关于java:一Spring事务开启原理

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Spring 事务简略实现步骤

在事务配置类上申明 @EnableTransactionManagement 注解开启事务
在事务配置类上定义数据源
在事务配置类上定义事务管理器
在相干类或者办法上应用 @Transactional 申明事务

代码如下：

@Configuration
@EnableTransactionManagement
public class RootConfig{
    
    @Bean
    public DataSource dataSource(){DruidDataSource dataSource = new DruidDataSource();
        dataSource.setXXX();
        ...
        
        return dataSource;
    }
    
    @Bean
    public PlatfromTransactionManager txManager(){return new DataSourceTransactionManager(dataSource());
    }
}

@Service
public class UserService{

    @Autowired
    private UserRepository userRepository;
    
    @Transactional
    public void addUser(User user){userRepository.save(user);
    }
}

@EnableTransactionManagement 开启事务原理解析

@EnableTransactionManagement 源码如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {boolean proxyTargetClass() default false;
    AdviceMode mode() default AdviceMode.PROXY;
    int order() default Ordered.LOWEST_PRECEDENCE;}

能够看到，@EnableTransactionManagement 接口类次要 Import 了 TransactionManagementConfigurationSelector 来实现其注入，而 TransactionManagementConfigurationSelector 又次要应用 selectImport 办法来实现其注入，代码如下：

@Override
public final String[] selectImports(AnnotationMetadata importingClassMetadata) {Class<?> annoType = GenericTypeResolver.resolveTypeArgument(getClass(), AdviceModeImportSelector.class);
    AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType);
    if (attributes == null) {
        throw new IllegalArgumentException(String.format(
            "@%s is not present on importing class'%s'as expected",
            annoType.getSimpleName(), importingClassMetadata.getClassName()));
    }

    AdviceMode adviceMode = attributes.getEnum(this.getAdviceModeAttributeName());
    // 依据 AdviceMode 返回不同的类型，默认是 AdviceMode.PROXY。String[] imports = selectImports(adviceMode);
    if (imports == null) {throw new IllegalArgumentException(String.format("Unknown AdviceMode:'%s'", adviceMode));
    }
    return imports;
}

@Override
protected String[] selectImports(AdviceMode adviceMode) {switch (adviceMode) {
        case PROXY:
            return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()};
        case ASPECTJ:
            return new String[] {TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
        default:
            return null;
    }
}

其中次要性能点为依据 AdviceMode 抉择创立不同的 bean，AdviceMode 的默认代理形式是 PROXY，jdk 代理。所以返回的是 AutoProxyRegistrar 和 ProxyTransactionManagementConfiguration。

咱们先剖析 AutoProxyRegistrar，AutoProxyRegistrar 实现了 ImportBeanDefinitionRegistrar，那在创立 bean 的时候会调用 registerBeanDefinitions 办法。registerBeanDefinitions 办法的实现：

@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    boolean candidateFound = false;
    Set<String> annoTypes = importingClassMetadata.getAnnotationTypes();
    for (String annoType : annoTypes) {AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType);
        if (candidate == null) {continue;}
        Object mode = candidate.get("mode");
        Object proxyTargetClass = candidate.get("proxyTargetClass");
        if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
                Boolean.class == proxyTargetClass.getClass()) {
            candidateFound = true;
            // 只有 @EnableTransactionManagement 注解才会走到这里
            if (mode == AdviceMode.PROXY) {AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
                if ((Boolean) proxyTargetClass) {AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
                    return;
                }
            }
        }
    }
    //...
}

public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, Object source) {return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
}

能够看到，它通过注册 InfrastructureAdvisorAutoProxyCreator 来启动 Spring Aop。

接下来再看 ProxyTransactionManagementConfiguration 的作用，代码如下：

@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
        advisor.setTransactionAttributeSource(transactionAttributeSource());
        advisor.setAdvice(transactionInterceptor());
        advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
        return advisor;
    }

    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionAttributeSource transactionAttributeSource() {return new AnnotationTransactionAttributeSource();
    }

    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionInterceptor transactionInterceptor() {TransactionInterceptor interceptor = new TransactionInterceptor();
        interceptor.setTransactionAttributeSource(transactionAttributeSource());
        if (this.txManager != null) {interceptor.setTransactionManager(this.txManager);
        }
        return interceptor;
    }

}

ProxyTransactionManagementConfiguration 是一个配置文件，注册了三个 bean，BeanFactoryTransactionAttributeSourceAdvisor、AnnotationTransactionAttributeSource、TransactionInterceptor，而这三个类别离继承 Advisor、Advice 和 Pointcut。即切面所需组件。

总结
@EnableTransactionManagement 利用 AutoProxyRegistrar 启动 Spring Aop, 应用 ProxyTransactionManagementConfiguration 配置对应切面部件。

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