Common questions and best practices of Jakarta Persistence API

Jakarta Persistence API (JPA) is a specification that is used in the Java EE platform to implement object relationship mapping (ORM).It provides a standard API and grammar that enables developers to easily map Java objects to the table structure in the relationship database.This article will answer some common questions about JPA and provide some best practices and Java code examples. Question 1: What is JPA? Answer: JPA is a standard specification for achieving ORM.It defines a set of APIs and grammar, so that developers can perform database operations through object operation without writing tedious SQL statements.JPA allows developers to perform database operations in an object -oriented way, which improves development efficiency and readability of code. Question 2: How to configure the JPA entity class? Answer: The JPA physical class is an ordinary Java class that is used to mappore to the table structure in the database.You can configure the physical class with annotations or xml.The following is an example of configuration in the annotation method: @Entity @Table(name = "employee") public class Employee { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; @Column(name = "name") private String name; // Getters and setters } In the above example, `@Entity` Note indicates that this class is a JPA entity class,`@table` annotations are used to specify the name of the database table of the mapping, `@ID` indicate that this is the primary key of the physical class,` `` `@` `@` `@` `@GeneratedValue` specifies the automatic generating strategy of the primary key, and `@column` is used to specify the mapping relationship between the attributes and the database field. Question 3: How to perform basic additions, deletion and change operations? Answer: JPA provides a set of standard APIs and methods to perform basic additions, deletion, modification operations.Here are some common examples: Create a physical object and save it in the database: EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("my-persistence-unit"); EntityManager entityManager = entityManagerFactory.createEntityManager(); Employee employee = new Employee(); employee.setName("John Doe"); entityManager.getTransaction().begin(); entityManager.persist(employee); entityManager.getTransaction().commit(); Inquire from the database and update the physical object: Employee employee = entityManager.find(Employee.class, 1L); entityManager.getTransaction().begin(); employee.setName("Jane Smith"); entityManager.getTransaction().commit(); Delete the physical object from the database: Employee employee = entityManager.find(Employee.class, 1L); entityManager.getTransaction().begin(); entityManager.remove(employee); entityManager.getTransaction().commit(); Question 4: How to perform complex query operations? Answer: JPA provides a powerful query language called JPQL (Java Persistence Query Language), which is similar to SQL but pays more attention to objects.You can use JPQL to perform complex query operations and get the required results.The following is an example: String jpql = "SELECT e FROM Employee e WHERE e.name LIKE :name"; TypedQuery<Employee> query = entityManager.createQuery(jpql, Employee.class); query.setParameter("name", "%Doe%"); List<Employee> employees = query.getResultList(); In the above example, we define a JPQL query that will find all the Employee objects containing "DOE" in the name.We use the `Like` operator and parameters` name` to perform fuzzy matching.Finally, we obtain the result set by calling the `GetResultList ()` method. Question 5: How to manage transaction? Answer: JPA provides a simple way to manage transactions to ensure the consistency of database operations.You can use the `Gettransaction () method of the` EntityManager` class to obtain transactions, and use the method of `Begin (),`), and `Rollback ()` to control the start, submission, and rollback of the transaction.The following is an example: EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("my-persistence-unit"); EntityManager entityManager = entityManagerFactory.createEntityManager(); EntityTransaction transaction = entityManager.getTransaction(); transaction.begin(); try { // Execute the database operation // Use Persist, Merge, Remove and other methods to perform operations transaction.commit(); } catch (Exception e) { transaction.rollback(); throw e; } In the above example, we get a transaction and start it. After performing the database operation, call the `Commit ()" method to submit the transaction.If abnormalities occur, we will roll back the transaction and re -throw an abnormality. Best Practice 1: Make sure the physical class has a parameter constructor. JPA requires the physical class to have a parameter constructor because it requires instantiated objects and sets attributes.Therefore, ensure that your physical class has a public constructor. Best Practice 2: Use delay loading to improve performance. Delay loading is a technology that can only load related data when needed, instead of loading all data at one time.This can improve performance and avoid loading unnecessary data.You can configure delay loads with annotations such as `@Manytoone`,@Onetomany`, and` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` `` ` Best Practice 3: Use cache to improve performance. JPA provides a cache mechanism that caches commonly used data and improves query performance.You can enable the cache by configure the `@cacheable` annotation of the physical class.But please use the cache carefully to ensure that the cache is updated in time when updating the data. This article answers some common questions about Jakarta Persistence API, and provides some best practice and Java code examples.By learning and following these best practices, you can better use JPA for data persistence operations.