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：

        
     
      UTF-8
         
     
      1.8
     
    
        
     
      org.springframework.boot
         
     
      spring-boot-starter-parent
         
     
      1.4.2.RELEASE
         
     
    
        
             
      
       org.springframework.boot
              
      
       spring-boot-starter
          
         
             
      
       org.springframework.boot
              
      
       spring-boot-starter-actuator
          
         
             
      
       org.springframework.boot
              
      
       spring-boot-starter-test
              
      
       test
          
     
    

JPA和MySQL的依赖如下所示：

        
             
      
       org.springframework.boot
              
      
       spring-boot-starter-data-jpa
          
         
             
      
       mysql
              
      
       mysql-connector-java
              
      
       runtime
          
     
    

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
    
      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
     
       getRoles() {        return roles;    }    public void setRoles(List
      
        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;@Repositorypublic interface UserRepository extends JpaRepository
    
      {}
    

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;@Repositorypublic interface RoleRepository extends JpaRepository
    
      {}
    

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 {    @Bean    PersistenceExceptionTranslationPostProcessor persistenceExceptionTranslationPostProcessor(){        return new PersistenceExceptionTranslationPostProcessor();    }    @Bean    public 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;    }    @Bean    public 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;    }    @Bean    public PlatformTransactionManager transactionManager() {        return new JpaTransactionManager();    }    @Bean    public 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 {    @Autowired    UserRepository userRepository;    @Autowired    RoleRepository roleRepository;    @Before    public 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
    
      roles = roleRepository.findAll();        user.setRoles(roles);        userRepository.save(user);        Assert.notNull(user.getId());    }    @Test    public void findPage(){        Pageable pageable = new PageRequest(0, 10, new Sort(Sort.Direction.ASC, "id"));        Page
     
       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：