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SpringBoot中的事务处理

作者：xcbeyond
疯狂源自梦想，技术成就辉煌！微信公众号：《程序猿技术大咖》号主，专注后端开发多年，拥有丰富的研发经验，乐于技术输出、分享，现阶段从事微服务架构项目的研发工作，涉及架构设计、技术选型、业务研发等工作。对于Java、微服务、数据库、Docker有深入了解，并有大量的调优经验。

前两章节主要讲解了在SpringBoot中关于对数据的操作，本章节将介绍如何进行事务处理。所有的数据访问技术都离不开事务处理，否则将会造成数据不一致。事务是一系列的动作，一旦其中有一个动作出现错误，必须全部回滚，系统将事务中对数据库的所有已完成的操作全部撤消，滚回到事务开始的状态，避免出现由于数据不一致而导致的接下来一系列的错误。事务的出现是为了确保数据的完整性和一致性，在目前企业级应用开发中，事务管理是必不可少的。

1、SpringBoot事务机制
事务处理机制都会提供API来开启事务、提交事务来完成数据操作，或者在发生错误的时候回滚数据，避免数据的不完整性、不一致性。

SpringBoot事务机制实质上就是Spring的事务机制，是采用统一的机制处理来自不同数据访问技术的事务处理，提供了一个接口 PlatformTransactionManager，已经为不同数据访问技术可以进行不同的实现，如下表。

涉及到接口关系如下：

接口PlatformTransactionManager源码如下：

/*
* Copyright 2002-2012 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.springframework.transaction;

import org.springframework.lang.Nullable;

/**
* This is the central interface in Spring's transaction infrastructure.
* Applications can use this directly, but it is not primarily meant as API:
* Typically, applications will work with either TransactionTemplate or
* declarative transaction demarcation through AOP.
*
* For implementors, it is recommended to derive from the provided
* {@link org.springframework.transaction.support.AbstractPlatformTransactionManager}
* class, which pre-implements the defined propagation behavior and takes care
* of transaction synchronization handling. Subclasses have to implement
* template methods for specific states of the underlying transaction,
* for example: begin, suspend, resume, commit.
*
* The default implementations of this strategy interface are
* {@link org.springframework.transaction.jta.JtaTransactionManager} and
* {@link org.springframework.jdbc.datasource.DataSourceTransactionManager},
* which can serve as an implementation guide for other transaction strategies.
*
* @author Rod Johnson
* @author Juergen Hoeller
* @since 16.05.2003
* @see org.springframework.transaction.support.TransactionTemplate
* @see org.springframework.transaction.interceptor.TransactionInterceptor
* @see org.springframework.transaction.interceptor.TransactionProxyFactoryBean
*/
public interface PlatformTransactionManager {

 /**
 * Return a currently active transaction or create a new one, according to
 * the specified propagation behavior.
 * Note that parameters like isolation level or timeout will only be applied
 * to new transactions, and thus be ignored when participating in active ones.
 * Furthermore, not all transaction definition settings will be supported
 * by every transaction manager: A proper transaction manager implementation
 * should throw an exception when unsupported settings are encountered.
 * An exception to the above rule is the read-only flag, which should be
 * ignored if no explicit read-only mode is supported. Essentially, the
 * read-only flag is just a hint for potential optimization.
 * @param definition TransactionDefinition instance (can be {@code null} for defaults),
 * describing propagation behavior, isolation level, timeout etc.
 * @return transaction status object representing the new or current transaction
 * @throws TransactionException in case of lookup, creation, or system errors
 * @throws IllegalTransactionStateException if the given transaction definition
 * cannot be executed (for example, if a currently active transaction is in
 * conflict with the specified propagation behavior)
 * @see TransactionDefinition#getPropagationBehavior
 * @see TransactionDefinition#getIsolationLevel
 * @see TransactionDefinition#getTimeout
 * @see TransactionDefinition#isReadOnly
 */
 TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException;

