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1.2 事务管理器TransactionManager

org.springframework.transaction.TransactionManager没有任何方法，仅作为类型标识，通常使用接口org.springframework.transaction.PlatformTransactionManager，该接口提供三个方法处理事务。事务管理器的类图如下：

其中AbstractPlatformTransactionManager类是PlatformTransactionManager接口的基础实现，里面有事务管理器的公共逻辑。在不同场景使用的事务管理器在它的基础上完善细节。

1.2.1 TransactionStatus

org.springframework.transaction.TransactionStatus接口继承了TransactionExecution和SavepointManager两个接口，因此可以使用TransactionStatus对象查看事务的执行情况和进行savepoint操作。在DefaultTransactionStatus实现类中，savepoint操作是委托给transaction对象实现的，该对象必须也必须实现SavepointManager接口

1.3 AbstractPlatformTransactionManager

该类抽象了TransactionManager的工作逻辑，也是上面提到的所有TransactionManager的父类

1.3.1 getTransaction

	@Overridepublic final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)throws TransactionException {// Use defaults if no transaction definition given.TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());Object transaction = doGetTransaction(); // 调用子类的实现获取事务对象boolean debugEnabled = logger.isDebugEnabled();// 判断当前是否已经存在事务。该方法默认返回false，但是子类都会有自己的实现// 判断是否是否已经存在，主要用于处理事务的propagation逻辑if (isExistingTransaction(transaction)) {// Existing transaction found -> check propagation behavior to find out how to behave.return handleExistingTransaction(def, transaction, debugEnabled);}// PROPAGATION_MANDATORY表示需要有事务，没有事务将报错if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {throw new IllegalTransactionStateException("No existing transaction found for transaction marked with propagation 'mandatory'");}else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {// 暂停前一个事务，通常为null，返回SuspendedResourcesHolder对象，用于恢复前一个事务SuspendedResourcesHolder suspendedResources = suspend(null);try {return startTransaction(def, transaction, debugEnabled, suspendedResources);}catch (RuntimeException | Error ex) {// 如果异常了，恢复前一个事务resume(null, suspendedResources);throw ex;}}else { // 用户自定义的传播级别// Create "empty" transaction: no actual transaction, but potentially synchronization.if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {logger.warn("Custom isolation level specified but no actual transaction initiated; " +"isolation level will effectively be ignored: " + def);}boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);}}

开始一个事务

/*** Start a new transaction.*/
private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {// 默认为trueboolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);// 获取status对象DefaultTransactionStatus status = newTransactionStatus(definition, transaction, true