Spring源码剖析-Transactional 事务执行流程

傷城~ 2022-10-12 05:48 269阅读 0赞

# 前言 #

上一篇《[Transactional源码解析][Transactional]》我们介绍了Spring对Transactional的解析，也就是事务的初始化工作，这一篇我们接着来分析事务的执行流程。

# 事务拦截器：TransactionInterceptor #

`TransactionInterceptor`是事务拦截器，该类实现了`TransactionAspectSupport` ， `TransactionAspectSupport` 中持有 `TransactionManager` ,拥有处理事务的能力。同时该类还实现了 `MethodInterceptor` 接口 ，它也作为AOP的拦截器。拦截器链中每个拦截器都有一个invoke方法，该方法就是对某个方法进行事务增强的入口，因此主要看invoke方法的实现逻辑！ 源码如下：

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable { 
    
    	
    	public TransactionInterceptor() { 
    	}
    
    	//PlatformTransactionManager：事务管理器
    	public TransactionInterceptor(PlatformTransactionManager ptm, Properties attributes) { 
    		setTransactionManager(ptm);
    		setTransactionAttributes(attributes);
    	}
    
    	//根据事务管理器PlatformTransactionManager，和事务注解源构造一个TransactionInterceptor
    	public TransactionInterceptor(PlatformTransactionManager ptm, TransactionAttributeSource tas) { 
    		setTransactionManager(ptm);
    		setTransactionAttributeSource(tas);
    	}
    
    	//当程序执行事务方法的时候会走invoke ，MethodInvocation：方法调用的描述，在方法调用时提供给拦截器
    	@Override
    	@Nullable
    	public Object invoke(MethodInvocation invocation) throws Throwable { 
    		// Work out the target class: may be {@code null}.
    		// The TransactionAttributeSource should be passed the target class
    		// as well as the method, which may be from an interface.
    		//获取目标类：也就是被代理的那个原生类
    		Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
    
    		// Adapt to TransactionAspectSupport's invokeWithinTransaction...
    		//调用方法，有事务支持
    		return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
    	}
    
    }

该类首先会通过 MethodInvocation(方法调用描述) 得到目标类 ，然后以事务的方式执行方法 invokeWithinTransaction。该方法是其父类的方法

public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean { 
    
    /** * General delegate for around-advice-based subclasses, delegating to several other template * methods on this class. Able to handle {@link CallbackPreferringPlatformTransactionManager} * as well as regular {@link PlatformTransactionManager} implementations. * @param method the Method being invoked * @param targetClass the target class that we're invoking the method on * @param invocation the callback to use for proceeding with the target invocation * @return the return value of the method, if any * @throws Throwable propagated from the target invocation */
    	@Nullable
    	protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
    			final InvocationCallback invocation) throws Throwable { 
    
    		// If the transaction attribute is null, the method is non-transactional.
    		//获取事务属性源：即 @Transactional注解的属性
    		TransactionAttributeSource tas = getTransactionAttributeSource();
    		//获取事务属性
    		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
    		//事务管理器
    		final PlatformTransactionManager tm = determineTransactionManager(txAttr);
    		//方法名，类名+方法名：如 cn.xx.UserServiceImpl.save
    		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
    
    		//声明式事务处理 @Transactional
    		if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) { 
    			// 【标记1】Standard transaction demarcation with getTransaction and commit/rollback calls.
    			//创建TransactionInfo，事务详情对象，其中包括事务管理器（transactionManager），
    			//事务属性（transactionAttribute），方法名（joinpointIdentification），事务状态(transactionStatus)
    			TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
    			Object retVal = null;
    			try { 
    				// This is an around advice: Invoke the next interceptor in the chain.
    				// This will normally result in a target object being invoked.
    				//【标记2】执行方法，这是一个环绕通知，通常会导致目标对象被调用
    				retVal = invocation.proceedWithInvocation();
    			}
    			catch (Throwable ex) { 
    				// target invocation exception
    				//回滚事务：AfterThrowing
    				completeTransactionAfterThrowing(txInfo, ex);
    				throw ex;
    			}
    			finally { 
    				//清理事务
    				cleanupTransactionInfo(txInfo);
    			}
    			//AfterReturning：后置通知，提交事务
    			commitTransactionAfterReturning(txInfo);
    			return retVal;
    		}
    
    		else { 
    			final ThrowableHolder throwableHolder = new ThrowableHolder();
    
