事务Transaction的理解(一)

目录

什么是事务？

局部事务和全局事务

可编程事务管理和Java注解型事务管理

Spring抽象事务层

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什么是事务？

数据库事务就是将数据库的一系列操作作为一个工作单元。这一系列的操作要么一起生效要么就全都不起作用。事务管理是数据库中非常重要的组成部分，它能确保数据的一致性和完整性。

事务具有如下几个特点：

• 原子性
• 一致性
• 隔离性
• 牢固性

一个关系型数据库的事务管理必须满足上边的四个特性，拿一个简单的SQL例子来展示一下，

• 用 begin transaction 来开始事务；

• 用SQL执行deleted, update or insert操作；

• 如果所有的这些操作执行成功，那就执行commit 否则的话 rollback所有的操作。

Spring或者Spring Boot在各种各样的事务Transaction管理API上封装了一层抽象层。Spring Boot 提供了两种事务管理方式：可编程型和Java注解型事务管理， 在下一系列来分别阐述其使用详解。

局部事务和全局事务

局部事务就是指单一资源的事务管理，比方说一个JDBC连接。而全局事务扩展到多个事务资源，比如分布式事务。

如果应用程序和数据库部署在单一位置上（application，resource可不在同一机器上），事务也仅仅是在一个机器上进行的数据管理，在这种情况下，局部事务是非常有效的。

而在分布式环境里， 所有的资源会部署到多个系统里，在这种情况下，事务管理要在局部层和全局层都要生效。一个分布式事务或者全局事务运行在多个系统里，这就要求全局事务管理和局部的数据管理必须协调一致地运行下去。

可编程事务管理和Java注解型事务管理

Spring和Spring Boot 支持两种类型的事务管理：

• 可编程事务管理：可以通过自己编程来实现事务管理，在一定程度上带来复杂性，相对而言较难维护。
• 注解型事务管理：可以将事务管理与业务逻辑相分离，可以通过注解或者XML配置来实现。

关于这一节内容，会在Transaction理解（二）和（三）里分别阐述。

Spring抽象事务层

Spring抽象事务层的关键部分是org.springframework.transaction.PlatformTransactionManager 接口

public interface PlatformTransactionManager {

	TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException;

	void commit(TransactionStatus status) throws TransactionException;

	void rollback(TransactionStatus status) throws TransactionException;

}

PlatformTransactionManager

Method	Description
getTransaction	Return a currently active transaction or create a new one, according to the specified propagation behavior.
commit	Commit the given transaction, with regard to its status. If the transaction has been marked rollback-only programmatically, perform a rollback.
rollback	Perform a rollback of the given transaction.

 

 

 

 

 

 

 

另外一个核心组成部分是TransactionDefinition接口

public interface TransactionDefinition {

	int PROPAGATION_REQUIRED = 0;

	int PROPAGATION_SUPPORTS = 1;

	int PROPAGATION_MANDATORY = 2;

	int PROPAGATION_REQUIRES_NEW = 3;

	int PROPAGATION_NOT_SUPPORTED = 4;

	int PROPAGATION_NEVER = 5;

	int PROPAGATION_NESTED = 6;

	int ISOLATION_DEFAULT = -1;

	int ISOLATION_READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;

	int ISOLATION_READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED;

	int ISOLATION_REPEATABLE_READ = Connection.TRANSACTION_REPEATABLE_READ;

	int ISOLATION_SERIALIZABLE = Connection.TRANSACTION_SERIALIZABLE;

	int TIMEOUT_DEFAULT = -1;

	int getPropagationBehavior();

	int getIsolationLevel();

	int getTimeout();

	boolean isReadOnly();

	String getName();

}

Method in TransactionDefinition

Method	Description
getPropagationBehavior	Return the propagation behavior. Must return one of the PROPAGATION_XXX constants defined on this interface.
getIsolationLevel	Return the isolation level.
getTimeout	Return the transaction timeout.
isReadOnly	Return whether to optimize as a read-only transaction.
getName	Return the name of this transaction. Can be null.

