Spring Boot的事务支持

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一 自动配置的事务管理器

1 说明

在使用JDBC作为数据访问技术的时候，Spring Boot为我们定义了PlatformTransactionManager的实现DataSourceTransactionManager的Bean。

在使用JPA作为数据库访问技术的时候，Spring Boot为我们定义了PlatformTransactionManager的实现JpaBaseConfiguration的Bean。

2 源码

//JDBC事务配置的源码
@Configuration
@ConditionalOnClass({ JdbcTemplate.class, PlatformTransactionManager.class })
@AutoConfigureOrder(Ordered.LOWEST_PRECEDENCE)
public class DataSourceTransactionManagerAutoConfiguration {

    @Autowired(required = false)
    private DataSource dataSource;

    @Bean
    @ConditionalOnMissingBean(PlatformTransactionManager.class)
    @ConditionalOnBean(DataSource.class)
    public DataSourceTransactionManager transactionManager() {
        return new DataSourceTransactionManager(this.dataSource);
    }

    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    @Configuration
    @EnableTransactionManagement
    protected static class TransactionManagementConfiguration {

    }

}
//JPA事务配置的源码
@EnableConfigurationProperties(JpaProperties.class)
@Import(DataSourceInitializedPublisher.Registrar.class)
public abstract class JpaBaseConfiguration implements BeanFactoryAware {

    private static final String[] NO_PACKAGES = new String[0];

    private ConfigurableListableBeanFactory beanFactory;

    @Autowired
    private DataSource dataSource;

    @Autowired(required = false)
    private PersistenceUnitManager persistenceUnitManager;

    @Autowired
    private JpaProperties jpaProperties;

    @Autowired(required = false)
    private JtaTransactionManager jtaTransactionManager;

    @Bean
    @ConditionalOnMissingBean(PlatformTransactionManager.class)
    public PlatformTransactionManager transactionManager() {
        return new JpaTransactionManager();
    }
    ......
}

二 自动开启注解事务的支持

Spring Boot专门用于配置事务的类为：TransactionAutoConfiguration，此配置类依赖于JpaBaseConfiguration和DataSourceTransactionManagerAutoConfiguration，代码如下：

@Configuration
@ConditionalOnClass({ TransactionTemplate.class, PlatformTransactionManager.class })
@ConditionalOnSingleCandidate(PlatformTransactionManager.class)
@AutoConfigureAfter({ JtaAutoConfiguration.class, JpaBaseConfiguration.class,
        DataSourceTransactionManagerAutoConfiguration.class })
public class TransactionAutoConfiguration {

    @Autowired
    private PlatformTransactionManager transactionManager;

    @Bean
    @ConditionalOnMissingBean
    public TransactionTemplate transactionTemplate() {
        return new TransactionTemplate(this.transactionManager);
    }

}

而在DataSourceTransactionManagerAutoConfiguration配置里还开启了声明式事务的支持，代码如下：

@Configuration
@ConditionalOnClass({ JdbcTemplate.class, PlatformTransactionManager.class })
@AutoConfigureOrder(Ordered.LOWEST_PRECEDENCE)
public class DataSourceTransactionManagerAutoConfiguration {

    @Autowired(required = false)
    private DataSource dataSource;

    @Bean
    @ConditionalOnMissingBean(PlatformTransactionManager.class)
    @ConditionalOnBean(DataSource.class)
    public DataSourceTransactionManager transactionManager() {
        return new DataSourceTransactionManager(this.dataSource);
    }

    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    @Configuration
    @EnableTransactionManagement
    protected static class TransactionManagementConfiguration {

    }

}

所以在Spring Boot中，无须显示开启使用@EnableTransactionManagement注解。