Transaction .OMG.ugly _Web前端

Transaction ....OMG...ugly

一、什么是Java事务

??? 通常的观念认为，事务仅与数据库相关。

??? 事务必须服从ISO/IEC所制定的ACID原则。ACID是原子性（atomicity）、一致性（consistency）、隔离性（isolation）和持久性（durability）的缩写。事务的原子性表示事务执行过程中的任何失败都将导致事务所做的任何修改失效。一致性表示当事务执行失败时，所有被该事务影响的数据都应该恢复到事务执行前的状态。隔离性表示在事务执行过程中对数据的修改，在事务提交之前对其他事务不可见。持久性表示已提交的数据在事务执行失败时，数据的状态都应该正确。

??? 通俗的理解，事务是一组原子操作单元，从数据库角度说，就是一组SQL指令，要么全部执行成功，若因为某个原因其中一条指令执行有错误，则撤销先前执行过的所有指令。更简答的说就是：要么全部执行成功，要么撤销不执行。

??? 既然事务的概念从数据库而来，那Java事务是什么？之间有什么联系？

??? 实际上，一个Java应用系统，如果要操作数据库，则通过JDBC来实现的。增加、修改、删除都是通过相应方法间接来实现的，事务的控制也相应转移到Java程序代码中。因此，数据库操作的事务习惯上就称为Java事务。

????二、为什么需要事务

??? 事务是为解决数据安全操作提出的，事务控制实际上就是控制数据的安全访问。具一个简单例子：比如银行转帐业务，账户A要将自己账户上的1000元转到B账户下面，A账户余额首先要减去1000元，然后B账户要增加1000元。假如在中间网络出现了问题，A账户减去1000元已经结束，B因为网络中断而操作失败，那么整个业务失败，必须做出控制，要求A账户转帐业务撤销。这才能保证业务的正确性，完成这个操走就需要事务，将A账户资金减少和B账户资金增加方到一个事务里面，要么全部执行成功，要么操作全部撤销，这样就保持了数据的安全性。

????三、Java事务的类型

??? Java事务的类型有三种：JDBC事务、JTA(Java Transaction API)事务、容器事务。

??? 1、JDBC事务

??? JDBC 事务是用 Connection 对象控制的。JDBC Connection 接口( java.sql.Connection )提供了两种事务模式：自动提交和手工提交。 java.sql.Connection 提供了以下控制事务的方法：

??? public void setAutoCommit(boolean)

??? public boolean getAutoCommit()

??? public void commit()

??? public void rollback()

??? 使用 JDBC 事务界定时，您可以将多个 SQL 语句结合到一个事务中。JDBC 事务的一个缺点是事务的范围局限于一个数据库连接。一个 JDBC 事务不能跨越多个数据库。......

http://news.newhua.com/news1/program_net/2009/1010/091010153538CIKK498BFF2JIF0C46361I3407JDE99FEJKFJA914B03F.html?lt=common

11.5.7?Transaction propagation

This section describes some semantics of transaction propagation in Spring. Please note that this section is not an introduction to transaction propagation proper; rather it details some of the semantics regarding transaction propagation in Spring.

In Spring-managed transactions, be aware of the difference between physical and logical transactions, and how the propagation setting applies to this difference.

11.5.7.1?Required

11.5.7?Transaction propagation

In Spring-managed transactions, be aware of the difference between physical and logical transactions, and how the propagation setting applies to this difference.

11.5.7.1?Required

PROPAGATION_REQUIRED

When the propagation setting is PROPAGATION_REQUIRED , a logical transaction scope is created for each method upon which the setting is applied. Each such logical transaction scope can determine rollback-only status individually, with an outer transaction scope being logically independent from the inner transaction scope. Of course, in case of standard PROPAGATION_REQUIRED behavior, all these scopes will be mapped to the same physical transaction. So a rollback-only marker set in the inner transaction scope does affect the outer transaction's chance to actually commit (as you would expect it to).

However, in the case where an inner transaction scope sets the rollback-only marker, the outer transaction has not decided on the rollback itself, and so the rollback (silently triggered by the inner transaction scope) is unexpected. A corresponding UnexpectedRollbackException is thrown at that point. This is expected behavior so that the caller of a transaction can never be misled to assume that a commit was performed when it really was not. So if an inner transaction (of which the outer caller is not aware) silently marks a transaction as rollback-only, the outer caller still calls commit. The outer caller needs to receive an UnexpectedRollbackException to indicate clearly that a rollback was performed instead.

PROPAGATION_REQUIRED

read dirty data : tx1 read tx2's update but not commited data
can not re-readable : tx1 can not get the same return data from one field (tx1 can read the updated and commited data from tx2 )
imaginary read : tx1 read the data from which tx2 inserted and commited

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if i use try-catch?surround the below code snippet? the spring container can not do the controll of transaction , if not spring container can do it for us. just throw !!!

@Transactional(rollbackFor=Exception.class)
?public void delPerson(Integer id) throws Exception {

try{
???String sql = "delete from person where id = ?";
???jdbcTemplate.update(sql, new Object[] {id}, new int []{java.sql.Types.INTEGER});
???throw new Exception ( "Exception");

}catch(){}
?}

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Spring 事务详解
http://developer.51cto.com/art/200906/129854.htm