    			// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
    			try { 
    				Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> { 
    					TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
    					try { 
    						return invocation.proceedWithInvocation();
    					}
    					catch (Throwable ex) { 
    						if (txAttr.rollbackOn(ex)) { 
    							// A RuntimeException: will lead to a rollback.
    							if (ex instanceof RuntimeException) { 
    								throw (RuntimeException) ex;
    							}
    							else { 
    								throw new ThrowableHolderException(ex);
    							}
    						}
    						else { 
    							// A normal return value: will lead to a commit.
    							throwableHolder.throwable = ex;
    							return null;
    						}
    					}
    					finally { 
    						cleanupTransactionInfo(txInfo);
    					}
    				});
    
    				// Check result state: It might indicate a Throwable to rethrow.
    				if (throwableHolder.throwable != null) { 
    					throw throwableHolder.throwable;
    				}
    				return result;
    			}
    			catch (ThrowableHolderException ex) { 
    				throw ex.getCause();
    			}
    			catch (TransactionSystemException ex2) { 
    				if (throwableHolder.throwable != null) { 
    					logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
    					ex2.initApplicationException(throwableHolder.throwable);
    				}
    				throw ex2;
    			}
    			catch (Throwable ex2) { 
    				if (throwableHolder.throwable != null) { 
    					logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
    				}
    				throw ex2;
    			}
    		}
    	}

上面方法是事务处理的宏观流程，支持编程时和声明式的事务处理，这里是使用了模板模式，细节交给子类去实现，这里我们总结一下上面方法的流程

*  获取事务属性源TransactionAttributeSource 在上一章节有说道该类
 *  加载TransactionManager 事务管理器
 *  对声明式或者编程式的事务处理
 *  创建 TransactionInfo 事务信息对象，其中包括事务管理器（transactionManager），事务属性（transactionAttribute），方法唯一标识（joinpointIdentification），事务状态(transactionStatus)
 *  执行目标方法：invocation.proceedWithInvocation
 *  出现异常，则回滚事务：completeTransactionAfterThrowing，默认是对RuntimeException异常回滚
 *  清理事务信息：cleanupTransactionInfo
 *  提交事务：commitTransactionAfterReturning

下面来分析具体的每个步骤流程

## 创建事务信息：createTransactionIfNecessary ##

createTransactionIfNecessary 是事务流程中的第一个方法，目的是根据给定的 TransactionAttribute 创建一个事务，其中包括事务实例的创建，事务传播行为处理，开启事务等。

protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
    			@Nullable TransactionAttribute txAttr, final String joinpointIdentification) { 
    		//如果未指定名称，则应用方法标识作为事务名称
    		// If no name specified, apply method identification as transaction name.
    		if (txAttr != null && txAttr.getName() == null) { 
    			txAttr = new DelegatingTransactionAttribute(txAttr) { 
    				@Override
    				public String getName() { 
    					return joinpointIdentification;
    				}
    			};
    		}
    
    		TransactionStatus status = null;
    		if (txAttr != null) { 
    			if (tm != null) { 
    				//根据TransactionAttribute 事务属性创建一个事务状态对象
    				status = tm.getTransaction(txAttr);
    			}
    			else { 
    				if (logger.isDebugEnabled()) { 
    					logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
    							"] because no transaction manager has been configured");
    				}
    			}
    		}
    		return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
    	}

### 获取事务状态：tm.getTransaction ###

见：`AbstractPlatformTransactionManager#getTransaction`

@Override
    	public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException { 
    		//开启一个事务，创建了一个DataSourceTransactionObject对象，其中绑定了ConnectionHolder
    		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();
    		}
    		//判断是否已经存在事务，会进行事务传播机制的判断
    		//判断连接不为空且连接(connectionHolder)中的 transactionActive 不为空
    		if (isExistingTransaction(transaction)) { 
    			//如果存在事务，走这里
    			// Existing transaction found -> check propagation behavior to find out how to behave.
    			return handleExistingTransaction(definition, transaction, debugEnabled);
    		}
    		//事务超时时间判断
    		// Check definition settings for new transaction.
    		if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) { 
    			throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
    		}
    		//【PROPAGATION_MANDATORY处理】 如果当前事务不存在，PROPAGATION_MANDATORY要求必须已有事务，则抛出异常
    
