本文相关源码包使用的是spring-tx-5.2.8.RELEASE.jar

一、事务的实现方式

1、JDBC

public static void main(String[] args) {
     Connection conn = null;
      Statement stmt = null;
      try {
          // 注册 JDBC 驱动
          Class.forName("com.mysql.cj.jdbc.Driver");
          // 打开连接
          conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test?characterEncoding=utf-8&serverTimezone=UTC", "root", "xxx");
          // 执行查询
          stmt = conn.createStatement();
          // 关闭自动提交
          conn.setAutoCommit(false); 
		  // 执行更新
          String sql1 = "INSERT INTO test(id,user_name)values(1,'forlan1')";
          stmt.executeUpdate(sql1);
          String sql1 = "INSERT INTO test(id,user_name)values(2,'forlan2')";
          stmt.executeUpdate(sql2);

          // 事务提交
          conn.commit(); // 上面两个操作都没有问题就提交
      } catch (Exception e) {
          e.printStackTrace();
          // 事务回滚
          conn.rollback();
      } finally {
         // 关闭资源，stmt.close();conn.close();
      }
   }

其实关键就是3个动作：关闭自动提交、commit、rollback

2、Spring

基于xml配置

• 创建事务管理器
• 配置事务方法
• 配置AOP

	 <bean class="org.springframework.jdbc.datasource.DataSourceTransactionManager" id="transactionManager">
	 	<property name="dataSource" ref="dataSource"/>
	 </bean>
	 <tx:advice id="advice" transaction-manager="transactionManager">
	 	<tx:attributes>
	 		<tx:method name="forlan*" propagation="REQUIRED"/>
	 	</tx:attributes>
	 </tx:advice>
	 <!-- aop配置 -->
	 <aop:config>
		 <aop:pointcut expression="execution(* *..service.*.*(..))" id="tx"/>
	 	 <aop:advisor advice-ref="advice" pointcut-ref="tx"/>
	 </aop:config>

编程式事务

@Autowired
private PlatformTransactionManager txManager;

public void forlanTest() {
	// 1、创建事务定义
	DefaultTransactionDefinition definition = new DefaultTransactionDefinition();
	// 2、根据定义开启事务
	TransactionStatus transaction = txManager.getTransaction(definition);
	try {
		//TODO 业务逻辑、SQL操作
		// 3、提交事务
		txManager.commit(transaction);
	} catch (Throwable e) {
		e.printStackTrace();
		// 4、异常了，回滚事务
		txManager.rollback(transaction);
	}
}

声明式事务

启动类加@EnableTransactionManagement，开启注解版本的事务
方法头部加@Transactional，开启事务

二、源码设计

通过编程式事务的代码我们可以发现，关键在于PlatformTransactionManager，TransactionDefinition，TransactionStatus这几个接口，下面我们就来深挖下这几个接口是怎么设计的

1、TransactionManager

1）TransactionManager

顶层接口，空代码

public interface TransactionManager {}

2）PlatformTransactionManager

这个是平台事务管理器，比较常用的，像我们一般的命令式编程就是用它

public interface PlatformTransactionManager extends TransactionManager {
    TransactionStatus getTransaction(@Nullable TransactionDefinition var1) throws TransactionException;

    void commit(TransactionStatus var1) throws TransactionException;

    void rollback(TransactionStatus var1) throws TransactionException;
}

PlatformTransactionManager有个核心的实现抽象类AbstractPlatformTransactionManager，实现了公共的方法
- JtaTransactionManager：支持分布式事务【本身服务中的多数据源】
- DataSourceTransactionManager：数据源事务管理器，提供了相关事务操作的方法

3）ReactiveTransactionManager

响应式编程的事务管理器，响应式编程会用，一般比较少使用

2、TransactionDefinition

事务定义，作为PlatformTransactionManager接口getTransactio()方法的入参，得到事务TransactionStatus；

public interface TransactionDefinition {
    int PROPAGATION_REQUIRED = 0;
    int PROPAGATION_SUPPORTS = 1;
    int PROPAGATION_MANDATORY = 2;
    int PROPAGATION_REQUIRES_NEW = 3;
    int PROPAGATION_NOT_SUPPORTED = 4;
    int PROPAGATION_NEVER = 5;
    int PROPAGATION_NESTED = 6;
    int ISOLATION_DEFAULT = -1;
    int ISOLATION_READ_UNCOMMITTED = 1;
    int ISOLATION_READ_COMMITTED = 2;
    int ISOLATION_REPEATABLE_READ = 4;
    int ISOLATION_SERIALIZABLE = 8;
    int TIMEOUT_DEFAULT = -1;

