Spring 事务模板方法设计模式

接上一篇文章

上一篇讲到了doGetTransaction方法

一、模板方法设计模式

这里涉及到了一个经典的设计模式：模板方法

如下图：

AbstractPlatformTransactionManager实现了PlatformTranscationManager接口

DatasourceTransactionManager,HibernateTransactionManager等继承了AbstractPlatformTransactionManager类

AbstractPlatformTransactionManager类中定义了事务处理的若干方法。 其中获得事务，提交事务，回滚事务由具体的子类实现。

二、进入doGetTransaction的其中一个实现类Hibernate3中的HibernateTransactionManager 类

进入doGetTransaction方法

	@Override
	protected Object doGetTransaction() {
		HibernateTransactionObject txObject = new HibernateTransactionObject();
		txObject.setSavepointAllowed(isNestedTransactionAllowed());

		SessionHolder sessionHolder =
				(SessionHolder) TransactionSynchronizationManager.getResource(getSessionFactory());
		if (sessionHolder != null) {
			if (logger.isDebugEnabled()) {
				logger.debug("Found thread-bound Session [" +
						SessionFactoryUtils.toString(sessionHolder.getSession()) + "] for Hibernate transaction");
			}
			txObject.setSessionHolder(sessionHolder);
		}
		else if (this.hibernateManagedSession) {
			try {
				Session session = getSessionFactory().getCurrentSession();
				if (logger.isDebugEnabled()) {
					logger.debug("Found Hibernate-managed Session [" +
							SessionFactoryUtils.toString(session) + "] for Spring-managed transaction");
				}
				txObject.setExistingSession(session);
			}
			catch (HibernateException ex) {
				throw new DataAccessResourceFailureException(
						"Could not obtain Hibernate-managed Session for Spring-managed transaction", ex);
			}
		}

		if (getDataSource() != null) {
			ConnectionHolder conHolder = (ConnectionHolder)
					TransactionSynchronizationManager.getResource(getDataSource());
			txObject.setConnectionHolder(conHolder);
		}

		return txObject;
	}

　　

1、创建HibernateTransactionObject对象

HibernateTransactionObject txObject = new HibernateTransactionObject();

HibernateTransactionObject 是一个Hibernate事务对象，代表一个SessionHolder。被HibernateTransactionManger用作事务对象。

2、进入SessionHolder

SessionHolder： session持有者，包装了一个Hibernate Session和一个Hibernate事务

三 、进入doGetTransaction的其中一个实现类JDBC中的DataSourceTransactionManager 类

进入doGetTransaction方法

	@Override
	protected Object doGetTransaction() {
		DataSourceTransactionObject txObject = new DataSourceTransactionObject();
		txObject.setSavepointAllowed(isNestedTransactionAllowed());
		ConnectionHolder conHolder =
		    (ConnectionHolder) TransactionSynchronizationManager.getResource(this.dataSource);
		txObject.setConnectionHolder(conHolder, false);
		return txObject;
	}

　　

1、创建DataSourceTransactionObject实例

DataSourceTransactionObject txObject = new DataSourceTransactionObject();
进入DataSourceTransactionObject

DataSourceTransactionObject也是继承自JdbcTransactionObjectSupport

三、回到AbstractPlatformTransactionManager类的getTransaction方法
进入如下代码

如果已经存在事务，则检查事务的传播行为

2、进入handleExistingTransaction方法

/**
	 * Create a TransactionStatus for an existing transaction.
	 */
	private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {

		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
			throw new IllegalTransactionStateException(
					"Existing transaction found for transaction marked with propagation 'never'");
		}

		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);
		}

		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;
			}
		}

		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);
	}

　　其中的doBegin(transaction, definition);方法，是开启事务

进入最后一行的prepareTransactionStatus方法

3、返回AbstractPlatformTransactionManager类的getTransaction方法，进入如下代码

4、进入Hibernate的doBegin开启事务方法实现

protected void doBegin(Object transaction, TransactionDefinition definition) {
		HibernateTransactionObject txObject = (HibernateTransactionObject) transaction;

		if (txObject.hasConnectionHolder() && !txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
			throw new IllegalTransactionStateException(
					"Pre-bound JDBC Connection found! HibernateTransactionManager does not support " +
					"running within DataSourceTransactionManager if told to manage the DataSource itself. " +
					"It is recommended to use a single HibernateTransactionManager for all transactions " +
					"on a single DataSource, no matter whether Hibernate or JDBC access.");
		}