    		// 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'");
    		}
    		//【如果没有事务，对于下面三种事务传播行为都需要新开事务】
    		else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
    				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
    				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) { 
    				//这三种事务传播机制都需要新开事务，先挂起事务
    			SuspendedResourcesHolder suspendedResources = suspend(null);
    			if (debugEnabled) { 
    				logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
    			}
    			try { 
    				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
    				//创建一个新的TransactionStatus
    				DefaultTransactionStatus status = newTransactionStatus(
    						definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
    				//开始事务，创建一个DataSourceTransactionObject
    				//设置 ConnectionHolder，设置隔离级别、设置timeout， 切换事务手动提交
    				//如果是新连接，绑定到当前线程
    				doBegin(transaction, definition);
    				//针对当期线程的新事务同步设置
    				prepareSynchronization(status, definition);
    				return status;
    			}
    			catch (RuntimeException | Error ex) { 
    				resume(null, suspendedResources);
    				throw ex;
    			}
    		}
    		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);
    			}
    			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
    			//创建一个newTransactionStatus
    			return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
    		}
    	}

该方法主要做了如下事情：

*  开启一个事务 创建了一个DataSourceTransactionObject 事务实例对象，其中绑定了ConnectionHolder，ConnectionHolder底层是ThreadLocal保存了当前线程的数据库连接信息。
 *  如果当前线程存在事务，则转向嵌套事务的处理。
 *  校验事务超时时间
 *  如果事务传播机制是 PROPAGATION\_MANDATORY ，如果不存在事务就抛异常
 *  创建一个新的TransactionStatus：DefaultTransactionStatus。
 *  完善事务信息设置ConnectionHolder、设置隔离级别、设置timeout，连接绑定到当前线程。

回顾一下事务传播行为：

<table> 
 <thead> 
  <tr> 
   <th>事务传播行为类型</th> 
   <th>说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>PROPAGATION_REQUIRED</td> 
   <td>如果当前没有事务，就新建一个事务，如果已经存在一个事务中，加入到这个事务中。这是最常见的选择。</td> 
  </tr> 
  <tr> 
   <td>PROPAGATION_SUPPORTS</td> 
   <td>支持当前事务，如果当前没有事务，就以非事务方式执行。</td> 
  </tr> 
  <tr> 
   <td>PROPAGATION_MANDATORY</td> 
   <td>使用当前的事务，如果当前没有事务，就抛出异常。</td> 
  </tr> 
  <tr> 
   <td>PROPAGATION_REQUIRES_NEW</td> 
   <td>新建事务，如果当前存在事务，把当前事务挂起。</td> 
  </tr> 
  <tr> 
   <td>PROPAGATION_NOT_SUPPORTED</td> 
   <td>以非事务方式执行操作，如果当前存在事务，就把当前事务挂起。</td> 
  </tr> 
  <tr> 
   <td>PROPAGATION_NEVER</td> 
   <td>以非事务方式执行，如果当前存在事务，则抛出异常。</td> 
  </tr> 
  <tr> 
   <td>PROPAGATION_NESTED</td> 
   <td>如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则执行与PROPAGATION_REQUIRED类似的操作。</td> 
  </tr> 
 </tbody> 
</table>

### 处理嵌套事务 ：handleExistingTransaction ###

//为现有事务创建 TransactionStatus。
    private TransactionStatus handleExistingTransaction(
    			TransactionDefinition definition, Object transaction, boolean debugEnabled)
    			throws TransactionException { 
    		//以非事务方式执行，如果当前存在事务，则抛出异常。
    		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) { 
    			throw new IllegalTransactionStateException(
    					"Existing transaction found for transaction marked with propagation 'never'");
    		}
    		//以非事务方式执行操作，如果当前存在事务，就把当前事务挂起。并把事务信息设置为null
    		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);
    		}
    		//新建事务，如果当前存在事务，把当前事务挂起。当然会创建新的连接，让业务在新的事务中完成，之后恢复挂起的事务。
    		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);
    				//新开一个事务
    				DefaultTransactionStatus status = newTransactionStatus(
    						definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
    				doBegin(transaction, definition);
    				//初始化事务同步
    				prepareSynchronization(status, definition);
    				return status;
    			}
    			catch (RuntimeException | Error beginEx) { 
    				resumeAfterBeginException(transaction, suspendedResources, beginEx);
    				throw beginEx;
    			}
    		}
    		//如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则执行与PROPAGATION_REQUIRED类似的操作。
    		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) { 
    			//是否运行嵌套事务
    			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() + "]");
    			}
    			//对嵌套事务使用保存点
    			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.
    				//如果是JDBC，使用保存点方式支持事务回滚
    				DefaultTransactionStatus status =
    						prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
    				status.createAndHoldSavepoint();
    				return status;
    			}
    			else { 
    				//如果是类似于JTA这种还无法使用保存点，处理方式如同PROPAGATION_REQUIRES_NEW
    				// 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);
    	}

