spring事务概念与获取事务时事务传播行为源码分析

一、事务状态：org.springframework.transaction.TransactionStatus

isNewTransaction 是否是新事务

hasSavepoint 是否有保存点

setRollbackOnly 设置回滚

isRollbackOnly 是否回滚

flush 刷新

isCompleted 是否完成

二、传播行为

1.传播行为分类

类别	简称	无事务	已有事务
PROPAGATION_REQUIRED	Required（必须-默认）	创建一个新的事务	使用该事务
PROPAGATION_SUPPORTS	Supports（支持）	以非事务执行	使用该事务
PROPAGATION_MANDATORY	Mandatory（强制）	抛异常	使用该事务
PROPAGATION_REQUIRES_NEW	requires_new（必须新的）	创建一个新的事务	原事务挂起，创建一个新的事务
PROPAGATION_NOT_SUPPORTED	not_supported（不支持）	以非事务执行	原事务挂起，以非事务执行
PROPAGATION_NEVER	Never（从不）	以非事务执行	抛异常
PROPAGATION_NESTED	Nested（嵌套）	创建一个新事务	在嵌套事务内执行

2.传播行为源码分析

（1）获取事务org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(TransactionDefinition definition)

    //---------------------------------------------------------------------
    // Implementation of PlatformTransactionManager
    //---------------------------------------------------------------------

    /**
     * This implementation handles propagation behavior. Delegates to
     * <code>doGetTransaction</code>, <code>isExistingTransaction</code>
     * and <code>doBegin</code>.
     * @see #doGetTransaction
     * @see #isExistingTransaction
     * @see #doBegin
     */
    public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
        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();
        }
　　　　　// 判断是否存在事务
        if (isExistingTransaction(transaction)) {
　　　　　　　// 如果有事务，根据传播行为处理
            // Existing transaction found -> check propagation behavior to find out how to behave.
            return handleExistingTransaction(definition, transaction, debugEnabled);
        }
　　　　　 
        // Check definition settings for new transaction.
        if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
            throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
        }
　　　　　// 下面是不存在事务情况下，根据传播行为处理
　　　　　// 如果配置的是mandatory强制，抛异常　
        // No existing transaction found -> check propagation behavior to find out how to proceed.
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
            throw new IllegalTransactionStateException(
                    "No existing transaction found for transaction marked with propagation 'mandatory'");
        }
　　　　　// 如果是required必须、requires_new必须新的和nested嵌套时，创建一个新的事物
        else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
            definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
            SuspendedResourcesHolder suspendedResources = suspend(null);
            if (debugEnabled) {
                logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
            }
            try {
                boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                DefaultTransactionStatus status = newTransactionStatus(
                        definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
                doBegin(transaction, definition);
                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.
            boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
            return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
        }
    }

（2）已有事务时处理org.springframework.transaction.support.AbstractPlatformTransactionManager.TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)

    /**
     * Create a TransactionStatus for an existing transaction.
     */
    private TransactionStatus handleExistingTransaction(
            TransactionDefinition definition, Object transaction, boolean debugEnabled)
            throws TransactionException {
　　　　　// 下面是对已有事务，根据传播行为处理
　　　　
　　　　　// 如果配置的是never从不，抛异常
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
            throw new IllegalTransactionStateException(
                    "Existing transaction found for transaction marked with propagation 'never'");
        }
　　　　　// 如果配置的是not_support不支持，原事务挂起，以非事务执行
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
            if (debugEnabled) {
                logger.debug("Suspending current transaction");
            }
            Object suspendedResources = suspend(transaction);
            boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
            return prepareTransactionStatus(
                    definition, null, false, newSynchronization, debugEnabled, suspendedResources);
        }
　　　　　// 如果配置的是requires_new，原事务挂起，创建一个新事务执行
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
            if (debugEnabled) {
                logger.debug("Suspending current transaction, creating new transaction with name [" +
                        definition.getName() + "]");
            }
            SuspendedResourcesHolder suspendedResources = suspend(transaction);
            try {
                boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                DefaultTransactionStatus status = newTransactionStatus(
                        definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
                doBegin(transaction, definition);
                prepareSynchronization(status, definition);
                return status;
            }
            catch (RuntimeException beginEx) {
                resumeAfterBeginException(transaction, suspendedResources, beginEx);
                throw beginEx;
            }
            catch (Error beginErr) {
                resumeAfterBeginException(transaction, suspendedResources, beginErr);
                throw beginErr;
            }
        }
　　　　 // 如果配置是nested嵌套，在嵌套事务中执行
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
            if (!isNestedTransactionAllowed()) {
                throw new NestedTransactionNotSupportedException(
                        "Transaction manager does not allow nested transactions by default - " +
                        "specify 'nestedTransactionAllowed' property with value 'true'");
            }
            if (debugEnabled) {
                logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
            }
            if (useSavepointForNestedTransaction()) {
                // Create savepoint within existing Spring-managed transaction,
                // through the SavepointManager API implemented by TransactionStatus.
                // Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
                DefaultTransactionStatus status =
                        prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
                status.createAndHoldSavepoint();
                return status;
            }
            else {
                // Nested transaction through nested begin and commit/rollback calls.
                // Usually only for JTA: Spring synchronization might get activated here
                // in case of a pre-existing JTA transaction.
                boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
                DefaultTransactionStatus status = newTransactionStatus(
                        definition, transaction, true, newSynchronization, debugEnabled, null);
                doBegin(transaction, definition);
                prepareSynchronization(status, definition);
                return status;
            }
        }

        // Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
        if (debugEnabled) {
            logger.debug("Participating in existing transaction");
        }
        if (isValidateExistingTransaction()) {
            if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
                Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
                if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
                    Constants isoConstants = DefaultTransactionDefinition.constants;
                    throw new IllegalTransactionStateException("Participating transaction with definition [" +
                            definition + "] specifies isolation level which is incompatible with existing transaction: " +
                            (currentIsolationLevel != null ?
                                    isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
                                    "(unknown)"));
                }
            }
            if (!definition.isReadOnly()) {
                if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
                    throw new IllegalTransactionStateException("Participating transaction with definition [" +
                            definition + "] is not marked as read-only but existing transaction is");
                }
            }
        }
        boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
        return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
    }

三、 Spring隔离级别：

　　（1）、ISOLATION_DEFAULT：使用后端数据库默认的隔离级别

　　（2）、ISOLATION_READ_UNCOMMITTED：最低的隔离级别，允许读取尚未提交的数据变更，可能会导致脏读、幻读或不可重复读

　　（3）、ISOLATION_READ_COMMITTED：允许读取并发事务已经提交的数据，可以阻止脏读，但是幻读或不可重复读仍有可能发生

　　（4）、ISOLATION_REPEATABLE_READ：对同一字段的多次读取结果都是一致的，除非数据是被本身事务自己所修改，可以阻止脏读和不可重复读，但幻读仍有可能发生

　　（5）、ISOLATION_SERIALIZABLE：最高的隔离级别，完全服从ACID的隔离级别，确保阻止脏读、不可重复读以及幻读，也是最慢的事务隔离级别，因为它通常是通过完全锁定事务相关的数据库表来实现的

部分参考：https://www.cnblogs.com/ysocean/p/7617620.html