Spring: @Transactional中的propagation属性

在Spring的事务管理中，我们可以使用@Transactional这一annotation来对事务进行声明式的设定。具体而言，就是在类或者方法前添加@Transactional并传入属性参数以获取所需要的Transaction特性。Spring中的@Transactional有5个属性：Propagation、Isolation、Rollback Rules、Timeout和Read-Only，其中Propagation属性定义了Transaction的边界 — 是否使用Transaction、在Transaction已存在的情况下如何表现等。

在service类前加上@Transactional，声明这个service所有方法需要事务管理。每一个业务方法开始时都会打开一个事务。 

Spring默认情况下会对运行期例外(RunTimeException)进行事务回滚。这个例外是unchecked 

如果遇到checked意外就不回滚。 

如何改变默认规则： 

1 让checked例外也回滚：在整个方法前加上 @Transactional(rollbackFor=Exception.class) 

2 让unchecked例外不回滚： @Transactional(notRollbackFor=RunTimeException.class) 

3 不需要事务管理的(只查询的)方法：@Transactional(propagation=Propagation.NOT_SUPPORTED) 

在整个方法运行前就不会开启事务 

       还可以加上：@Transactional(propagation=Propagation.NOT_SUPPORTED,readOnly=true)，这样就做成一个只读事务，可以提高效率。 

       各种属性的意义： 

       REQUIRED:业务方法需要在一个容器里运行。如果方法运行时，已经处在一个事务中，那么加入到这个事务，否则自己新建一个新的事务。 

       NOT_SUPPORTED:声明方法不需要事务。如果方法没有关联到一个事务，容器不会为他开启事务，如果方法在一个事务中被调用，该事务会被挂起，调用结束后，原先的事务会恢复执行。 

       REQUIRESNEW:不管是否存在事务，该方法总汇为自己发起一个新的事务。如果方法已经运行在一个事务中，则原有事务挂起，新的事务被创建。 

       MANDATORY：该方法只能在一个已经存在的事务中执行，业务方法不能发起自己的事务。如果在没有事务的环境下被调用，容器抛出例外。 

       SUPPORTS:该方法在某个事务范围内被调用，则方法成为该事务的一部分。如果方法在该事务范围外被调用，该方法就在没有事务的环境下执行。 

       NEVER：该方法绝对不能在事务范围内执行。如果在就抛例外。只有该方法没有关联到任何事务，才正常执行。 

       NESTED:如果一个活动的事务存在，则运行在一个嵌套的事务中。如果没有活动事务，则按REQUIRED属性执行。它使用了一个单独的事务，这个事务拥有多个可以回滚的保存点。内部事务的回滚不会对外部事务造成影响。它只对DataSourceTransactionManager事务管理器起效。 

观察以下两个定义了@Transactional的方法，innerMethod()模拟了Transaction已经存在的情况，outMethod()则模拟了不存在已定义Transaction的情况：

@Transactional
public void outMethod() {
    exampleDAO.doSomething();
    innerMethod();
}

@Transactional
public void innerMethod() {
    exampleDAO.doElse();
}

对于这两个方法，定义不同的Propagation属性值所产生的效果如下(Propagation.NESTED的情况较为复杂，在此忽略)： 

Propagation属性	outMethod	innerMethod
Propagation.MANDATORY	.抛出异常	.在outMethod的Transaction中运行
Propagation.NEVER	.不在Transaction中运行	.抛出异常
Propagation.NOT_SUPPORTED	.不在Transaction中运行	.outMethod的Transaction暂停直至innerMethod执行完毕
Propagation.REQUIRED ( 默认值 )	.新开一个Transaction并在其中运行	.在outMethod的Transaction中运行
Propagation.REQUIRES_NEW	.新开一个Transaction并在其中运行	.outMethod的Transaction暂停直至innerMethod中新开的Transaction执行完毕
Propagation.SUPPORTS	.不在Transaction中运行	.在outMethod的Transaction中运行