### 新开启事务：DataSourceTransactionManager\#doBegin ###

/** * This implementation sets the isolation level but ignores the timeout. */
    	@Override
    	protected void doBegin(Object transaction, TransactionDefinition definition) { 
    		//创建 DataSource 事务对象
    		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
    		Connection con = null;
    
    		try { 
    			if (!txObject.hasConnectionHolder() ||
    					txObject.getConnectionHolder().isSynchronizedWithTransaction()) { 
    					//获取连接
    				Connection newCon = obtainDataSource().getConnection();
    				if (logger.isDebugEnabled()) { 
    					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
    				}
    				//把链接设置给DataSourceTransactionObject 
    				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
    			}
    			
    			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
    			con = txObject.getConnectionHolder().getConnection();
    			//设置事务隔离级别 ,使用, 以及ReadOnly
    			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");
    				}
    				//设置手动提交,由Spring来控制事务提交
    				con.setAutoCommit(false);
    			}
    
    			prepareTransactionalConnection(con, definition);
    			//设置事务Active为true
    			txObject.getConnectionHolder().setTransactionActive(true);
    			//设置事务超时时间
    			int timeout = determineTimeout(definition);
    			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) { 
    				txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
    			}
    
    			//把连接绑定到当前线程
    			// Bind the connection holder to the thread.
    			if (txObject.isNewConnectionHolder()) { 
    				TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
    			}
    		}
    
    		catch (Throwable ex) { 
    			if (txObject.isNewConnectionHolder()) { 
    				DataSourceUtils.releaseConnection(con, obtainDataSource());
    				txObject.setConnectionHolder(null, false);
    			}
    			throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
    		}
    	}

这个方法是事务开始的方法，因为它已经尝试和数据库进行连接了，然后又做了一些基础设置

*  设置事务的隔离级别，如果DB的隔离级别和事务属性源(TransactionAttribute )即：用户定义的事务隔离级别不一致，使用用户定义的隔离级别
 *  把事务自动提交改为false,由Spring来控制事务提交
 *  把 TransactionActive 状态设置为true,代表事务是active 激活状态
 *  设置事务超时时间
 *  把连接绑定到当前对象

到这， createTransactionIfNecessary 方法中的业务就分析完了，接下来就是 调用 `invocation.proceedWithInvocation()` 去执行目标类的方法，如果出现异常，会走catch中的回滚事务代码。

## 回滚事务：completeTransactionAfterThrowing ##

代码回到`TransactionAspectSupport#invokeWithinTransaction` ，我们跟一下`completeTransactionAfterThrowing(txInfo, ex);` 回滚事务代码，源码如下

protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) { 
    		if (txInfo != null && txInfo.getTransactionStatus() != null) { 
    			if (logger.isTraceEnabled()) { 
    				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
    						"] after exception: " + ex);
    			}
    			//判断异常类型决定是否要回滚
    			//默认判断异常是否是 RuntimeException 类型或者是 Error 类型
    			//可以指定异常处理类型，例如：@Transactional(rollbackFor=Exception.class)
    			if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) { 
    				try { 
    					//走事务管理器的rollback回滚事务
    					txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
    				}
    				catch (TransactionSystemException ex2) { 
    					logger.error("Application exception overridden by rollback exception", ex);
    					ex2.initApplicationException(ex);
    					throw ex2;
    				}
    				catch (RuntimeException | Error ex2) { 
    					logger.error("Application exception overridden by rollback exception", ex);
    					throw ex2;
    				}
    			}
    			else { 
    				// We don't roll back on this exception.
    				// Will still roll back if TransactionStatus.isRollbackOnly() is true.
    				try { 
    					//如果不满足回滚条件，即使抛出异常也同样会提交
    					txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
    				}
    				catch (TransactionSystemException ex2) { 
    					logger.error("Application exception overridden by commit exception", ex);
    					ex2.initApplicationException(ex);
    					throw ex2;
    				}
    				catch (RuntimeException | Error ex2) { 
    					logger.error("Application exception overridden by commit exception", ex);
    					throw ex2;
    				}
    			}
    		}
    	}

