Spring中声明式事务确实给我们带来了很大的便利，在Service层方法上带上@Transactional注解即可实现事务，首先看看@Transactional的默认配置：
1、默认使用的事务管理器名字为"transactionManager"。
2、默认事务隔离传播为Propagation.REQUIRED。
3、默认超时时长为底层事务系统的时长。
4、默认事务隔离级别为数据库的隔离级别。
5、默认事务类型为读写事务。
6、默认发生RuntimeExeption异常会触发事务回滚，而受检异常不会。
@Target({ElementType.TYPE, ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)@Inherited@Documentedpublic @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 {};} 二、事务传播机制介绍 public enum Propagation { /** * 如果当前线程执行的方法如果没有事务则创建新事务，否则在已存在事务中执行 * / REQUIRED(TransactionDefinition.PROPAGATION_REQUIRED), /** * 如果当前线程执行的方法如果有事务，则在事务中执行，否则无事务执行 * / SUPPORTS(TransactionDefinition.PROPAGATION_SUPPORTS), /** * 当前线程执行的方法必须有事务，否则抛出异常 * /MANDATORY(TransactionDefinition.PROPAGATION_MANDATORY), /** * 当前线程执行方法每次都会创建新的事务，如果当前方法已在事务中执行，则会挂起外部事务 * /REQUIRES_NEW(TransactionDefinition.PROPAGATION_REQUIRES_NEW), /** * 当前线程所执行的方法在无事务环境中执行，如果已有外部事务，则会挂起外部事务 * /NOT_SUPPORTED(TransactionDefinition.PROPAGATION_NOT_SUPPORTED), /** * 当前线程所执行的方法在无事务环境中执行，如果已有外部事务，则会抛出异常 * /NEVER(TransactionDefinition.PROPAGATION_NEVER), /** * 如果外部事务存在，则当前线程所在方法会在嵌套事务中执行。 * 如果外部事务不存在，则和REQUIRED效果一样。 * /NESTED(TransactionDefinition.PROPAGATION_NESTED);} 三、嵌套事务实例 1、Propagation.REQUIRED @Servicepublic class TransactionServiceB { @Autowired private OperationLogMapper operationLogMapper; @Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class) public void methodB() { OperationLogDo operationLog = new OperationLogDo(); operationLog.setOpType(1); operationLog.setOpContent("新增操作"); operationLog.setUserId(1); operationLog.setCreateTime(new Date()); operationLogMapper.saveOperationLog(operationLog); }}@Servicepublic class TransactionServiceA { @Autowired private TransactionServiceB serviceB; @Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class) public void methodA() { serviceB.methodB(); throw new RuntimeException("发生异常了!"); }}由于methodA和methodB事务的传播机制都是Propagation.REQUIRED，因此methodB会和methodA在同一个事务中执行，当methodA执行发生异常时，methodB会回滚，因此methodB中的插入操作不会生效。
2、Propagation.REQUIRES_NEW @Servicepublic class TransactionServiceA { @Autowired private TransactionServiceB serviceB; @Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class) public void methodA() { serviceB.methodB(); throw new RuntimeException("发生异常了!"); }}@Servicepublic class TransactionServiceB { @Autowired private OperationLogMapper operationLogMapper; @Transactional(propagation = Propagation.REQUIRES_NEW, rollbackFor = Exception.class) public void methodB() { OperationLogDo operationLog = new OperationLogDo(); operationLog.setOpType(1); operationLog.setOpContent("新增操作"); operationLog.setUserId(1); operationLog.setCreateTime(new Date()); operationLogMapper.saveOperationLog(operationLog); }}methodA方法上的事务传播机制是Propagation.REQUIRED，而methodB方法上的事务传播机制Propagation.REQUIRES_NEW，因此methodB方法会在一个新的事务中执行，同时外部事务(即methodA所在事务)会被挂起。这里我们看到的效果是数据库中仍然插入了一条记录，说明不受外部事务影响。
3、Propagation.NESTED @Servicepublic class TransactionServiceA { @Autowired private OperationLogMapper operationLogMapper; @Autowired private TransactionServiceB serviceB; @Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class) public void methodA() { serviceB.methodB(); OperationLogDo operationLog = new OperationLogDo(); operationLog.setOpType(1); operationLog.setOpContent("修改操作"); operationLog.setUserId(1); operationLog.setCreateTime(new Date()); operationLogMapper.saveOperationLog(operationLog); throw new RuntimeException("发生异常了!"); }}@Servicepublic class TransactionServiceB { @Autowired private OperationLogMapper operationLogMapper; @Transactional(propagation = Propagation.NESTED, rollbackFor = Exception.class) public void methodB() { OperationLogDo operationLog = new OperationLogDo(); operationLog.setOpType(1); operationLog.setOpContent("新增操作"); operationLog.setUserId(1); operationLog.setCreateTime(new Date()); operationLogMapper.saveOperationLog(operationLog); }}此时methodA方法在methodB方法执行完后抛出了异常，最后的结果是methodB中的插入操作回滚了。为什么呢？我们可以把嵌套事务看做是外部事务的一个子事务，既然外部事务发生异常回滚了，那么子事务也会回滚，但要注意子事务不会影响外部事务。而如果methodB上的隔离级别设为REQUIRES_NEW，那么methodB中的插入操作会生效,因为methodB方法执行会在一个新的事务环境中，不受其它事务影响。
有同学会发现，上面的例子即使methodB方法上的事务传播机制为Propagation.REQUIRED，methodB方法中的插入操作也会回滚，那我们再看看Propagation.NESTED和Propagation.REQUIRED的区别，下面再进行改写：
@Servicepublic class TransactionServiceA { @Autowired private OperationLogMapper operationLogMapper; @Autowired private TransactionServiceB serviceB; @Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class) public void methodA() { try { serviceB.methodB(); } catch (Exception e) { e.printStackTrace(); } OperationLogDo operationLog = new OperationLogDo(); operationLog.setOpType(1); operationLog.setOpContent("修改操作"); operationLog.setUserId(1); operationLog.setCreateTime(new Date()); operationLogMapper.saveOperationLog(operationLog); }}@Servicepublic class TransactionServiceB { @Autowired private OperationLogMapper operationLogMapper; @Transactional(propagation = Propagation.NESTED, rollbackFor = Exception.class) public void methodB() { OperationLogDo operationLog = new OperationLogDo(); operationLog.setOpType(1); operationLog.setOpContent("新增操作"); operationLog.setUserId(1); operationLog.setCreateTime(new Date()); operationLogMapper.saveOperationLog(operationLog); throw new RuntimeException("发生异常了!"); }}上面这个例子看到的效果是methodB方法中的插入操作会回滚，而methodA方法在捕获methodB方法抛出的异常后执行插入操作，因此数据库会新增一条记录。这也说明当methodB方法的传播机制为Propagation.NESTED时，methodB方法所在的事务为外部事务的一个子事务，子事务不会影响外部事务。当然如果methodA方法不捕获异常，那么methodA方法中的插入操作同样也会回滚。