SpringBoot 中的事务处理
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- 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
*
* /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.
*
* <p>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.
*
* <p>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.
* @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.
* <p>Note that parameters like isolation level or timeout will only be applied
* to new transactions, and thus be ignored when participating in active ones.
* <p>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.
* <p>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.
* <p>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.
* <p>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.
* <p>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.
* <p>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.
* <p><b>Do not call rollback on a transaction if commit threw an exception.</b>
* 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。