    default int getPropagationBehavior() {
        return PROPAGATION_REQUIRED;
    }
    default int getIsolationLevel() {
        return ISOLATION_DEFAULT;
    }
    default int getTimeout() {
        return TIMEOUT_DEFAULT;
    }
    default boolean isReadOnly() {
        return false;
    }
    @Nullable
    default String getName() {
        return null;
    }
    static TransactionDefinition withDefaults() {
        return StaticTransactionDefinition.INSTANCE;
    }
}

主要参数：传播行为、隔离级别，是否只读，超时时间
DefaultTransactionDefinition：是事务定义的默认实现

 private int propagationBehavior = 0;// PROPAGATION_REQUIRED
 private int isolationLevel = -1;// 跟随数据库
 private int timeout = -1;// 无超时时间
 private boolean readOnly = false;// 可读写

3、TransactionStatus

public interface TransactionStatus extends TransactionExecution, SavepointManager, Flushable {
    boolean hasSavepoint(); //是否有保存点
    void flush(); // 刷新
}

4、 核心方法说明

1）getTransaction

public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
		throws TransactionException {

	// 如果未给出事务定义，则使用默认值
	TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

	// 关键方法1：获取数据源事务对象DataSourceTransactionObject(是否允许保存点，ConnectionHolder)
	Object transaction = doGetTransaction();
	boolean debugEnabled = logger.isDebugEnabled();

	/**
	 * 判断当前线程是否存在事务：connectionHolder != null && transactionActive
	 * 第1次调用，connectionHolder为null
	 * 第2次调用，connectionHolder的transactionActive=true
	 */
	if (isExistingTransaction(transaction)) {
		// Existing transaction found -> check propagation behavior to find out how to behave.
		// 关键方法4：处理已经存在的事务
		return handleExistingTransaction(def, transaction, debugEnabled);
	}

	// 超时，抛出异常
	if (def.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
		throw new InvalidTimeoutException("Invalid transaction timeout", def.getTimeout());
	}

	// 传播行为是PROPAGATION_MANDATORY且当前线程不存在事务，就抛出异常
	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) {
		// 关键方法2：挂起事务
		SuspendedResourcesHolder suspendedResources = suspend(null);
		if (debugEnabled) {
			logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
		}
		try {
			 // 关键方法3：创建新事务，transaction = xxx;newTransaction = true;newSynchronization = true
			return startTransaction(def, transaction, debugEnabled, suspendedResources);
		}
		catch (RuntimeException | Error ex) {
			// 恢复挂起的事务
			resume(null, suspendedResources);
			throw ex;
		}
	}
	else {// PROPAGATION_SUPPORTS、PROPAGATION_NOT_SUPPORTED、PROPAGATION_NEVER这些传播行为进到这里
		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);
		// 创建空事务，transaction = null;newTransaction = true;newSynchronization = true
		return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
	}
}

protected boolean isExistingTransaction(Object transaction) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
	return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}

关键方法1:doGetTransaction()

创建一个数据源事务对象DataSourceTransactionObject，设置SavepointAllowed和ConnectionHolder，ConnectionHolder里面会存放的是JDBC连接（第一次进来是null）

protected Object doGetTransaction() {
	// 创建一个数据源事务对象
    DataSourceTransactionManager.DataSourceTransactionObject txObject = new DataSourceTransactionManager.DataSourceTransactionObject();
    // 设置是否允许保存点，DataSourceTransactionManager里面为true
    txObject.setSavepointAllowed(this.isNestedTransactionAllowed());
	/**
	 * TransactionSynchronizationManager是事务同步管理器对象，用来保存当前事务信息
	 * 第一次进来获取的话，因为是通过数据源去获取，但事务同步管理器中还没有存放，所以此时获取的conHolder为null
	 */
    ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(this.obtainDataSource());
    // 非新创建连接则写false,newConnectionHolder=false
    txObject.setConnectionHolder(conHolder, false);
    return txObject;
}

