SpringFramework源码解析——@Transactional是如何生效的？

1. 背景

相比于JDBC的编程式事务，Spring提供了一种更便捷的处理事务的方式，即声明式事务。我们只需要在方法或类上标注@Transactional注解便可完成事务的声明，如下示例：

@Transactional(rollbackFor = Exception.class)
public void pay() {
	jdbcTemplate.execute("update balance = balance + 1 where id = 1");
	jdbcTemplate.execute("update balance = balance - 1 where id = 2");
}

当上面的方法抛出异常时，整个事务会进行回滚。下面我们来看@Transactional注解是如何生效的？

2. 声明式事务是如何实现的？

声明式事务是在事务切面中完成了事务的创建、提交和回滚。

2.1. 事务切面

事务切面在配置类ProxyTransactionManagementConfiguration中配置，切面相关要素如下：

• 事务切点：事务属性来源切点TransactionAttributeSourcePointcut。
• 事务通知：事务拦截器TransactionInterceptor。
• 事务切面：Bean工厂事务属性来源切面BeanFactoryTransactionAttributeSourceAdvisor。

2.2. 事务切点

事务切点由事务属性来源切点TransactionAttributeSourcePointcut实现，在以下几种情况会进行方法的拦截：

• 当类能够被事务属性来源TransactionAttributeSource认定为候选类时。
• 当方法和类能够被事务属性来源TransactionAttributeSource获取到事务属性TransactionAttribute时。

@Override
public boolean matches(Method method, Class<?> targetClass) {
    // 事务属性来源为空或存在事务属性时，切点匹配
    return (this.transactionAttributeSource == null ||
            this.transactionAttributeSource.getTransactionAttribute(method, targetClass) != null);
}

private class TransactionAttributeSourceClassFilter implements ClassFilter {

    @Override
    public boolean matches(Class<?> clazz) {
        if (TransactionalProxy.class.isAssignableFrom(clazz) ||
                TransactionManager.class.isAssignableFrom(clazz) ||
                PersistenceExceptionTranslator.class.isAssignableFrom(clazz)) {
            return false;
        }
        // 是否是候选类
        return (transactionAttributeSource == null || transactionAttributeSource.isCandidateClass(clazz));
    }
}

对应源码所在位置：TransactionAttributeSourcePointcut#matches。

2.3. 事务通知

事务通知由事务拦截器TransactionInterceptor实现，其完成了事务方法的拦截处理。

3. 事务拦截器是如何执行的？

事务拦截器的执行流程：

1. 获取事务属性；
2. 如果必要则创建事务；
3. 执行方法；
4. 抛出异常时决定是否需要回滚事务；
5. 执行正常返回时提交事务。

对应源码所在位置：TransactionAspectSupport#invokeWithinTransaction。

3.1. 获取事务属性

3.1.1. 事务属性

事务属性TransactionAttribute是对事务信息的抽象，其包含以下属性：

• propagationBehavior：事务的传播行为。
• isolationLevel：事务的隔离级别。
• timeout：事务的超时时间。
• readOnly：事务是否只读。
• rollbackOn：事务需要回滚的异常。

3.1.2. 事务属性来源

事务属性来源TransactionAttributeSource提供了getTransactionAttribute方法，其拥有从方法和类上获取事务属性TransactionAttribute的能力。

从实现上看，注解事务属性来源AnnotationTransactionAttributeSource将获取事务属性TransactionAttribute的逻辑委派给了事务注解转换器TransactionAnnotationParser。

• 从方法Method上获取事务属性TransactionAttribute。
• 从类Class上获取事务属性TransactionAttribute。
• 退路到接口方法获取事务属性TransactionAttribute。

protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
	// Don't allow non-public methods, as configured.
	if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
		// 事务仅允许公共方法，且方法的修饰符不是公共的，返回空
		return null;
	}

	// The method may be on an interface, but we need attributes from the target class.
	// If the target class is null, the method will be unchanged.
	// 获取具体的方法
	Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);

	// First try is the method in the target class.
	// 从方法上获取事务属性
	TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
	if (txAttr != null) {
		return txAttr;
	}

