SpringBoot (15)---事务处理

SpringBoot 中的事务处理

        前两章节主要讲解了在SpringBoot中关于对数据的操作，本章节将介绍如何进行事务处理。所有的数据访问技术都离不开事务处理，否则将会造成数据不一致。事务是一系列的动作，一旦其中有一个动作出现错误，必须全部回滚，系统将事务中对数据库的所有已完成的操作全部撤消，滚回到事务开始的状态，避免出现由于数据不一致而导致的接下来一系列的错误。事务的出现是为了确保数据的完整性和一致性，在目前企业级应用开发中，事务管理是必不可少的。

1、SpringBoot事务机制

       事务处理机制都会提供API来开启事务、提交事务来完成数据操作，或者在发生错误的时候回滚数据，避免数据的不完整性、不一致性。

       SpringBoot事务机制实质上就是Spring的事务机制，是采用统一的机制处理来自不同数据访问技术的事务处理，提供了一个接口 PlatformTransactionManager，已经为不同数据访问技术可以进行不同的实现，如下表。

数据访问技术及实现

数据访问技术	实现类
JDBC	DataSourceTransactionManager
JPA	JpaTransactionManager
Hibernate	HibernateTransactionManager
JDO	JdoTransactionManager
分布式事务	JtaTransactionManager

涉及到接口关系如下：

接口PlatformTransactionManager源码如下：

1. /*

2. * Copyright 2002-2012 the original author or authors.

3. *

4. * Licensed under the Apache License, Version 2.0 (the "License");

5. * you may not use this file except in compliance with the License.

6. * You may obtain a copy of the License at

7. *

8. * http://www.apache.org/licenses/LICENSE-2.0

9. *

10. * Unless required by applicable law or agreed to in writing, software

11. * distributed under the License is distributed on an "AS IS" BASIS,

12. * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

13. * See the License for the specific language governing permissions and

14. * limitations under the License.

15. */

16.  

17. package org.springframework.transaction;

18.  

19. import org.springframework.lang.Nullable;

20.  

21. /**

22. * This is the central interface in Spring's transaction infrastructure.

23. * Applications can use this directly, but it is not primarily meant as API:

24. * Typically, applications will work with either TransactionTemplate or

25. * declarative transaction demarcation through AOP.

26. *

27. * <p>For implementors, it is recommended to derive from the provided

28. * {@link org.springframework.transaction.support.AbstractPlatformTransactionManager}

29. * class, which pre-implements the defined propagation behavior and takes care

30. * of transaction synchronization handling. Subclasses have to implement

31. * template methods for specific states of the underlying transaction,

32. * for example: begin, suspend, resume, commit.

33. *

34. * <p>The default implementations of this strategy interface are

35. * {@link org.springframework.transaction.jta.JtaTransactionManager} and

36. * {@link org.springframework.jdbc.datasource.DataSourceTransactionManager},

37. * which can serve as an implementation guide for other transaction strategies.

38. *

39. * @author Rod Johnson

40. * @author Juergen Hoeller

41. * @since 16.05.2003

42. * @see org.springframework.transaction.support.TransactionTemplate

43. * @see org.springframework.transaction.interceptor.TransactionInterceptor

44. * @see org.springframework.transaction.interceptor.TransactionProxyFactoryBean

45. */

46. public interface PlatformTransactionManager {

47.  

48. /**

49. * Return a currently active transaction or create a new one, according to

50. * the specified propagation behavior.

51. * <p>Note that parameters like isolation level or timeout will only be applied

52. * to new transactions, and thus be ignored when participating in active ones.

53. * <p>Furthermore, not all transaction definition settings will be supported

54. * by every transaction manager: A proper transaction manager implementation

55. * should throw an exception when unsupported settings are encountered.

56. * <p>An exception to the above rule is the read-only flag, which should be

57. * ignored if no explicit read-only mode is supported. Essentially, the

58. * read-only flag is just a hint for potential optimization.

59. * @param definition TransactionDefinition instance (can be {@code null} for defaults),

60. * describing propagation behavior, isolation level, timeout etc.

61. * @return transaction status object representing the new or current transaction

62. * @throws TransactionException in case of lookup, creation, or system errors

63. * @throws IllegalTransactionStateException if the given transaction definition

64. * cannot be executed (for example, if a currently active transaction is in

65. * conflict with the specified propagation behavior)

66. * @see TransactionDefinition#getPropagationBehavior

67. * @see TransactionDefinition#getIsolationLevel

68. * @see TransactionDefinition#getTimeout

69. * @see TransactionDefinition#isReadOnly

70. */

71. TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException;

72.  

73. /**

74. * Commit the given transaction, with regard to its status. If the transaction

75. * has been marked rollback-only programmatically, perform a rollback.

76. * <p>If the transaction wasn't a new one, omit the commit for proper

77. * participation in the surrounding transaction. If a previous transaction

78. * has been suspended to be able to create a new one, resume the previous

79. * transaction after committing the new one.

80. * <p>Note that when the commit call completes, no matter if normally or

81. * throwing an exception, the transaction must be fully completed and

82. * cleaned up. No rollback call should be expected in such a case.

83. * <p>If this method throws an exception other than a TransactionException,

84. * then some before-commit error caused the commit attempt to fail. For

85. * example, an O/R Mapping tool might have tried to flush changes to the

86. * database right before commit, with the resulting DataAccessException

87. * causing the transaction to fail. The original exception will be

88. * propagated to the caller of this commit method in such a case.

