Detailed explanation of the technical principles of the Persistence API framework in the Java class library
Detailed explanation of the technical principles of the Persistence API framework in the Java class library
Overview
Persistence API (hereinafter referred to as JPA) is a framework on the Java platform to simplify database access and management.It provides a set of standard interfaces and specifications to map objects into relational databases and achieve durable data.
The core idea of JPA is the object relationship mapping (ORM).In traditional development, we often need to manually write SQL statements in the program to operate the database.After using the ORM framework, we can hide the details and operations of the database in the framework, and perform database operations through simple object operation syntax.
Technical principle
The technical principles of JPA can be divided into the following aspects:
1. Note and XML mapping configuration
In JPA, we can define the mapping relationship between the physical class and the database table by annotating or xml configuration files.Through annotations, we can use@Column,@Table,@Entity, etc. to specify the structure of the database table and the associated relationship with the physical class attributes at the attributes or class levels.When using XML configuration, we describe the metadata of the physical class in the XML file, and the framework is processed based on these configurations.
2. EntityManager
The entity manager is a core component in JPA. It is responsible for managing the creation, preservation, renewal, and deletion of the entity object.We can perform persistent operations through a physical manager, such as saving objects into databases, querying objects from the database, etc.
3. Object query language (JPQL)
JPQL is a query language similar to SQL, which is used to perform query operations for physical objects.JPQL provides statements similar to SQL for querying physical objects, but it is an object -oriented query language, which can directly query the attributes and related relationships of the entity.JPQL query can flexibly generate specific SQL statements, making the query process more concise and easy to understand.
4. Affairs management
The JPA framework also provides the function of transaction management to ensure the consistency and integrity of the data operation.By using annotations or programming methods, we can organize a set of associated data operations in one transaction to ensure their atomicity, consistency and isolation.
5. cache mechanism
The JPA framework also provides a cache mechanism to improve the performance of data access.When querying data, if the data already exists in the cache, the framework will read the data directly from the cache instead of requesting a database.This can significantly reduce access to databases and improve system performance.
Code example
Below is a simple example code that demonstrates how to use the JPA framework for database addition, deletion, and check operation.
1. Create a physical class
@Entity
@Table(name = "users")
public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
private String name;
// omit Getters and Setters
}
2. Create data access object (DAO)
public class UserDao {
@PersistenceContext
private EntityManager entityManager;
public User findById(int id) {
return entityManager.find(User.class, id);
}
public void save(User user) {
entityManager.persist(user);
}
public void update(User user) {
entityManager.merge(user);
}
public void delete(User user) {
entityManager.remove(user);
}
}
3. Configuration file
In the configuration file, we need to specify the implementation providers of JPA, such as Hibernate, Eclipseelink, etc.The following is an example of the configuration file of using Hibernate as a JPA to implement the provider:
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="myPersistenceUnit" transaction-type="RESOURCE_LOCAL">
<provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
<class>com.example.User</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/mydb"/>
<property name="javax.persistence.jdbc.user" value="user"/>
<property name="javax.persistence.jdbc.password" value="password"/>
</properties>
</persistence-unit>
</persistence>
The above code example demonstrates the basic use of JPA. Through the cooperation of physical objects and DAO, we can easily perform the database addition, deletion, and inspection operation.
in conclusion
JPA is a framework for simplifying database access and management. It is based on ORM ideas and realizes the mapping relationship between the physical class and the database table through annotation or XML configuration.JPA provides functions such as physical manager, object query language, transaction management, and cache mechanism, which simplifies the development of the developers' database.By using JPA, we can pay more attention to business logic and improve the maintenance of development efficiency and code.
