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JPA在MySQL中自动建表
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;@Repositorypublic 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;@Repositorypublic 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 { @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<Role> roles = roleRepository.findAll(); roles.add(role); 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<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：
(原文：https://my.oschina.net/syic/blog/798994)
JPA在MySQL中自动建表
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