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();

}