首先是判断了异常的类型符不符合回滚条件，如果符合就调用事务管理器的回滚逻辑，如果不符合回滚条件就走事务管理器的commit提交事务，下面是回滚逻辑：`AbstractPlatformTransactionManager#rollback`

@Override
    	public final void rollback(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");
    		}
    		//事务状态
    		DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
    		//处理回滚
    		processRollback(defStatus, false);
    	}
    
    	/** * Process an actual rollback. * The completed flag has already been checked. * @param status object representing the transaction * @throws TransactionException in case of rollback failure */
    	private void processRollback(DefaultTransactionStatus status, boolean unexpected) { 
    		try { 
    			boolean unexpectedRollback = unexpected;
    
    			try { 
    				//触发事务同步器TransactionSynchronization中的beforeCompletion回调
    				//比如调用SqlSessionSynchronization#beforeCompletion 释放资源，sqlSession close等
    				triggerBeforeCompletion(status);
    				//如果有保存点，回滚到保存点
    				if (status.hasSavepoint()) { 
    					if (status.isDebug()) { 
    						logger.debug("Rolling back transaction to savepoint");
    					}
    					status.rollbackToHeldSavepoint();
    				}
    				else if (status.isNewTransaction()) { 
    				//如果是新开的事务，直接触发回滚，
    					if (status.isDebug()) { 
    						logger.debug("Initiating transaction rollback");
    					}
    					//走的是 Connection.rollback回滚
    					doRollback(status);
    				}
    				else { 
    					// Participating in larger transaction
    					// 如果当前事务不是独立的事务，那么只能标记状态，等到事务链执行完毕后统一回滚
    
    					if (status.hasTransaction()) { 
    						if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) { 
    							if (status.isDebug()) { 
    								logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
    							}
    							doSetRollbackOnly(status);
    						}
    						else { 
    							if (status.isDebug()) { 
    								logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
    							}
    						}
    					}
    					else { 
    						logger.debug("Should roll back transaction but cannot - no transaction available");
    					}
    					// Unexpected rollback only matters here if we're asked to fail early
    					if (!isFailEarlyOnGlobalRollbackOnly()) { 
    						unexpectedRollback = false;
    					}
    				}
    			}
    			catch (RuntimeException | Error ex) { 
    				triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
    				throw ex;
    			}
                // 触发所有 TransactionSynchronization 同步器中对应的 afterCompletion 方法
                //比如调用 SqlSessionSynchronization#beforeCompletion 释放资源，重置SqlSession等
    			triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
    
    			// Raise UnexpectedRollbackException if we had a global rollback-only marker
    			if (unexpectedRollback) { 
    				throw new UnexpectedRollbackException(
    						"Transaction rolled back because it has been marked as rollback-only");
    			}
    		}
    		finally { 
    			//执行清空资源 ， 恢复挂起的资源
    			cleanupAfterCompletion(status);
    		}
    	}

总结一下回滚逻辑

*  触发TransactionSynchronization中的beforeCompletion回调 ， TransactionSynchronization定义是事务同步逻辑。比如：SqlSessionSynchronization\#beforeCompletion 就用来释放资源，sqlSession close等
 *  如果有保存点，就使用保存点信息进行回滚
 *  如果是新开的是事务，使用底层数据库的API回滚
 *  其他情况比如JTA模式就标记回滚，等到提交的时候统一回滚

### 事务清理：cleanupAfterCompletion ###

private void cleanupAfterCompletion(DefaultTransactionStatus status) { 
    		//设置完成状态
    		status.setCompleted();
    		if (status.isNewSynchronization()) { 
    			//事务同步管理器清理
    			TransactionSynchronizationManager.clear();
    		}
    		if (status.isNewTransaction()) { 
    			doCleanupAfterCompletion(status.getTransaction());
    		}
    		if (status.getSuspendedResources() != null) { 
    			if (status.isDebug()) { 
    				logger.debug("Resuming suspended transaction after completion of inner transaction");
    			}
    			Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
    			//恢复挂起的事务：
    			resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
    		}
    	}