关键方法2:suspend

挂起事务，暂停事务同步或解绑数据源的数据库连接对象，返回这些资源

protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {
	if (TransactionSynchronizationManager.isSynchronizationActive()) {// 事务和同步中有东西，说明事务和同步都激活
		// 暂停所有当前同步并停用当前线程的事务同步，返回挂起的事务同步对象列表
		List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
		try {
			Object suspendedResources = null;
			if (transaction != null) {
				// 解绑数据源的数据库连接，返回该连接
				suspendedResources = doSuspend(transaction);
			}
			String name = TransactionSynchronizationManager.getCurrentTransactionName();
			TransactionSynchronizationManager.setCurrentTransactionName(null);
			boolean readOnly = TransactionSynchronizationManager.isCurrentTransactionReadOnly();
			TransactionSynchronizationManager.setCurrentTransactionReadOnly(false);
			Integer isolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
			TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(null);
			boolean wasActive = TransactionSynchronizationManager.isActualTransactionActive();
			TransactionSynchronizationManager.setActualTransactionActive(false);
			// 返回挂起的ResourcesHolder
			return new SuspendedResourcesHolder(
					suspendedResources, suspendedSynchronizations, name, readOnly, isolationLevel, wasActive);
		}
		catch (RuntimeException | Error ex) {
			// 挂起失败，重新激活当前线程的事务同步并恢复所有给定的同步
			doResumeSynchronization(suspendedSynchronizations);
			throw ex;
		}
	}
	else if (transaction != null) {// 事务激活，但没激活同步
		// 解绑数据源的数据库连接，返回该连接
		Object suspendedResources = doSuspend(transaction);
		// 返回挂起的ResourcesHolder
		return new SuspendedResourcesHolder(suspendedResources);
	}
	else {
		// 事务和同步都没激活
		return null;
	}
}

private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations =
		new NamedThreadLocal<>("Transaction synchronizations");
public static boolean isSynchronizationActive() {
	return (synchronizations.get() != null);
}

关键方法3:startTransaction

创建TransactionStatus实例，填充属性值，获取数据库连接，关闭自动提交，绑定数据源和数据库连接到当前线程，激活事务+同步

private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled, @Nullable AbstractPlatformTransactionManager.SuspendedResourcesHolder suspendedResources) {
    // 这里的transactionSynchronization = 0，所以newSynchronization =true
    boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
    // 创建指定参数的TransactionStatus实例，true指的是newTransaction，创建后，事务提交和回滚状态都为false
    DefaultTransactionStatus status = newTransactionStatus(
		definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
    // 获取数据库连接，关闭自动提交，绑定数据源和数据库连接到当前线程，激活事务
    doBegin(transaction, definition);
    // newSynchronization=true,激活事务+同步
    prepareSynchronization(status, definition);
    return status;
}

1、newTransactionStatus方法：创建TransactionStatus实例，填充属性值

/**
 * status = {DefaultTransactionStatus@10391}
 *  completed = false
 *  debug = false
 *  newSynchronization = true
 *  newTransaction = true
 *  readOnly = false
 *  rollbackOnly = false
 *  savepoint = null
 *  suspendedResources = null
 *  transaction = {DataSourceTransactionManager$DataSourceTransactionObject@10383}
 *   connectionHolder = null
 *   mustRestoreAutoCommit = false
 *   newConnectionHolder = false
 *   previousIsolationLevel = null
 *   readOnly = false
 *   savepointAllowed = true
 */
protected DefaultTransactionStatus newTransactionStatus(
		TransactionDefinition definition, @Nullable Object transaction, boolean newTransaction,
		boolean newSynchronization, boolean debug, @Nullable Object suspendedResources) {
	// newSynchronization = true并且之前没激活同步（synchronizations线程对象没东西）
	boolean actualNewSynchronization = newSynchronization &&
			!TransactionSynchronizationManager.isSynchronizationActive();
	return new DefaultTransactionStatus(
			transaction, newTransaction, actualNewSynchronization,
			definition.isReadOnly(), debug, suspendedResources);
}

public DefaultTransactionStatus(
			@Nullable Object transaction, boolean newTransaction, boolean newSynchronization,
			boolean readOnly, boolean debug, @Nullable Object suspendedResources) {

		this.transaction = transaction;
		this.newTransaction = newTransaction;
		this.newSynchronization = newSynchronization;
		this.readOnly = readOnly;
		this.debug = debug;
		this.suspendedResources = suspendedResources;
	}