@Transactional的代码定义：

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

     /**
      * A qualifier value for the specified transaction.
      * <p>May be used to determine the target transaction manager,
      * matching the qualifier value (or the bean name) of a specific
      * {@link org.springframework.transaction.PlatformTransactionManager}
      * bean definition.
      */
     String value() default "";

     /**
      * The transaction propagation type.
      * Defaults to {@link Propagation#REQUIRED}.
      * @see org.springframework.transaction.interceptor.TransactionAttribute#getPropagationBehavior()
      */
     Propagation propagation() default Propagation.REQUIRED;

     /**
      * The transaction isolation level.
      * Defaults to {@link Isolation#DEFAULT}.
      * @see org.springframework.transaction.interceptor.TransactionAttribute#getIsolationLevel()
      */
     Isolation isolation() default Isolation.DEFAULT;

     /**
      * The timeout for this transaction.
      * Defaults to the default timeout of the underlying transaction system.
      * @see org.springframework.transaction.interceptor.TransactionAttribute#getTimeout()
      */
     int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;

     /**
      * {@code true} if the transaction is read-only.
      * Defaults to {@code false}.
      * <p>This just serves as a hint for the actual transaction subsystem;
      * it will <i>not necessarilyi> cause failure of write access attempts.
      * A transaction manager which cannot interpret the read-only hint will
      * <i>noti> throw an exception when asked for a read-only transaction.
      * @see org.springframework.transaction.interceptor.TransactionAttribute#isReadOnly()
      */
     boolean readOnly() default false;

     /**
      * Defines zero (0) or more exception {@link Class classes}, which must be a
      * subclass of {@link Throwable}, indicating which exception types must cause
      * a transaction rollback.
      * <p>This is the preferred way to construct a rollback rule, matching the
      * exception class and subclasses.
      * <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(Class clazz)}
      */
     Class extends Throwable>[] rollbackFor() default {};

     /**
      * Defines zero (0) or more exception names (for exceptions which must be a
      * subclass of {@link Throwable}), indicating which exception types must cause
      * a transaction rollback.
      * <p>This can be a substring, with no wildcard support at present.
      * A value of "ServletException" would match
      * {@link javax.servlet.ServletException} and subclasses, for example.
      * <p><b>NB: b>Consider carefully how specific the pattern is, and whether
      * to include package information (which isn't mandatory). For example,
      * "Exception" will match nearly anything, and will probably hide other rules.
      * "java.lang.Exception" would be correct if "Exception" was meant to define
      * a rule for all checked exceptions. With more unusual {@link Exception}
      * names such as "BaseBusinessException" there is no need to use a FQN.
      * <p>Similar to {@link org.springframework.transaction.interceptor.RollbackRuleAttribute#RollbackRuleAttribute(String exceptionName)}
      */
     String[] rollbackForClassName() default {};

     /**
      * Defines zero (0) or more exception {@link Class Classes}, which must be a
      * subclass of {@link Throwable}, indicating which exception types must <b>notb>
      * cause a transaction rollback.
      * <p>This is the preferred way to construct a rollback rule, matching the
      * exception class and subclasses.
      * <p>Similar to {@link org.springframework.transaction.interceptor.NoRollbackRuleAttribute#NoRollbackRuleAttribute(Class clazz)}
      */
     Class extends Throwable>[] noRollbackFor() default {};

     /**
      * Defines zero (0) or more exception names (for exceptions which must be a
      * subclass of {@link Throwable}) indicating which exception types must <b>notb>
      * cause a transaction rollback.
      * <p>See the description of {@link #rollbackForClassName()} for more info on how
      * the specified names are treated.
      * <p>Similar to {@link org.springframework.transaction.interceptor.NoRollbackRuleAttribute#NoRollbackRuleAttribute(String exceptionName)}
      */
     String[] noRollbackForClassName() default {};

 }

  基于源代码，我们可以发现在@Transactional，原来有这么多的属性可以进行配置，从而达到复杂应用控制的目的。

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