1.  如果是新的同步状态，则走`TransactionSynchronizationManager.clear();`清除当前线程的整个事务同步状态
2.  如果是新开的事务，则走`doCleanupAfterCompletion`清理资源，该方法做了如下事情：

*  把数据库连接和当前线程解绑
 *  重置连接，设置自动提交为true
 *  如果是新开的事务，就释放连接对象
 *  清空ConnectionHolder

1.  如果之前有挂起的事务，就走resume恢复

## 事务提交：commitTransactionAfterReturning ##

protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) { 
    		if (txInfo != null && txInfo.getTransactionStatus() != null) { 
    			if (logger.isTraceEnabled()) { 
    				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
    			}
    			txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
    		}
    	}
    
    
    @Override
    	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;
    		//如果事务被标记回滚，直接回滚
    		if (defStatus.isLocalRollbackOnly()) { 
    			if (defStatus.isDebug()) { 
    				logger.debug("Transactional code has requested rollback");
    			}
    			//走回滚流程
    			processRollback(defStatus, false);
    			return;
    		}
    
    		if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) { 
    			if (defStatus.isDebug()) { 
    				logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
    			}
    			processRollback(defStatus, true);
    			return;
    		}
    		//执行提交流程
    		processCommit(defStatus);
    	}

提交事务之前会先判断是否要回滚，然后触发回滚，否则就正常走提交事务流程

private void processCommit(DefaultTransactionStatus status) throws TransactionException { 
    		try { 
    			boolean beforeCompletionInvoked = false;
    
    			try { 
    				boolean unexpectedRollback = false;
    				
    				prepareForCommit(status);
    				//调用 同步器的beforeCommit ：比如走 SqlSessionSynchronization#beforeCommit执行 SqlSession().commit() 提交事务
    				triggerBeforeCommit(status);
    				//触发同步器的beforeCompletion，比如走：SqlSessionSynchronization#beforeCompletion 解绑资源，执行sqlSession.close 
    				triggerBeforeCompletion(status);
    				beforeCompletionInvoked = true;
    				//如果有保存点
    				if (status.hasSavepoint()) { 
    					if (status.isDebug()) { 
    						logger.debug("Releasing transaction savepoint");
    					}
    					unexpectedRollback = status.isGlobalRollbackOnly();
    					//释放保存点
    					status.releaseHeldSavepoint();
    				}
    				//如果是新开事务
    				else if (status.isNewTransaction()) { 
    					if (status.isDebug()) { 
    						logger.debug("Initiating transaction commit");
    					}
    					unexpectedRollback = status.isGlobalRollbackOnly();
    					//调用collection.commit 提交事务
    					doCommit(status);
    				}
    				else if (isFailEarlyOnGlobalRollbackOnly()) { 
    					unexpectedRollback = status.isGlobalRollbackOnly();
    				}
    
    				// Throw UnexpectedRollbackException if we have a global rollback-only
    				// marker but still didn't get a corresponding exception from commit.
    				if (unexpectedRollback) { 
    					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 | Error ex) { 
    				if (!beforeCompletionInvoked) { 
    					triggerBeforeCompletion(status);
    				}
    				doRollbackOnCommitException(status, ex);
    				throw ex;
    			}
    
    			// Trigger afterCommit callbacks, with an exception thrown there
    			// propagated to callers but the transaction still considered as committed.
    			try { 
    			//触发 afterCommit 回调
    				triggerAfterCommit(status);
    			}
    			finally { 
    				// 释放资源：this.holder.getSqlSession().close();
    
    				triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
    			}
    
    		}
    		finally { 
    			cleanupAfterCompletion(status);
    		}
    	}

1.  当事务有保存点不会去提交事务，而是释放保存点
2.  如果是新开的事务就调用collection.comit 提交事务
3.  然后就是各种释放资源，关闭SqlSession等操作

# 总结 #

就分析到这里吧，总体来说，当Spring启动就会为标记了@Transcational的方法生成代理，然后代理对象在执行方法的时候会通过TransactionInterceptor来执行事务增强，而事务的业务主要是调用TransactionManager 来完成。

深夜码字，实在太困了，还是早点休息吧明天还上班，喜欢的话给个好评，你的鼓励是我最大的动力，谢谢！！！

[Transactional]: https://blog.csdn.net/u014494148/article/details/118398677