	// Second try is the transaction attribute on the target class.
	// 从类上获取事务属性
	txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
	if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
		return txAttr;
	}

	if (specificMethod != method) {
		// 退路到接口方法获取事务属性
		// Fallback is to look at the original method.
		txAttr = findTransactionAttribute(method);
		if (txAttr != null) {
			return txAttr;
		}
		// Last fallback is the class of the original method.
		txAttr = findTransactionAttribute(method.getDeclaringClass());
		if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
			return txAttr;
		}
	}

	return null;
}

@Override
@Nullable
protected TransactionAttribute findTransactionAttribute(Class<?> clazz) {
	return determineTransactionAttribute(clazz);
}

@Nullable
protected TransactionAttribute determineTransactionAttribute(AnnotatedElement element) {
	for (TransactionAnnotationParser parser : this.annotationParsers) {
		TransactionAttribute attr = parser.parseTransactionAnnotation(element);
		if (attr != null) {
			return attr;
		}
	}
	return null;
}

3.1.3. 事务注解转换器

事务注解转换器TransactionAnnotationParser提供了parseTransactionAnnotation方法，其拥有将注解转换为事务属性TransactionAttribute的能力。从实现上看，Spring事务注解转换器SpringTransactionAnnotationParser实现了将@Transactional注解转为事务属性TransactionAttribute`的逻辑。

@Override
@Nullable
public TransactionAttribute parseTransactionAnnotation(AnnotatedElement element) {
	// 获取事务注解
	AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
			element, Transactional.class, false, false);
	if (attributes != null) {
		return parseTransactionAnnotation(attributes);
	}
	else {
		return null;
	}
}
protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
	// 创建基于规则的事务属性
	RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();

	Propagation propagation = attributes.getEnum("propagation");
	rbta.setPropagationBehavior(propagation.value());
	Isolation isolation = attributes.getEnum("isolation");
	rbta.setIsolationLevel(isolation.value());

	rbta.setTimeout(attributes.getNumber("timeout").intValue());
	String timeoutString = attributes.getString("timeoutString");
	Assert.isTrue(!StringUtils.hasText(timeoutString) || rbta.getTimeout() < 0,
			"Specify 'timeout' or 'timeoutString', not both");
	rbta.setTimeoutString(timeoutString);

	rbta.setReadOnly(attributes.getBoolean("readOnly"));
	rbta.setQualifier(attributes.getString("value"));
	rbta.setLabels(Set.of(attributes.getStringArray("label")));

	List<RollbackRuleAttribute> rollbackRules = new ArrayList<>();
	for (Class<?> rbRule : attributes.getClassArray("rollbackFor")) {
		rollbackRules.add(new RollbackRuleAttribute(rbRule));
	}
	for (String rbRule : attributes.getStringArray("rollbackForClassName")) {
		rollbackRules.add(new RollbackRuleAttribute(rbRule));
	}
	for (Class<?> rbRule : attributes.getClassArray("noRollbackFor")) {
		rollbackRules.add(new NoRollbackRuleAttribute(rbRule));
	}
	for (String rbRule : attributes.getStringArray("noRollbackForClassName")) {
		rollbackRules.add(new NoRollbackRuleAttribute(rbRule));
	}
	rbta.setRollbackRules(rollbackRules);

	return rbta;
}

对应源码所在位置：AbstractFallbackTransactionAttributeSource#getTransactionAttribute。

3.2. 如果必要则创建事务

事务的传播行为决定了当存在或不存在事务时执行的行为，如下表格：

@Override
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
		throws TransactionException {

	// Use defaults if no transaction definition given.
	TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

	// 获取当前事务
	Object transaction = doGetTransaction();
	boolean debugEnabled = logger.isDebugEnabled();

	// 如果存在事务，进行相应处理
	if (isExistingTransaction(transaction)) {
		// Existing transaction found -> check propagation behavior to find out how to behave.
		return handleExistingTransaction(def, transaction, debugEnabled);
	}

	// Check definition settings for new transaction.
	if (def.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
		throw new InvalidTimeoutException("Invalid transaction timeout", def.getTimeout());
	}