2、doBegin方法：获取数据库连接，关闭自动提交，绑定数据源和ConnectionHolder到当前线程，设置DataSourceTransactionObject 信息（激活事务、同步），
可能的操作（设置只读、隔离解绑）

protected void doBegin(Object transaction, TransactionDefinition definition) {
    DataSourceTransactionManager.DataSourceTransactionObject txObject = (DataSourceTransactionManager.DataSourceTransactionObject
    Connection con = null;
    try {
    	// ConnectionHolder为空 或 已经激活事务同步
        if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
            // 通过数据源获取数据库连接对象
			Connection newCon = this.obtainDataSource().getConnection();
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
            }
			// 把数据库连接对象包装为ConnectionHolder对象，然后设置到txObject对象中
            txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
        }
		// 设置ConnectionHolder的synchronizedWithTransaction为true
        txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
        // 获取数据库连接对象
		con = txObject.getConnectionHolder().getConnection();
        // 如果有是否只读和隔离级别！=-1，设置con.setReadOnly(true);con.setTransactionIsolation(xxx);
		Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
        // 设置txObject的隔离级别
        txObject.setPreviousIsolationLevel(previousIsolationLevel);
        // 设置txObject是否只读
		txObject.setReadOnly(definition.isReadOnly());
        if (con.getAutoCommit()) {// 如果是自动提交
            txObject.setMustRestoreAutoCommit(true);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
            }
            con.setAutoCommit(false);// 关闭自动提交
        }
        // 准备事务连接,如果“enforceReadOnly”标志设置为true，并且事务定义指示只读事务，则默认实现将执行“SET TRANSACTION IONLY”语句
        prepareTransactionalConnection(con, definition);
        // 设置ConnectionHolder的transactionActive为true，表示已经存在事务，后面就会进入有事务的逻辑
        txObject.getConnectionHolder().setTransactionActive(true);
        int timeout = this.determineTimeout(definition);
        if (timeout != -1) {
            txObject.getConnectionHolder().setTimeoutInSeconds(timeout);// 设置事务超时时间
        }
        if (txObject.isNewConnectionHolder()) {
			// 将资源绑定到当前线程，key=数据源,value=ConnectionHolder
            TransactionSynchronizationManager.bindResource(this.obtainDataSource(), txObject.getConnectionHolder());
        }
    } catch (Throwable var7) {
        if (txObject.isNewConnectionHolder()) {
			// 关闭数据库连接
            DataSourceUtils.releaseConnection(con, this.obtainDataSource());
            txObject.setConnectionHolder((ConnectionHolder)null, false);
        }
        throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", var7);
    }
}

3、prepareSynchronization方法：如果newSynchronization = true，初始化事务同步管理器

protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
	if (status.isNewSynchronization()) {
		TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
		TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
				definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
						definition.getIsolationLevel() : null);
		TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
		TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
		TransactionSynchronizationManager.initSynchronization();
	}
}

关键方法4:handleExistingTransaction

处理存在事务的情况下，不同传播行为的逻辑，主要看是否抛出异常，是否挂起资源，是执行startTransaction
startTransaction和prepareTransactionStatus的区别，startTransaction多执行了doBegin()方法，开启了新事物，返回的参数为true（newSynchronization，newTransaction，mustRestoreAutoCommit，newConnectionHolder）
说明：newTransaction：true表示新开事务，false参与到当前事务
newSynchronization：后面提交和回滚有用

private TransactionStatus handleExistingTransaction(
		TransactionDefinition definition, Object transaction, boolean debugEnabled)
		throws TransactionException {
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
		// 当前存在事务，则抛出异常
		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");
		}
		// 挂起当前事务,返回挂起资源
		Object suspendedResources = suspend(transaction);
		boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
		// 创建空事务，并携带挂起资源，transaction = null;suspendedResources=xxx; newTransaction = false;newSynchronization = true;
		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() + "]");
		}
		// 挂起当前事务,返回挂起资源
		SuspendedResourcesHolder suspendedResources = suspend(transaction);
		try {
			// 创建新事务，并携带挂起资源，transaction = xxx;suspendedResources=xxx; newTransaction = true;newSynchronization = true;
			return startTransaction(definition, transaction, debugEnabled, suspendedResources);
		}
		catch (RuntimeException | Error beginEx) {
			resumeAfterBeginException(transaction, suspendedResources, beginEx);
			throw beginEx;
		}
	}
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
		if (!isNestedTransactionAllowed()) {// 不允许嵌套事务就报错
			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.
			