		Session session = null;

		try {
			if (txObject.getSessionHolder() == null || txObject.getSessionHolder().isSynchronizedWithTransaction()) {
				Interceptor entityInterceptor = getEntityInterceptor();
				Session newSession = (entityInterceptor != null ?
						getSessionFactory().openSession(entityInterceptor) : getSessionFactory().openSession());
				if (logger.isDebugEnabled()) {
					logger.debug("Opened new Session [" + SessionFactoryUtils.toString(newSession) +
							"] for Hibernate transaction");
				}
				txObject.setSession(newSession);
			}

			session = txObject.getSessionHolder().getSession();

			if (this.prepareConnection && isSameConnectionForEntireSession(session)) {
				// We're allowed to change the transaction settings of the JDBC Connection.
				if (logger.isDebugEnabled()) {
					logger.debug(
							"Preparing JDBC Connection of Hibernate Session [" + SessionFactoryUtils.toString(session) + "]");
				}
				Connection con = session.connection();
				Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
				txObject.setPreviousIsolationLevel(previousIsolationLevel);
			}
			else {
				// Not allowed to change the transaction settings of the JDBC Connection.
				if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
					// We should set a specific isolation level but are not allowed to...
					throw new InvalidIsolationLevelException(
							"HibernateTransactionManager is not allowed to support custom isolation levels: " +
							"make sure that its 'prepareConnection' flag is on (the default) and that the " +
							"Hibernate connection release mode is set to 'on_close' (SpringTransactionFactory's default). " +
							"Make sure that your LocalSessionFactoryBean actually uses SpringTransactionFactory: Your " +
							"Hibernate properties should *not* include a 'hibernate.transaction.factory_class' property!");
				}
				if (logger.isDebugEnabled()) {
					logger.debug(
							"Not preparing JDBC Connection of Hibernate Session [" + SessionFactoryUtils.toString(session) + "]");
				}
			}

			if (definition.isReadOnly() && txObject.isNewSession()) {
				// Just set to NEVER in case of a new Session for this transaction.
				session.setFlushMode(FlushMode.MANUAL);
			}

			if (!definition.isReadOnly() && !txObject.isNewSession()) {
				// We need AUTO or COMMIT for a non-read-only transaction.
				FlushMode flushMode = session.getFlushMode();
				if (flushMode.lessThan(FlushMode.COMMIT)) {
					session.setFlushMode(FlushMode.AUTO);
					txObject.getSessionHolder().setPreviousFlushMode(flushMode);
				}
			}

			Transaction hibTx;

			// Register transaction timeout.
			int timeout = determineTimeout(definition);
			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
				// Use Hibernate's own transaction timeout mechanism on Hibernate 3.1+
				// Applies to all statements, also to inserts, updates and deletes!
				hibTx = session.getTransaction();
				hibTx.setTimeout(timeout);
				hibTx.begin();
			}
			else {
				// Open a plain Hibernate transaction without specified timeout.
				hibTx = session.beginTransaction();
			}

			// Add the Hibernate transaction to the session holder.
			txObject.getSessionHolder().setTransaction(hibTx);

			// Register the Hibernate Session's JDBC Connection for the DataSource, if set.
			if (getDataSource() != null) {
				Connection con = session.connection();
				ConnectionHolder conHolder = new ConnectionHolder(con);
				if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
					conHolder.setTimeoutInSeconds(timeout);
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Exposing Hibernate transaction as JDBC transaction [" + con + "]");
				}
				TransactionSynchronizationManager.bindResource(getDataSource(), conHolder);
				txObject.setConnectionHolder(conHolder);
			}

			// Bind the session holder to the thread.
			if (txObject.isNewSessionHolder()) {
				TransactionSynchronizationManager.bindResource(getSessionFactory(), txObject.getSessionHolder());
			}
			txObject.getSessionHolder().setSynchronizedWithTransaction(true);
		}

		catch (Exception ex) {
			if (txObject.isNewSession()) {
				try {
					if (session.getTransaction().isActive()) {
						session.getTransaction().rollback();
					}
				}
				catch (Throwable ex2) {
					logger.debug("Could not rollback Session after failed transaction begin", ex);
				}
				finally {
					SessionFactoryUtils.closeSession(session);
				}
			}
			throw new CannotCreateTransactionException("Could not open Hibernate Session for transaction", ex);
		}
	}