	// 如果不存在事务，进行相应处理
	// No existing transaction found -> check propagation behavior to find out how to proceed.
	// 当前不存在事务，且传播行为为 MANDATORY，则抛出异常
	if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
		throw new IllegalTransactionStateException(
				"No existing transaction found for transaction marked with propagation 'mandatory'");
	}
	// 当前不存在事务，且传播行为为 REQUIRED 或 REQUIRES_NEW 或 NESTED，则启动新的事务
	else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
			def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
			def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
		SuspendedResourcesHolder suspendedResources = suspend(null);
		if (debugEnabled) {
			logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
		}
		try {
			return startTransaction(def, transaction, debugEnabled, suspendedResources);
		}
		catch (RuntimeException | Error ex) {
			resume(null, suspendedResources);
			throw ex;
		}
	}
	// 当前不存在事务，且传播行为为 SUPPORTS 或 NOT_SUPPORTED 或 NEVER，则以非事务的方式执行
	else {
		// Create "empty" transaction: no actual transaction, but potentially synchronization.
		if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
			logger.warn("Custom isolation level specified but no actual transaction initiated; " +
					"isolation level will effectively be ignored: " + def);
		}
		boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
		return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
	}
}

private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {

	// 当前存在事务，且传播行为为 NEVER，则抛出异常
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
		throw new IllegalTransactionStateException(
				"Existing transaction found for transaction marked with propagation 'never'");
	}
	// 当前存在事务，且传播行为为 NOT_SUPPORTED，则挂起当前事务，以非事务的方式执行
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
		if (debugEnabled) {
			logger.debug("Suspending current transaction");
		}
		Object suspendedResources = suspend(transaction);
		boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
		return prepareTransactionStatus(
				definition, null, false, newSynchronization, debugEnabled, suspendedResources);
	}

	// 当前存在事务，且传播行为为 REQUIRES_NEW，则挂起当前事务，并启动新事务
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
		if (debugEnabled) {
			logger.debug("Suspending current transaction, creating new transaction with name [" +
					definition.getName() + "]");
		}
		SuspendedResourcesHolder suspendedResources = suspend(transaction);
		try {
			return startTransaction(definition, transaction, debugEnabled, suspendedResources);
		}
		catch (RuntimeException | Error beginEx) {
			resumeAfterBeginException(transaction, suspendedResources, beginEx);
			throw beginEx;
		}
	}
	// 当前存在事务，且传播行为为 NESTED，则启用嵌套事务
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
		if (!isNestedTransactionAllowed()) {
			throw new NestedTransactionNotSupportedException(
					"Transaction manager does not allow nested transactions by default - " +
					"specify 'nestedTransactionAllowed' property with value 'true'");
		}
		if (debugEnabled) {
			logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
		}
		if (useSavepointForNestedTransaction()) {
			// Create savepoint within existing Spring-managed transaction,
			// through the SavepointManager API implemented by TransactionStatus.
			// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
			DefaultTransactionStatus status =
					prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
			status.createAndHoldSavepoint();
			return status;
		}
		else {
			// Nested transaction through nested begin and commit/rollback calls.
			// Usually only for JTA: Spring synchronization might get activated here
			// in case of a pre-existing JTA transaction.
			return startTransaction(definition, transaction, debugEnabled, null);
		}
	}

	// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
	// 当前存在事务，且传播行为为 SUPPORTS 或 REQUIRED 或 MANDATORY，则加入到当前事务中
	if (debugEnabled) {
		logger.debug("Participating in existing transaction");
	}
	if (isValidateExistingTransaction()) {
		if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
			Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
			if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
				Constants isoConstants = DefaultTransactionDefinition.constants;
				throw new IllegalTransactionStateException("Participating transaction with definition [" +
						definition + "] specifies isolation level which is incompatible with existing transaction: " +
						(currentIsolationLevel != null ?
								isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
								"(unknown)"));
			}
		}
		if (!definition.isReadOnly()) {
			if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
				throw new IllegalTransactionStateException("Participating transaction with definition [" +
						definition + "] is not marked as read-only but existing transaction is");
			}
		}
	}
	boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
	return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}

