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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。

数据访问技术	实现类
JDBC	DataSourceTransactionManager
JPA	JpaTransactionManager
Hibernate	HibernateTransactionManager
JDO	JdoTransactionManager
分布式事务	JtaTransactionManager

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.
 *
 * <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 16.05.2003
 * @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.
	 * <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：

属性	类型	描述
value	String	可选的限定描述符，指定使用的事务管理器
propagation	enum: Propagation	定义事务的生命周期，有REQUIRED、SUPPORTS、MANDATORY、REQUIRES_NEW、NOT_SUPPORTED、NEVER、NESTED，详细含义可查阅枚举类org.springframework.transaction.annotation.Propagation源码。
isolation	enum: Isolation	可选的事务隔离级别设置，决定了事务的完整性
readOnly	boolean	读写或只读事务，默认读写
timeout	int (in seconds granularity)	事务超时时间设置
rollbackFor	Class对象数组，必须继承自Throwable	导致事务回滚的异常类数组
rollbackForClassName	类名数组，必须继承自Throwable	导致事务回滚的异常类名字数组
noRollbackFor	Class对象数组，必须继承自Throwable	不会导致事务回滚的异常类数组
noRollbackForClassName	类名数组，必须继承自Throwable	不会导致事务回滚的异常类名字数组

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