　　关键代码

5、进入JDBC的doBegin方法实现方式（DataSourceTransactionManager类中）

@Override
	protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			if (txObject.getConnectionHolder() == null ||
					txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
				Connection newCon = this.dataSource.getConnection();
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
			}

			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
			con = txObject.getConnectionHolder().getConnection();

			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			txObject.setPreviousIsolationLevel(previousIsolationLevel);

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
				con.setAutoCommit(false);
			}
			txObject.getConnectionHolder().setTransactionActive(true);

			int timeout = determineTimeout(definition);
			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
				txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
			}

			// Bind the session holder to the thread.
			if (txObject.isNewConnectionHolder()) {
				TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
			}
		}

		catch (Exception ex) {
			DataSourceUtils.releaseConnection(con, this.dataSource);
			throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
		}
	}

　　

开启事务： con.setAutoCommit(false);，关闭自动提交，换句话说就是手工提交，开启了事务。

6、返回TransactionAspectSupport类createTransactionIfNecessary方法

进入prepareTransactionInfo方法

7、定位到txInfo.bindToThread();

进入bindToThread方法

公开当前的TransactionStatus，当前事务完成之后，恢复之前的事务。 我们可以理解成，如果存在嵌套事务，将外层事务保存在oldTransactionInfo里面，里面的事务执行完成后再获取外层的。实际上就是将外层的事务挂起。

8、回到TransactionInterceptor类的invoke方法

commitTransactionAfterReturning 最终提交事务。

进入commitTransactionAfterReturning 方法

进入txInfo.getTransactionManager().commit(txInfo.getTransactionStatus()); 方法的实现

	/**
	 * This implementation of commit handles participating in existing
	 * transactions and programmatic rollback requests.
	 * Delegates to <code>isRollbackOnly</code>, <code>doCommit</code>
	 * and <code>rollback</code>.
	 * @see org.springframework.transaction.TransactionStatus#isRollbackOnly()
	 * @see #doCommit
	 * @see #rollback
	 */
	public final void commit(TransactionStatus status) throws TransactionException {
		if (status.isCompleted()) {
			throw new IllegalTransactionStateException(
					"Transaction is already completed - do not call commit or rollback more than once per transaction");
		}

		DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
		if (defStatus.isLocalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Transactional code has requested rollback");
			}
			processRollback(defStatus);
			return;
		}
		if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
			}
			processRollback(defStatus);
			// Throw UnexpectedRollbackException only at outermost transaction boundary
			// or if explicitly asked to.
			if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
				throw new UnexpectedRollbackException(
						"Transaction rolled back because it has been marked as rollback-only");
			}
			return;
		}

		processCommit(defStatus);
	}

　　

进入最后一行processCommit(defStatus);

	/**
	 * Process an actual commit.
	 * Rollback-only flags have already been checked and applied.
	 * @param status object representing the transaction
	 * @throws TransactionException in case of commit failure
	 */
	private void processCommit(DefaultTransactionStatus status) throws TransactionException {
		try {
			boolean beforeCompletionInvoked = false;
			try {
				prepareForCommit(status);
				triggerBeforeCommit(status);
				triggerBeforeCompletion(status);
				beforeCompletionInvoked = true;
				boolean globalRollbackOnly = false;
				if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
					globalRollbackOnly = status.isGlobalRollbackOnly();
				}
				if (status.hasSavepoint()) {
					if (status.isDebug()) {
						logger.debug("Releasing transaction savepoint");
					}
					status.releaseHeldSavepoint();
				}
				else if (status.isNewTransaction()) {
					if (status.isDebug()) {
						logger.debug("Initiating transaction commit");
					}
					doCommit(status);
				}
				// Throw UnexpectedRollbackException if we have a global rollback-only
				// marker but still didn't get a corresponding exception from commit.
				if (globalRollbackOnly) {
					throw new UnexpectedRollbackException(
							"Transaction silently rolled back because it has been marked as rollback-only");
				}
			}
			catch (UnexpectedRollbackException ex) {
				// can only be caused by doCommit
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
				throw ex;
			}
			catch (TransactionException ex) {
				// can only be caused by doCommit
				if (isRollbackOnCommitFailure()) {
					doRollbackOnCommitException(status, ex);
				}
				else {
					triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
				}
				throw ex;
			}
			catch (RuntimeException ex) {
				if (!beforeCompletionInvoked) {
					triggerBeforeCompletion(status);
				}
				doRollbackOnCommitException(status, ex);
				throw ex;
			}
			catch (Error err) {
				if (!beforeCompletionInvoked) {
					triggerBeforeCompletion(status);
				}
				doRollbackOnCommitException(status, err);
				throw err;
			}

