spring事务传播属性，隔离级别

package org.springframework.transaction;

import java.sql.Connection;

/**
* Interface that defines Spring-compliant transaction properties.
* Based on the propagation behavior definitions analogous to EJB CMT attributes.
*
* <p>Note that isolation level and timeout settings will not get applied unless
* an actual new transaction gets started. As only {@link #PROPAGATION_REQUIRED},
* {@link #PROPAGATION_REQUIRES_NEW} and {@link #PROPAGATION_NESTED} can cause
* that, it usually doesn't make sense to specify those settings in other cases.
* Furthermore, be aware that not all transaction managers will support those
* advanced features and thus might throw corresponding exceptions when given
* non-default values.
*
* <p>The {@link #isReadOnly() read-only flag} applies to any transaction context,
* whether backed by an actual resource transaction or operating non-transactionally
* at the resource level. In the latter case, the flag will only apply to managed
* resources within the application, such as a Hibernate <code>Session</code>.
*
* @author Juergen Hoeller
* @since 08.05.2003
* @see PlatformTransactionManager#getTransaction(TransactionDefinition)
* @see org.springframework.transaction.support.DefaultTransactionDefinition
* @see org.springframework.transaction.interceptor.TransactionAttribute
*/
public interface TransactionDefinition {
//事务的传播属性
/**
* Support a current transaction; create a new one if none exists.
* 如果存在一个事务，则支持当前事务。如果没有事务则开启一个新的事务。 REQUIRED(必需的)
*/
int PROPAGATION_REQUIRED = 0;

/**
* Support a current transaction; execute non-transactionally if none exists.
* 支持当前事务，如果当前没有事务，就以非事务方式执行
*/
int PROPAGATION_SUPPORTS = 1;

/**
* Support a current transaction; throw an exception if no current transaction
* exists.
* 支持当前事务，如果当前没有事务，就抛出异常   MANDATORY(强制的)
*/
int PROPAGATION_MANDATORY = 2;

/**
* Create a new transaction, suspending the current transaction if one exists.
* 新建事务，如果当前存在事务，把当前事务挂起
* 一旦内层事务进行了提交后，外层事务不能对其进行回滚
* 两个事务互不影响
*/
int PROPAGATION_REQUIRES_NEW = 3;

/**
* Do not support a current transaction; rather always execute non-transactionally.
* 以非事务方式执行操作，如果当前存在事务，就把当前事务挂起
*/
int PROPAGATION_NOT_SUPPORTED = 4;

/**
* Do not support a current transaction; throw an exception if a current transaction
* exists.
* 以非事务方式执行，如果当前存在事务，则抛出异常。
*/
int PROPAGATION_NEVER = 5;

/**
* Execute within a nested transaction if a current transaction exists,
* 支持当前事务，新增Savepoint点，与当前事务同步提交或回滚。
* 嵌套事务一个非常重要的概念就是内层事务依赖于外层事务。
* 外层事务失败时，会回滚内层事务所做的动作，而内层事务操作失败并不会引起外层事务的回滚。
*/
int PROPAGATION_NESTED = 6;

//事务的隔离级别

/**
* 使用数据库默认的事务隔离级别.另外四个与JDBC的隔离级别相对应
*/
int ISOLATION_DEFAULT = -1;

/**
*	读未提交 (脏读)这是事务最低的隔离级别，
它允许另外一个事务可以看到这个事务未提交的数据。
这种隔离级别会产生脏读，不可重复读和幻像读。
*/
int ISOLATION_READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;

/**
*保证一个事务修改的数据提交后才能被另外一个事务读取。
另外一个事务不能读取该事务未提交的数据。
这种事务隔离级别可以避免脏读出现，但是可能会出现不可重复读和幻像读。
*/
int ISOLATION_READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED;

/**
这种事务隔离级别可以防止脏读，不可重复读。
但是可能出现幻像读。
它除了保证一个事务不能读取另一个事务未提交的数据外，还保证了不可重复读
*/
int ISOLATION_REPEATABLE_READ = Connection.TRANSACTION_REPEATABLE_READ;

/**
这是花费最高代价但是最可靠的事务隔离级别。
事务被处理为顺序执行。除了防止脏读，不可重复读外，还避免了幻像读。
*/
int ISOLATION_SERIALIZABLE = Connection.TRANSACTION_SERIALIZABLE;

/**
* Use the default timeout of the underlying transaction system,
* or none if timeouts are not supported.
*/
int TIMEOUT_DEFAULT = -1;

/**
* Return the propagation behavior.
* <p>Must return one of the <code>PROPAGATION_XXX</code> constants
* defined on {@link TransactionDefinition this interface}.
* @return the propagation behavior
* @see #PROPAGATION_REQUIRED
* @see org.springframework.transaction.support.TransactionSynchronizationManager#isActualTransactionActive()
*/
int getPropagationBehavior();

/**
* Return the isolation level.
* <p>Must return one of the <code>ISOLATION_XXX</code> constants
* defined on {@link TransactionDefinition this interface}.
* <p>Only makes sense in combination with {@link #PROPAGATION_REQUIRED}
* or {@link #PROPAGATION_REQUIRES_NEW}.
* <p>Note that a transaction manager that does not support custom isolation levels
* will throw an exception when given any other level than {@link #ISOLATION_DEFAULT}.
* @return the isolation level
*/
int getIsolationLevel();

/**
* Return the transaction timeout.
* <p>Must return a number of seconds, or {@link #TIMEOUT_DEFAULT}.
* <p>Only makes sense in combination with {@link #PROPAGATION_REQUIRED}
* or {@link #PROPAGATION_REQUIRES_NEW}.
* <p>Note that a transaction manager that does not support timeouts will throw
* an exception when given any other timeout than {@link #TIMEOUT_DEFAULT}.
* @return the transaction timeout
*/
int getTimeout();

/**
* Return whether to optimize as a read-only transaction.
* <p>The read-only flag applies to any transaction context, whether
* backed by an actual resource transaction
* ({@link #PROPAGATION_REQUIRED}/{@link #PROPAGATION_REQUIRES_NEW}) or
* operating non-transactionally at the resource level
* ({@link #PROPAGATION_SUPPORTS}). In the latter case, the flag will
* only apply to managed resources within the application, such as a
* Hibernate <code>Session</code>.
<<	 * <p>This just serves as a hint for the actual transaction subsystem;
* it will <i>not necessarily</i> cause failure of write access attempts.
* A transaction manager which cannot interpret the read-only hint will
* <i>not</i> throw an exception when asked for a read-only transaction.
* @return <code>true</code> if the transaction is to be optimized as read-only
* @see org.springframework.transaction.support.TransactionSynchronization#beforeCommit(boolean)
* @see org.springframework.transaction.support.TransactionSynchronizationManager#isCurrentTransactionReadOnly()
*/
boolean isReadOnly();

/**
* Return the name of this transaction. Can be <code>null</code>.
* <p>This will be used as the transaction name to be shown in a
* transaction monitor, if applicable (for example, WebLogic's).
* <p>In case of Spring's declarative transactions, the exposed name will be
* the <code>fully-qualified class name + "." + method name</code> (by default).
* @return the name of this transaction
* @see org.springframework.transaction.interceptor.TransactionAspectSupport
* @see org.springframework.transaction.support.TransactionSynchronizationManager#getCurrentTransactionName()
*/
String getName();

}