 /**
 * Commit the given transaction, with regard to its status. If the transaction
 * has been marked rollback-only programmatically, perform a rollback.
 * If the transaction wasn't a new one, omit the commit for proper
 * participation in the surrounding transaction. If a previous transaction
 * has been suspended to be able to create a new one, resume the previous
 * transaction after committing the new one.
 * Note that when the commit call completes, no matter if normally or
 * throwing an exception, the transaction must be fully completed and
 * cleaned up. No rollback call should be expected in such a case.
 * If this method throws an exception other than a TransactionException,
 * then some before-commit error caused the commit attempt to fail. For
 * example, an O/R Mapping tool might have tried to flush changes to the
 * database right before commit, with the resulting DataAccessException
 * causing the transaction to fail. The original exception will be
 * propagated to the caller of this commit method in such a case.
 * @param status object returned by the {@code getTransaction} method
 * @throws UnexpectedRollbackException in case of an unexpected rollback
 * that the transaction coordinator initiated
 * @throws HeuristicCompletionException in case of a transaction failure
 * caused by a heuristic decision on the side of the transaction coordinator
 * @throws TransactionSystemException in case of commit or system errors
 * (typically caused by fundamental resource failures)
 * @throws IllegalTransactionStateException if the given transaction
 * is already completed (that is, committed or rolled back)
 * @see TransactionStatus#setRollbackOnly
 */
 void commit(TransactionStatus status) throws TransactionException;

 /**
 * Perform a rollback of the given transaction.
 * If the transaction wasn't a new one, just set it rollback-only for proper
 * participation in the surrounding transaction. If a previous transaction
 * has been suspended to be able to create a new one, resume the previous
 * transaction after rolling back the new one.
 * Do not call rollback on a transaction if commit threw an exception.
 * The transaction will already have been completed and cleaned up when commit
 * returns, even in case of a commit exception. Consequently, a rollback call
 * after commit failure will lead to an IllegalTransactionStateException.
 * @param status object returned by the {@code getTransaction} method
 * @throws TransactionSystemException in case of rollback or system errors
 * (typically caused by fundamental resource failures)
 * @throws IllegalTransactionStateException if the given transaction
 * is already completed (that is, committed or rolled back)
 */
 void rollback(TransactionStatus status) throws TransactionException;

}
2、声明式事务
 建立在AOP之上的，其本质是对方法前后进行拦截，然后在目标方法开始之前创建或者加入一个事务，在执行完目标方法之后根据执行情况提交或者回滚事务。声明式事务最大的优点就是不需要通过编程的方式管理事务，这样就不需要在业务逻辑代码中掺杂事务管理的代码，只需在配置文件中做相关的事务规则声明(或通过基于@Transactional注解的方式)，便可以将事务规则应用到业务逻辑中。

Spring支持声明式事务，被注解的方法在被调用时，Spring开启一个新的事务，当方法无异常结束后，Spring会提交这个事务。

@Transactional
public void insertUser(User user) {
//数据库表的操作
……
}
注：

（1）@Transactional是来自org.springframework.transaction.annotation包的。

（2）@Transactional不仅可以注解在方法上，也可以注解在类上。当注解在类上时，意味着此类的所有public方法都是开启事务的。如果类级别和方法级别同时使用了@Transactional注解，则使用在类级别的注解会重载方法级别的注解。

以下为注解@Transactional源码：

(为了缩小所占篇数，故去掉注释部分)

package org.springframework.transaction.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.core.annotation.AliasFor;
import org.springframework.transaction.TransactionDefinition;

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

 @AliasFor("transactionManager")
 String value() default "";

 @AliasFor("value")
 String transactionManager() default "";

 Propagation propagation() default Propagation.REQUIRED;

 Isolation isolation() default Isolation.DEFAULT;

 int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;

 boolean readOnly() default false;

 Class<? extends Throwable>[] rollbackFor() default {};

 String[] rollbackForClassName() default {};

 Class<? extends Throwable>[] noRollbackFor() default {};

 String[] noRollbackForClassName() default {};
}
属性说明如下表：

在SpringBoot中，建议采用注解@Transactional进行事务的控制。

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