			// Trigger afterCommit callbacks, with an exception thrown there
			// propagated to callers but the transaction still considered as committed.
			try {
				triggerAfterCommit(status);
			}
			finally {
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
			}

		}
		finally {
			cleanupAfterCompletion(status);
		}
	}

　　真正提交事务的方法

doCommit是一个抽象类，由具体的子类实现

9、进入Hibernate的doCommit实现

10.进入JDBC实现的doCommit实现

11、回到TransactionInterceptor类的invoke方法，定位到抛出异常代码completeTransactionAfterThrowing(txInfo, ex);

进入completeTransactionAfterThrowing(txInfo, ex);方法

/**
	 * Handle a throwable, completing the transaction.
	 * We may commit or roll back, depending on the configuration.
	 * @param txInfo information about the current transaction
	 * @param ex throwable encountered
	 */
	protected void completeTransactionAfterThrowing(TransactionInfo txInfo, Throwable ex) {
		if (txInfo != null && txInfo.hasTransaction()) {
			if (logger.isTraceEnabled()) {
				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
						"] after exception: " + ex);
			}
			if (txInfo.transactionAttribute.rollbackOn(ex)) {
				try {
					txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
				}
				catch (TransactionSystemException ex2) {
					logger.error("Application exception overridden by rollback exception", ex);
					ex2.initApplicationException(ex);
					throw ex2;
				}
				catch (RuntimeException ex2) {
					logger.error("Application exception overridden by rollback exception", ex);
					throw ex2;
				}
				catch (Error err) {
					logger.error("Application exception overridden by rollback error", ex);
					throw err;
				}
			}
			else {
				// We don't roll back on this exception.
				// Will still roll back if TransactionStatus.isRollbackOnly() is true.
				try {
					txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
				}
				catch (TransactionSystemException ex2) {
					logger.error("Application exception overridden by commit exception", ex);
					ex2.initApplicationException(ex);
					throw ex2;
				}
				catch (RuntimeException ex2) {
					logger.error("Application exception overridden by commit exception", ex);
					throw ex2;
				}
				catch (Error err) {
					logger.error("Application exception overridden by commit error", ex);
					throw err;
				}
			}
		}
	}

　　根据当前事务的配置，来确定是提交还是回滚。

12、回到TransactionInterceptor类的invoke方法，定位到cleanupTransactionInfo(txInfo);方法

进入cleanupTransactionInfo方法

重置TransactionInfo ThreadLocal

	/**
	 * Reset the TransactionInfo ThreadLocal.
	 * <p>Call this in all cases: exception or normal return!
	 * @param txInfo information about the current transaction (may be <code>null</code>)
	 */
	protected void cleanupTransactionInfo(TransactionInfo txInfo) {
		if (txInfo != null) {
			txInfo.restoreThreadLocalStatus();
		}
	}

　　

进入restoreThreadLocalStatus方法

相当于里层的事务执行完了，恢复外层的事务

transactionInfoHolder是一个ThreadLocal类型的变量。

总结，Spring事务用到的几个类

TransactionDefinition 事务的定义，下面有一个重要的子接口TransactionAttribute，主要描述事务的配置信息

TransactionInfo 是一个内部类，里面包含了TransactionStatus和自身TransactionInfo （用于嵌套事务，供恢复使用）

TransactionStatus： 当前事务运行期的一些状态。

PlatformTransactionManager： 平台事务管理器

　　AbstractPlatformTransactionManager 抽象平台事务管理器的实现类，里面用到了大量的模板方法（模板方法设计模式）

　　AbstractPlatformTransactionManager  抽象平台事务管理器下面，就是各个平台相关的事务管理器。如下图：