Spring事务注解@Transactional的实现原理

@Transactional 注解元数据驱动的声明式事务

基本原理是：

将对应的方法通过注解元数据，标注在业务方法或者所在的对象上，然后在业务执行期间，通过AOP拦截器反射读取元数据信息，最终将根据读取的业务信息构建事务管理支持。

不同的方法之间的事务传播保证在同一个事务内，是通过统一的数据源来实现的，事务开始时将数据源绑定到ThreadLocal中，后续加入的事务从ThreadLocal获取数据源来保证数据源的统一。

@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Transactional {

    @AliasFor("transactionManager")
    String value() default "";
    //事务管理器名称
    @AliasFor("value")
    String transactionManager() default "";
    //事务传播模式
    Propagation propagation() default Propagation.REQUIRED;
    //事务隔离级别
    Isolation isolation() default Isolation.DEFAULT;
    //超时时间
    int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;
    //是否是只读事务
    boolean readOnly() default false;
    //需要回滚的异常类
    Class<? extends Throwable>[] rollbackFor() default {};
    //需要回滚的异常类名称
    String[] rollbackForClassName() default {};
    //排除回滚的异常类
    Class<? extends Throwable>[] noRollbackFor() default {};
    //排除回滚的异常类名称
    String[] noRollbackForClassName() default {};
}

这里通过SpringBoot代码来分析实现过程，源码中删除了部分代码，只保留了一些重要部分

// 事务自动配置类 注意类里面使用的@EnableTransactionManagement注解
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration

public class TransactionAutoConfiguration {
    @Configuration
    @ConditionalOnBean(PlatformTransactionManager.class)
    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    public static class EnableTransactionManagementConfiguration {

        //注意这里使用的@EnableTransactionManagement注解
        @Configuration
        @EnableTransactionManagement(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
                havingValue = "false", matchIfMissing = false)
        public static class JdkDynamicAutoProxyConfiguration {
        }
        //注意这里使用的@EnableTransactionManagement注解
        @Configuration
        @EnableTransactionManagement(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
                havingValue = "true", matchIfMissing = true)
        public static class CglibAutoProxyConfiguration {
        }
    }
}

这里只分析proxy模式

@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());
        if (this.enableTx != null) {
            advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
        }
        return advisor;
    }

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

    //这里注入了TransactionInterceptor拦截器bean~~~~
    @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;
    }

}

TransactionInterceptor拦截器通过元数据获取事务定义信息TransactionDefinition，根据Definition信息获取PlatformTransactionManager（TM），tm接口抽象了事务的实现流程，默认的tm是DataSourceTransactionManager（通过DataSourceTransactionManagerAutoConfiguration初始化的），tm中的getTransaction根据事务的传播方式，开启、加入、挂起事务

@Override
    public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
        Object transaction = doGetTransaction();

        boolean debugEnabled = logger.isDebugEnabled();

        if (definition == null) {
            // 使用默认的Definition
            definition = new DefaultTransactionDefinition();
        }

        if (isExistingTransaction(transaction)) {
            //已经存在事务，进入单独的方法处理
            return handleExistingTransaction(definition, transaction, debugEnabled);
        }

        // 检查timeout参数
        if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
            throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
        }

        // 当前必须存在事务，否则抛出异常
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
            throw new IllegalTransactionStateException(
                    "No existing transaction found for transaction marked with propagation 'mandatory'");
        }
        else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
            //获取当前的一些事务信息，用于当前事务执行完后恢复
            SuspendedResourcesHolder suspendedResources = suspend(null);
            try {
                boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                //构造一个新事务的TransactionStatus（包含嵌套事务SavePoint的支持）
                DefaultTransactionStatus status = newTransactionStatus(
                        definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
                //开启新的事务
                doBegin(transaction, definition);
                prepareSynchronization(status, definition);
                return status;
            }
            catch (RuntimeException | Error ex) {
                //异常，恢复挂起的事务信息
                resume(null, suspendedResources);
                throw ex;
            }
        }
        else {
            //空的事务
            boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
            return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
        }
    }