			// 创建新事务，不挂起资源，transaction = xxx;suspendedResources=null; newTransaction = false;newSynchronization = false;
			DefaultTransactionStatus status =
					prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
			// 创建保存点，savepoint=xxx
			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.
			// 创建新事务
			return startTransaction(definition, transaction, debugEnabled, null);
		}
	}
	// 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");
			}
		}
	}
	// 传播行为：PROPAGATION_REQUIRED，PROPAGATION_SUPPORTS，PROPAGATION_MANDATORY，会走到这里的逻辑
	boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
	// 创建新事务，不挂起资源，transaction = xxx;suspendedResources=null; newTransaction = false;newSynchronization = false;
	return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}

（1）实验

我们通过以下代码来分析源码做了什么

@Transactional/*(propagation = Propagation.SUPPORTS)*/
public void m1(){
	forlanDao.update(xxx);
	forlanService.m2();
}

@Transactional(propagation = Propagation.REQUIRED)
// @Transactional(propagation = Propagation.NESTED)
// @Transactional(propagation = Propagation.REQUIRES_NEW)
// @Transactional(propagation = Propagation.NOT_SUPPORTED)
public void m2(){
	forlanDao.insert(xxx);
}

a、执行m1()方法过程：

• 调用doGetTransaction()方法，创建DataSourceTransactionObjec
• 调用isExistingTransaction()方法，判断当前线程是否存在事务，最外层方法，肯定不存在
  • REQUIRED
    • 调用suspend()方法，什么都没做
    • 调用startTransaction()方法，创建TransactionStatus对象，获取数据库连接，关闭自动提交，绑定数据源和数据库连接到当前线程，激活事务+同步
  • SUPPORTS
    • 调用prepareTransactionStatus()方法，创建TransactionStatus对象
      最终，返回的TransactionStatus对象，内容如下：

transactionStatus = {DefaultTransactionStatus@10569} 
 transaction = {DataSourceTransactionManager$DataSourceTransactionObject@10398} 
  newConnectionHolder = true
  mustRestoreAutoCommit = true
  connectionHolder = {ConnectionHolder@10577} 
  previousIsolationLevel = null
  readOnly = false
  savepointAllowed = true
 newTransaction = true
 newSynchronization = true
 readOnly = false
 debug = false
 suspendedResources = null
 rollbackOnly = false
 completed = false
 savepoint = null

放开注释的话，返回的TransactionStatus对象，内容如下：

transactionStatus = {DefaultTransactionStatus@10421} 
 transaction = null
 newTransaction = true
 newSynchronization = true
 readOnly = false
 debug = false
 suspendedResources = null
 rollbackOnly = false
 completed = false
 savepoint = null

主要区别在于transaction是否为空，为null表示没有事务，否则就是有事务

b、执行m2()方法过程如下：

• 调用doGetTransaction()方法，创建DataSourceTransactionObjec
• 调用isExistingTransaction()方法，判断当前线程是否存在事务，前面已经创建过事务了，肯定存在
• 调用handleExistingTransaction()方法，这里就有区分逻辑了，看我们的传播行为是什么
  • PROPAGATION_NEVER，抛异常
  • PROPAGATION_NOT_SUPPORTED，挂起当前事务，创建空事务（带挂起资源）
  • PROPAGATION_REQUIRES_NEW，挂起当前事务，创建新事务（带挂起资源）
  • PROPAGATION_NESTED，加入当前事务（带保存点）
  • 其它行为，比如REQUIRED，SUPPORTS，MANDATORY，加入当前事务

1）REQUIRED的话，返回的TransactionStatus对象，内容如下：

transactionStatus = {DefaultTransactionStatus@10678} 
 transaction = {DataSourceTransactionManager$DataSourceTransactionObject@10652} 
  newConnectionHolder = false
  mustRestoreAutoCommit = false
  connectionHolder = {ConnectionHolder@10683} 
  previousIsolationLevel = null
  readOnly = false
  savepointAllowed = true
 suspendedResources = null
 newTransaction = false
 newSynchronization = false
 readOnly = false
 debug = false
 rollbackOnly = false
 completed = false
 savepoint = null

