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.
*/ 必須有一個事務，如果目前已存在則用目前的，如果不存在則建立一個事務
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
*/ 使用目前事物，如果目前則以無事務模式運作
MANDATORY
/**
* Support a current transaction, throw an exception if none exists.
* Analogous to EJB transaction attribute of the same name.
*/ 表示強制需要一個事務，如果不存事務則抛出異常
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
*/ 每次都建立一個新的事務，如果目前存在事務則挂起目前事務運作新事務，兩個事務互相獨立，新事物復原也不會影響外部事務
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
*/ 已非事務模式運作，如果目前存在事務則挂起目前事務
NEVER
/**
* Execute non-transactionally, throw an exception if a transaction exists.
* Analogous to EJB transaction attribute of the same name.
*/ 從不需要事務，如果目前存在事務則抛出異常，剛好與 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
*/ 嵌入式事務模式，與PROPAGATION_REQUIRED相似，如果沒有就建立，如果有适用目前事務，但是NESTED具體目前事務多個儲存點，可以部分復原。僅适用于JDBC事務管理，内部可以獨立復原并且不影響外部。