 

 

 

 

 

 

 

 

 

接下来列举一下如何事务的隔离性

Isolation

Isolation	Description
ISOLATION_DEFAULT	Use the default isolation level of the underlying datastore. All other levels correspond to the JDBC isolation levels.
ISOLATION_READ_UNCOMMITTED	Indicates that dirty reads, non-repeatable reads and phantom reads can occur
ISOLATION_READ_COMMITTED	Indicates that dirty reads are prevented; non-repeatable reads and phantom reads can occur.
ISOLATION_REPEATABLE_READ	Indicates that dirty reads and non-repeatable reads are prevented; phantom reads can occur.
ISOLATION_SERIALIZABLE	Indicates that dirty reads, non-repeatable reads and phantom reads are prevented.

 

 

 

 

 

 

 

 

 

 

接下来还有事务之间的关系

Propagation

Propagation	Description
PROPAGATION_REQUIRED	Support a current transaction; create a new one if none exists. Analogous to the EJB transaction attribute of the same name.
PROPAGATION_SUPPORTS	Support a current transaction; execute non-transactionally if none exists. Analogous to the EJB transaction attribute of the same name.
PROPAGATION_MANDATORY	Support a current transaction; throw an exception if no current transaction exists. Analogous to the EJB transaction attribute of the same name.
PROPAGATION_REQUIRES_NEW	Create a new transaction, suspending the current transaction if one exists. Analogous to the EJB transaction attribute of the same name.
PROPAGATION_NOT_SUPPORTED	Do not support a current transaction; rather always execute non-transactionally. Analogous to the EJB transaction attribute of the same name.
PROPAGATION_NEVER	Do not support a current transaction; throw an exception if a current transaction exists. Analogous to the EJB transaction attribute of the same name.
PROPAGATION_NESTED	Execute within a nested transaction if a current transaction exists, behave like PROPAGATION_REQUIRED else. There is no analogous feature in EJB.
TIMEOUT_DEFAULT	Use the default timeout of the underlying transaction system, or none if timeouts are not supported.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TransactionStatus 接口提供了一系列管理事务和获取事务状态的方式方法。

public interface TransactionStatus extends SavepointManager, Flushable {

	/**
	 * Return whether the present transaction is new (else participating
	 * in an existing transaction, or potentially not running in an
	 * actual transaction in the first place).
	 */
	boolean isNewTransaction();

	/**
	 * Return whether this transaction internally carries a savepoint,
	 * that is, has been created as nested transaction based on a savepoint.
	 * 
This method is mainly here for diagnostic purposes, alongside
	 * {@link #isNewTransaction()}. For programmatic handling of custom
	 * savepoints, use SavepointManager's operations.
	 * @see #isNewTransaction()
	 * @see #createSavepoint
	 * @see #rollbackToSavepoint(Object)
	 * @see #releaseSavepoint(Object)
	 */
	boolean hasSavepoint();

	/**
	 * Set the transaction rollback-only. This instructs the transaction manager
	 * that the only possible outcome of the transaction may be a rollback, as
	 * alternative to throwing an exception which would in turn trigger a rollback.
	 * 
This is mainly intended for transactions managed by
	 * {@link org.springframework.transaction.support.TransactionTemplate} or
	 * {@link org.springframework.transaction.interceptor.TransactionInterceptor},
	 * where the actual commit/rollback decision is made by the container.
	 * @see org.springframework.transaction.support.TransactionCallback#doInTransaction
	 * @see org.springframework.transaction.interceptor.TransactionAttribute#rollbackOn
	 */
	void setRollbackOnly();

	/**
	 * Return whether the transaction has been marked as rollback-only
	 * (either by the application or by the transaction infrastructure).
	 */
	boolean isRollbackOnly();

	/**
	 * Flush the underlying session to the datastore, if applicable:
	 * for example, all affected Hibernate/JPA sessions.
	 * 
This is effectively just a hint and may be a no-op if the underlying
	 * transaction manager does not have a flush concept. A flush signal may
	 * get applied to the primary resource or to transaction synchronizations,
	 * depending on the underlying resource.
	 */
	@Override
	void flush();

	/**
	 * Return whether this transaction is completed, that is,
	 * whether it has already been committed or rolled back.
	 * @see PlatformTransactionManager#commit
	 * @see PlatformTransactionManager#rollback
	 */
	boolean isCompleted();

}