manager: The Entity Manager is one of the core interfaces of JPA, used to manage the interaction between applications and databases. It is responsible for creating, reading, updating, and deleting entity objects. Through the entity manager, developers can use the API provided by JPA to perform database related operations. The following is an example of using an entity manager for querying: EntityManagerFactory emf = Persistence.createEntityManagerFactory("myPU"); EntityManager em = emf.createEntityManager(); Customer customer = em.find(Customer.class, 1L); System.out.println("Customer name: " + customer.getName()); em.close(); emf.close(); Persistence Context: Persistence context is an important concept in the JPA framework used to manage the lifecycle of entity objects. It is a cache area that stores the state of entity objects managed by JPA. By persisting the context, JPA can track changes in entity objects and synchronize changes to the database at appropriate times. JPQL (Java Persistence Query Language): JPQL is a query language provided by JPA, similar to SQL but targeting entity objects rather than database tables. It can be queried through entity classes and attributes, and supports various query operations such as selection, filtering, and sorting. The following is an example of using JPQL for querying: TypedQuery<Customer> query = em.createQuery("SELECT c FROM Customer c WHERE c.name = :name", Customer.class); query.setParameter("name", "John Doe"); List<Customer> customers = query.getResultList(); The above are some key concepts in the JPA framework. By using JPA, developers can more easily handle the mapping between objects and databases, improve development efficiency, and reduce the workload of writing duplicate code.