Entities are classes used to represent persistent objects in JPA. It can be defined using the @Entity annotation and needs to implement the Serializable interface. Entity classes usually contain fields, properties and methods, which correspond to columns and tables in the database, and methods are used to operate these fields and properties.

3. What are the persistence strategies in JPA? Explain their advantages and disadvantages respectively.

JPA provides a variety of persistence strategies, including:

• Managed: Entities are managed through EntityManager, and the life cycle is controlled by JPA. The advantage is that it simplifies the management of entities, but the disadvantage is that it increases memory overhead.
• Detached: The entity is separated from the EntityManager and is not under the management of JPA. The advantage is that it reduces memory overhead, but the disadvantage is that the life cycle of the entity needs to be manually managed.
• Transient: The entity does not participate in any persistence operations. The advantage is that it will not be persisted to the database. The disadvantage is that it cannot be managed and queried by JPA.
• Deleted (Removed): The entity is marked for deletion and is deleted from the database after transaction is submitted. The advantage is to ensure the consistency of the data, but the disadvantage is that the entity needs to be managed manually. life cycle.

4. How to use query language (JPQL) in JPA to query data?

JPQL (Java Persistence Query Language) is the language used to query data in JPA. It is similar to SQL, but the syntax is simpler and closer to the Java language. JPQL queries can be divided into two categories:

• Select query: Used to retrieve data that meets specific conditions. The syntax is: SELECT [select_clause] FROM [entity_name] [where_clause]
• Update query: Used to update or delete data, the syntax is: UPDATE [entity_name] SET [assignment_clause] [where_clause] or DELETE FROM [entity_name] [where_clause]

5. How to implement relationship mapping in JPA?

JPA supports multiple relationship mapping types, including:

• One-to-one (OneToOne): An entity can establish a one-to-one relationship with another entity, which can be defined through the @OneToOne annotation.
• One-to-many (OneToMany): An entity can establish a one-to-many relationship with multiple entities, which can be defined through the @OneToMany annotation.
• Many-to-One (ManyToOne): Multiple entities can establish a many-to-one relationship with one entity, which can be defined through the @ManyToOne annotation.
• Many-to-Many (ManyToMany): Multiple entities can establish many-to-many relationships with multiple entities, which can be defined through the @ManyToMany annotation.

6. How to optimize performance in JPA?

JPA provides a variety of performance optimization technologies, including:

• Using caching: JPA can improve performance by caching entities and query results.
• Use indexes: You can create indexes in the database table to improve query performance.
• Use batch processing: JPA supports batch processing operations, which can improve the performance of batch data updates or deletions.
• Use asynchronous queries: JPA supports asynchronous queries, which can execute queries in the background without blocking the main thread.

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