REQUIRED
/**
* Support a current transaction, create a new one if none exists.
* Analogous to EJB transaction attribute of the same name.
* <p>This is the default setting of a transaction annotation.
*/必须有一个事务，如果当前已存在则用当前的，如果不存在则新建一个事务
SUPPORTS
/**
* Support a current transaction, execute non-transactionally if none exists.
* Analogous to EJB transaction attribute of the same name.
* <p>Note: For transaction managers with transaction synchronization,
* PROPAGATION_SUPPORTS is slightly different from no transaction at all,
* as it defines a transaction scope that synchronization will apply for.
* As a consequence, the same resources (JDBC Connection, Hibernate Session, etc)
* will be shared for the entire specified scope. Note that this depends on
* the actual synchronization configuration of the transaction manager.
* @see org.springframework.transaction.support.AbstractPlatformTransactionManager#setTransactionSynchronization
*/使用当前事物，如果当前则以无事务模式运行
MANDATORY
/**
* Support a current transaction, throw an exception if none exists.
* Analogous to EJB transaction attribute of the same name.
*/表示强制需要一个事务，如果不存事务则抛出异常
REQUIRES_NEW
/**
* Create a new transaction, and suspend the current transaction if one exists.
* Analogous to the EJB transaction attribute of the same name.
* <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
* on all transaction managers. This in particular applies to
* {@link org.springframework.transaction.jta.JtaTransactionManager},
* which requires the {@code javax.transaction.TransactionManager} to be
* made available it to it (which is server-specific in standard Java EE).
* @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
*/每次都创建一个新的事务，如果当前存在事务则挂起当前事务运行新事务，两个事务互相独立，新事物回滚也不会影响外部事务
NOT_SUPPORTED
/**
* Execute non-transactionally, suspend the current transaction if one exists.
* Analogous to EJB transaction attribute of the same name.
* <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
* on all transaction managers. This in particular applies to
* {@link org.springframework.transaction.jta.JtaTransactionManager},
* which requires the {@code javax.transaction.TransactionManager} to be
* made available it to it (which is server-specific in standard Java EE).
* @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
*/已非事务模式运行，如果当前存在事务则挂起当前事务
NEVER
/**
* Execute non-transactionally, throw an exception if a transaction exists.
* Analogous to EJB transaction attribute of the same name.
*/从不需要事务，如果当前存在事务则抛出异常，刚好与 MANDATORY 相反
NESTED
/**
* Execute within a nested transaction if a current transaction exists,
* behave like PROPAGATION_REQUIRED else. There is no analogous feature in EJB.
* <p>Note: Actual creation of a nested transaction will only work on specific
* transaction managers. Out of the box, this only applies to the JDBC
* DataSourceTransactionManager when working on a JDBC 3.0 driver.
* Some JTA providers might support nested transactions as well.
* @see org.springframework.jdbc.datasource.DataSourceTransactionManager
*/嵌入式事务模式，与PROPAGATION_REQUIRED相似，如果没有就新建，如果有适用当前事务，但是NESTED具体当前事务多个保存点，可以部分回滚。仅适用于JDBC事务管理，内部可以独立回滚并且不影响外部。
