JPA（Java Persistence API）是Spring Boot访问关系型数据库的一个标准接口，它使用ORM（Object-Relational Mapping）方式为数据对象建模和实现持久化。当然JAP也可以应用于其它支持Java的开发框架之中。

在JPA中使用MySQL数据库，只要为其进行建模，就可以自动生成表结构，省略了一些数据库设计的工作。而且，当数据模型更改的时候，应用程序也会对表结构进行更新。需要实现这一功能，主要取决你对使用JPA的配置。

例如，我们有一个数据的实体-关系模型如下图所示。

首先，我们创建项目工程，并引入Spring Boot和JPA的依赖。

其中，Spring Boot依赖如下所示：

<properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><java.version>1.8</java.version>
</properties><parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>1.4.2.RELEASE</version><relativePath/>
</parent><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-actuator</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency>
</dependencies>

JPA和MySQL的依赖如下所示：

<dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-jpa</artifactId></dependency><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><scope>runtime</scope></dependency>
</dependencies>

现在，可以对实体对象进行建模和持久化设计。

用户实体建模，其中User与Role的多对多关系使用一个关系表user_role来存储：

package com.demo.mysql.entity;import org.springframework.format.annotation.DateTimeFormat;import javax.persistence.*;
import java.util.Date;
import java.util.List;@Entity
@Table(name = "user")
public class User implements java.io.Serializable{@Id@GeneratedValue(strategy = GenerationType.IDENTITY)private Long id;private String name;@DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss")private Date createdate;@ManyToMany(cascade = {}, fetch = FetchType.EAGER)@JoinTable(name = "user_role",joinColumns = {@JoinColumn(name = "user_id")},inverseJoinColumns = {@JoinColumn(name = "roles_id")})private List<Role> roles;public User() {}public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Date getCreatedate() {return createdate;}public void setCreatedate(Date createdate) {this.createdate = createdate;}public List<Role> getRoles() {return roles;}public void setRoles(List<Role> roles) {this.roles = roles;}
}

角色建模：

package com.demo.mysql.entity;import javax.persistence.*;@Entity
@Table(name = "role")
public class Role implements java.io.Serializable{@Id@GeneratedValue(strategy = GenerationType.IDENTITY)private Long id;private String name;public Role() {}public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}
}

用户实体持久化：

package com.demo.mysql.repository;import com.demo.mysql.entity.User;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

角色实体持久化：

package com.demo.mysql.repository;import com.demo.mysql.entity.Role;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;@Repository
public interface RoleRepository extends JpaRepository<Role, Long> {
}

JPA及数据源配置，注意其中一行“hibernateProperties.setProperty("hibernate.hbm2ddl.auto", "update");”，将Hibernate的自动处理设置为“update”，表示表结构更改时即进行更新：

package com.demo.mysql.test;import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor;
import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
import org.springframework.jdbc.datasource.DriverManagerDataSource;
import org.springframework.orm.jpa.JpaTransactionManager;
import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
import org.springframework.orm.jpa.vendor.Database;
import org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter;
import org.springframework.transaction.PlatformTransactionManager;
import org.springframework.transaction.support.TransactionTemplate;import javax.sql.DataSource;
import java.util.Properties;@Configuration
@EnableJpaRepositories(basePackages = "com.**.repository")
public class JpaConfiguration {@BeanPersistenceExceptionTranslationPostProcessor persistenceExceptionTranslationPostProcessor(){return new PersistenceExceptionTranslationPostProcessor();}@Beanpublic DataSource dataSource() {DriverManagerDataSource dataSource = new DriverManagerDataSource();dataSource.setDriverClassName("com.mysql.jdbc.Driver");dataSource.setUrl("jdbc:mysql://localhost:3306/test?characterEncoding=utf8");dataSource.setUsername("root");dataSource.setPassword("12345678");return dataSource;}@Beanpublic LocalContainerEntityManagerFactoryBean entityManagerFactory() {LocalContainerEntityManagerFactoryBean entityManagerFactoryBean = new LocalContainerEntityManagerFactoryBean();entityManagerFactoryBean.setDataSource(dataSource());entityManagerFactoryBean.setPackagesToScan("com.**.entity");entityManagerFactoryBean.setJpaProperties(buildHibernateProperties());entityManagerFactoryBean.setJpaVendorAdapter(new HibernateJpaVendorAdapter() {{setDatabase(Database.MYSQL);}});return entityManagerFactoryBean;}protected Properties buildHibernateProperties(){Properties hibernateProperties = new Properties();hibernateProperties.setProperty("hibernate.dialect", "org.hibernate.dialect.MySQL5Dialect");hibernateProperties.setProperty("hibernate.show_sql", "true");hibernateProperties.setProperty("hibernate.use_sql_comments", "false");hibernateProperties.setProperty("hibernate.format_sql", "true");hibernateProperties.setProperty("hibernate.hbm2ddl.auto", "update");hibernateProperties.setProperty("hibernate.generate_statistics", "false");hibernateProperties.setProperty("javax.persistence.validation.mode", "none");hibernateProperties.setProperty("org.hibernate.envers.store_data_at_delete", "true");hibernateProperties.setProperty("org.hibernate.envers.global_with_modified_flag", "true");return hibernateProperties;}@Beanpublic PlatformTransactionManager transactionManager() {return new JpaTransactionManager();}@Beanpublic TransactionTemplate transactionTemplate() {return new TransactionTemplate(transactionManager());}
}

测试用例：

package com.demo.mysql.test;import com.demo.mysql.entity.Role;
import com.demo.mysql.entity.User;
import com.demo.mysql.repository.RoleRepository;
import com.demo.mysql.repository.UserRepository;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
import org.springframework.util.Assert;import java.util.Date;
import java.util.List;@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = {JpaConfiguration.class})
public class MysqlTest {@AutowiredUserRepository userRepository;@AutowiredRoleRepository roleRepository;@Beforepublic void initData(){Role role = new Role();role.setName("admins");roleRepository.save(role);Assert.notNull(role.getId());User user = new User();user.setName("user");user.setCreatedate(new Date());List<Role> roles = roleRepository.findAll();user.setRoles(roles);userRepository.save(user);Assert.notNull(user.getId());}@Testpublic void findPage(){Pageable pageable = new PageRequest(0, 10, new Sort(Sort.Direction.ASC, "id"));Page<User> page = userRepository.findAll(pageable);Assert.notNull(page);}
}

运行通过，在数据库test中即可以看到生成的表，并且具有上面的一些测试数据，如下图：