89. * @param status object returned by the {@code getTransaction} method

90. * @throws UnexpectedRollbackException in case of an unexpected rollback

91. * that the transaction coordinator initiated

92. * @throws HeuristicCompletionException in case of a transaction failure

93. * caused by a heuristic decision on the side of the transaction coordinator

94. * @throws TransactionSystemException in case of commit or system errors

95. * (typically caused by fundamental resource failures)

96. * @throws IllegalTransactionStateException if the given transaction

97. * is already completed (that is, committed or rolled back)

98. * @see TransactionStatus#setRollbackOnly

99. */

100. void commit(TransactionStatus status) throws TransactionException;

101.  

102. /**

103. * Perform a rollback of the given transaction.

104. * <p>If the transaction wasn't a new one, just set it rollback-only for proper

105. * participation in the surrounding transaction. If a previous transaction

106. * has been suspended to be able to create a new one, resume the previous

107. * transaction after rolling back the new one.

108. * <p><b>Do not call rollback on a transaction if commit threw an exception.</b>

109. * The transaction will already have been completed and cleaned up when commit

110. * returns, even in case of a commit exception. Consequently, a rollback call

111. * after commit failure will lead to an IllegalTransactionStateException.

112. * @param status object returned by the {@code getTransaction} method

113. * @throws TransactionSystemException in case of rollback or system errors

114. * (typically caused by fundamental resource failures)

115. * @throws IllegalTransactionStateException if the given transaction

116. * is already completed (that is, committed or rolled back)

117. */

118. void rollback(TransactionStatus status) throws TransactionException;

119.  

120. }

2、声明式事务

         建立在AOP之上的，其本质是对方法前后进行拦截，然后在目标方法开始之前创建或者加入一个事务，在执行完目标方法之后根据执行情况提交或者回滚事务。声明式事务最大的优点就是不需要通过编程的方式管理事务，这样就不需要在业务逻辑代码中掺杂事务管理的代码，只需在配置文件中做相关的事务规则声明(或通过基于@Transactional注解的方式)，便可以将事务规则应用到业务逻辑中。

      Spring支持声明式事务，被注解的方法在被调用时，Spring开启一个新的事务，当方法无异常结束后，Spring会提交这个事务。

1. @Transactional

2. public void insertUser(User user) {

3. //数据库表的操作

4. ……

5. }

注：

（1）@Transactional是来自org.springframework.transaction.annotation包的。

（2）@Transactional不仅可以注解在方法上，也可以注解在类上。当注解在类上时，意味着此类的所有public方法都是开启事务的。如果类级别和方法级别同时使用了@Transactional注解，则使用在类级别的注解会重载方法级别的注解。

以下为注解@Transactional源码：

(为了缩小所占篇数，故去掉注释部分)

1. package org.springframework.transaction.annotation;

2.  

3. import java.lang.annotation.Documented;

4. import java.lang.annotation.ElementType;

5. import java.lang.annotation.Inherited;

6. import java.lang.annotation.Retention;

7. import java.lang.annotation.RetentionPolicy;

8. import java.lang.annotation.Target;

9.  

10. import org.springframework.core.annotation.AliasFor;

11. import org.springframework.transaction.TransactionDefinition;

12.  
13.  

14. @Target({ElementType.METHOD, ElementType.TYPE})

15. @Retention(RetentionPolicy.RUNTIME)

16. @Inherited

17. @Documented

18. public @interface Transactional {

19.  

20. @AliasFor("transactionManager")

21. String value() default "";

22.  

23. @AliasFor("value")

24. String transactionManager() default "";

25.  

26. Propagation propagation() default Propagation.REQUIRED;

27.  

28. Isolation isolation() default Isolation.DEFAULT;

29.  

30. int timeout() default TransactionDefinition.TIMEOUT_DEFAULT;

31.  

32. boolean readOnly() default false;

33.  

34. Class<? extends Throwable>[] rollbackFor() default {};

35.  

36. String[] rollbackForClassName() default {};

37.  

38. Class<? extends Throwable>[] noRollbackFor() default {};

39.  

40. String[] noRollbackForClassName() default {};

41. }

属性说明如下表：

属性	类型	描述
value	String	可选的限定描述符，指定使用的事务管理器
propagation	enum: Propagation	定义事务的生命周期，有REQUIRED、SUPPORTS、MANDATORY、REQUIRES_NEW、NOT_SUPPORTED、NEVER、NESTED，详细含义可查阅枚举类org.springframework.transaction.annotation.Propagation源码。
isolation	enum: Isolation	可选的事务隔离级别设置，决定了事务的完整性
readOnly	boolean	读写或只读事务，默认读写
timeout	int (in seconds granularity)	事务超时时间设置
rollbackFor	Class对象数组，必须继承自Throwable	导致事务回滚的异常类数组
rollbackForClassName	类名数组，必须继承自Throwable	导致事务回滚的异常类名字数组
noRollbackFor	Class对象数组，必须继承自Throwable	不会导致事务回滚的异常类数组
noRollbackForClassName	类名数组，必须继承自Throwable	不会导致事务回滚的异常类名字数组

在SpringBoot中，建议采用注解@Transactional进行事务的控制。