3、Spring建立事务

接上节事务执行流程，这是第3点的解析。
建立事务主要两部分：

1. 获取事务状态
2. 构建事务信息

获取事务状态
代码以下：

@Override
    public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
    //1.获取事务
    Object transaction = doGetTransaction();

    // Cache debug flag to avoid repeated checks.
    boolean debugEnabled = logger.isDebugEnabled();

    if (definition == null) {
        // Use defaults if no transaction definition given.
        definition = new DefaultTransactionDefinition();
    }

    //判断当前线程是否存在事务，判断依据为当前线程记录链接不为空且链接中的(connectionHolder)中的transactionActive属性不为空
    if (isExistingTransaction(transaction)) {
        // Existing transaction found -> check propagation behavior to find out how to behave.
        return handleExistingTransaction(definition, transaction, debugEnabled);
    }

    // Check definition settings for new transaction.
    //事务超时设置验证
    if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
        throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
    }

    // No existing transaction found -> check propagation behavior to find out how to proceed.
    //若是当前线程不存在事务，可是@Transactional却声明事务为PROPAGATION_MANDATORY抛出异常
    if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
        throw new IllegalTransactionStateException(
                "No existing transaction found for transaction marked with propagation 'mandatory'");
    }
    //若是当前线程不存在事务，PROPAGATION_REQUIRED、PROPAGATION_REQUIRES_NEW、PROPAGATION_NESTED都得建立事务
    else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
            definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
            definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
        //空挂起
        SuspendedResourcesHolder suspendedResources = suspend(null);
        if (debugEnabled) {
            logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
        }
        try {
            //默认返回true
            boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
            //构建事务状态
            DefaultTransactionStatus status = newTransactionStatus(
                    definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
            //构造transaction、包括设置connectionHolder、隔离级别、timeout
            //若是是新事务，绑定到当前线程
            doBegin(transaction, definition);
            //新事务同步设置，针对当前线程
            prepareSynchronization(status, definition);
            return status;
        }
        catch (RuntimeException ex) {
            resume(null, suspendedResources);
            throw ex;
        }
        catch (Error err) {
            resume(null, suspendedResources);
            throw err;
        }
    }
    else {
        // Create "empty" transaction: no actual transaction, but potentially synchronization.
        if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
            logger.warn("Custom isolation level specified but no actual transaction initiated; " +
                    "isolation level will effectively be ignored: " + definition);
        }
        //声明事务是PROPAGATION_SUPPORTS
        boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
        return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
    }
}

构建事务信息

1. 获取事务，建立对应的事务实例，这里使用的是DataSourceTransactionManager中的doGetTransaction方法，建立基于JDBC的事务实例，若是当前线程中存在关于dataSoruce的链接，那么直接使用。这里有一个对保存点的设置，是否开启容许保存点取决因而否设置了容许嵌入式事务。DataSourceTransactionManager默认是开启的。
2. 若是当先线程存在事务，则转向嵌套的事务处理。是否存在事务在DataSourceTransactionManager的isExistingTransaction方法中
3. 事务超时设置验证
4. 事务PropagationBehavior属性的设置验证
5. 构建DefaultTransactionStatus。
6. 完善transaction，包括设置connectionHolder、隔离级别、timeout，若是是新事务，绑定到当前线程
7. 将事务信息记录在当前线程中