对应源码所在位置：TransactionAspectSupport#createTransactionIfNecessary。

3.3. 执行方法

执行被代理对象的方法。

try {
	// This is an around advice: Invoke the next interceptor in the chain.
	// This will normally result in a target object being invoked.
	// 执行方法
	retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
	// target invocation exception
	// 在捕获到异常后，决定是否需要回滚事务
	completeTransactionAfterThrowing(txInfo, ex);
	throw ex;
}
finally {
	// 清除事务信息
	cleanupTransactionInfo(txInfo);
}

对应源码所在位置：TransactionAspectSupport.InvocationCallback#proceedWithInvocation。

3.4. 抛出异常时决定是否需要回滚事务

当执行方法异常时，根据异常类型决定是否需要回滚事务。需要回滚时则回滚事务；不需要回滚时则提交事务。

protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {
	if (txInfo != null && txInfo.getTransactionStatus() != null) {
		if (logger.isTraceEnabled()) {
			logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
					"] after exception: " + ex);
		}
		// 判断是否需要回滚
		if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
			try {
				// 需要回滚则回滚事务
				txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
			}
			catch (TransactionSystemException ex2) {
				logger.error("Application exception overridden by rollback exception", ex);
				ex2.initApplicationException(ex);
				throw ex2;
			}
			catch (RuntimeException | Error ex2) {
				logger.error("Application exception overridden by rollback exception", ex);
				throw ex2;
			}
		}
		else {
			// We don't roll back on this exception.
			// Will still roll back if TransactionStatus.isRollbackOnly() is true.
			try {
				// 不需要回滚则提交事务
				txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
			}
			catch (TransactionSystemException ex2) {
				logger.error("Application exception overridden by commit exception", ex);
				ex2.initApplicationException(ex);
				throw ex2;
			}
			catch (RuntimeException | Error ex2) {
				logger.error("Application exception overridden by commit exception", ex);
				throw ex2;
			}
		}
	}
}

由基于规则的事务属性RuleBasedTransactionAttribute来决定是否需要回滚，其rollbackOn方法会获取最接近抛出异常的声明异常，决定是否回滚。

@Override
public boolean rollbackOn(Throwable ex) {
    RollbackRuleAttribute winner = null;
    int deepest = Integer.MAX_VALUE;

    if (this.rollbackRules != null) {
        for (RollbackRuleAttribute rule : this.rollbackRules) {
            int depth = rule.getDepth(ex);
            // 获取最接近的规则
            if (depth >= 0 && depth < deepest) {
                deepest = depth;
                winner = rule;
            }
        }
    }

    // User superclass behavior (rollback on unchecked) if no rule matches.
    if (winner == null) {
        return super.rollbackOn(ex);
    }

    return !(winner instanceof NoRollbackRuleAttribute);
}

回滚规则属性RollbackRuleAttribute提供了getDepth方法，以获取异常的深度。

// 获取指定异常进行规则匹配的深度
public int getDepth(Throwable exception) {
    return getDepth(exception.getClass(), 0);
}


private int getDepth(Class<?> exceptionType, int depth) {
    if (this.exceptionType != null) {
        if (this.exceptionType.equals(exceptionType)) {
            // Found it!
            return depth;
        }
    }
    // 包含规则即可
    else if (exceptionType.getName().contains(this.exceptionPattern)) {
        // Found it!
        return depth;
    }
    // If we've gone as far as we can go and haven't found it...
    if (exceptionType == Throwable.class) {
        return -1;
    }
    return getDepth(exceptionType.getSuperclass(), depth + 1);
}

对应源码所在位置：TransactionAspectSupport#completeTransactionAfterThrowing。

3.5. 执行正常返回时提交事务

执行方法正常结束时，提交事务。

protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) {
	if (txInfo != null && txInfo.getTransactionStatus() != null) {
		if (logger.isTraceEnabled()) {
			logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
		}
		txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
	}
}

对应源码所在位置：TransactionAspectSupport#commitTransactionAfterReturning。