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

    @AliasFor("transactionManager")
    String value() default "";
    //事務(wù)管理器名稱
    @AliasFor("value")
    String transactionManager() default "";
    //事務(wù)傳播模式
    Propagation propagation() default Propagation.REQUIRED;
    //事務(wù)隔離級(jí)別
    Isolation isolation() default Isolation.DEFAULT;
    //超時(shí)時(shí)間
    int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;
    //是否是只讀事務(wù)
    boolean readOnly() default false;
    //需要回滾的異常類
    Class<? extends Throwable>[] rollbackFor() default {};
    //需要回滾的異常類名稱
    String[] rollbackForClassName() default {};
    //排除回滾的異常類
    Class<? extends Throwable>[] noRollbackFor() default {};
    //排除回滾的異常類名稱
    String[] noRollbackForClassName() default {};
}

// 事務(wù)自動(dòng)配置類 注意類里面使用的@EnableTransactionManagement注解
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration
public class TransactionAutoConfiguration {
    @Configuration
    @ConditionalOnBean(PlatformTransactionManager.class)
    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    public static class EnableTransactionManagementConfiguration {
        //注意這里使用的@EnableTransactionManagement注解
        @Configuration
        @EnableTransactionManagement(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
                havingValue = "false", matchIfMissing = false)
        public static class JdkDynamicAutoProxyConfiguration {
        }
        //注意這里使用的@EnableTransactionManagement注解
        @Configuration
        @EnableTransactionManagement(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
                havingValue = "true", matchIfMissing = true)
        public static class CglibAutoProxyConfiguration {
        }
    }
}

@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();
    }
    //這里注入了TransactionInterceptor攔截器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;
    }
}

@Override
    public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
        Object transaction = doGetTransaction();
        boolean debugEnabled = logger.isDebugEnabled();
        if (definition == null) {
            // 使用默認(rèn)的Definition
            definition = new DefaultTransactionDefinition();
        }
        if (isExistingTransaction(transaction)) {
            //已經(jīng)存在事務(wù)，進(jìn)入單獨(dú)的方法處理
            return handleExistingTransaction(definition, transaction, debugEnabled);
        }
        // 檢查timeout參數(shù)
        if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
            throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
        }
        // 當(dāng)前必須存在事務(wù)，否則拋出異常
        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) {
            //獲取當(dāng)前的一些事務(wù)信息，用于當(dāng)前事務(wù)執(zhí)行完后恢復(fù)
            SuspendedResourcesHolder suspendedResources = suspend(null);
            try {
                boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                //構(gòu)造一個(gè)新事務(wù)的TransactionStatus（包含嵌套事務(wù)SavePoint的支持）
                DefaultTransactionStatus status = newTransactionStatus(
                        definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
                //開啟新的事務(wù)
                doBegin(transaction, definition);
                prepareSynchronization(status, definition);
                return status;
            }
            catch (RuntimeException | Error ex) {
                //異常，恢復(fù)掛起的事務(wù)信息
                resume(null, suspendedResources);
                throw ex;
            }
        }
        else {
            //空的事務(wù)
            boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
            return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
        }
    }