注：SUPPORTS，MANDATORY也是一样内容

2）NESTED的话，返回的TransactionStatus对象，内容如下：

transactionStatus = {DefaultTransactionStatus@10675} 
 transaction = {DataSourceTransactionManager$DataSourceTransactionObject@10652} 
  newConnectionHolder = false
  mustRestoreAutoCommit = false
  connectionHolder = {ConnectionHolder@10695} 
  previousIsolationLevel = null
  readOnly = false
  savepointAllowed = true
 suspendedResources = null
 newTransaction = false
 newSynchronization = false
 readOnly = false
 debug = false
 rollbackOnly = false
 completed = false
 savepoint = {MysqlSavepoint@10692} 
  savepointName = "SAVEPOINT_1"
  exceptionInterceptor = null

3）REQUIRES_NEW的话，返回的TransactionStatus对象，内容如下：

transactionStatus = {DefaultTransactionStatus@10672} 
 transaction = {DataSourceTransactionManager$DataSourceTransactionObject@10652} 
  newConnectionHolder = true
  mustRestoreAutoCommit = true
  connectionHolder = {ConnectionHolder@10678} 
  previousIsolationLevel = null
  readOnly = false
  savepointAllowed = true
 suspendedResources = {AbstractPlatformTransactionManager$SuspendedResourcesHolder@10677} 
  suspendedResources = {ConnectionHolder@10679} 
  suspendedSynchronizations = {Collections$UnmodifiableRandomAccessList@10680}  size = 1
  name = "cn.lw.service.impl.ShiwuServiceImpl.insert"
  readOnly = false
  isolationLevel = null
  wasActive = true
 newTransaction = true
 newSynchronization = true
 readOnly = false
 debug = false
 rollbackOnly = false
 completed = false
 savepoint = null

4）NOT_SUPPORTED的话，返回的TransactionStatus对象，内容如下：

transactionStatus = {DefaultTransactionStatus@10678} 
 transaction = null
 suspendedResources = {AbstractPlatformTransactionManager$SuspendedResourcesHolder@10666} 
  suspendedResources = {ConnectionHolder@10681} 
   connectionHandle = {SimpleConnectionHandle@10687} "SimpleConnectionHandle: HikariProxyConnection@961058126 wrapping com.mysql.jdbc.JDBC4Connection@4d0128fe"
   currentConnection = {HikariProxyConnection@10688} "HikariProxyConnection@961058126 wrapping com.mysql.jdbc.JDBC4Connection@4d0128fe"
   transactionActive = true
   savepointsSupported = null
   savepointCounter = 0
   synchronizedWithTransaction = true
   rollbackOnly = false
   deadline = null
   referenceCount = 1
   isVoid = false
  suspendedSynchronizations = {Collections$UnmodifiableRandomAccessList@10682}  size = 1
  name = "cn.lw.service.impl.ShiwuServiceImpl.insert"
  readOnly = false
  isolationLevel = null
  wasActive = true
 newTransaction = false
 newSynchronization = true
 readOnly = false
 debug = false
 rollbackOnly = false
 completed = false
 savepoint = null

（2）总结

总的区别：

• transaction是否为null
• suspendedResources是否为null
• savepoint是否为null
• newTransaction、newSynchronization布尔值不一样

属性值区别：

• NOT_SUPPORTED：transaction=null，suspendedResources=挂起资源，savepoint=null，newTransaction=false，newSynchronization= true
• REQUIRED，SUPPORTS，MANDATORY：transaction={newConnectionHolder = false，mustRestoreAutoCommit = false}，suspendedResources=null，savepoint=null，newTransaction=false，newSynchronization= false
• NESTED：transaction={newConnectionHolder = false，mustRestoreAutoCommit = false}，suspendedResources=null，savepoint=保存点，newTransaction=false，newSynchronization= false
• REQUIRES_NEW：transaction={newConnectionHolder = true，mustRestoreAutoCommit = true}，suspendedResources=挂起资源，savepoint=null，newTransaction=true，newSynchronization= true

transaction表示是否以事务的方式执行，newConnectionHolder表示是否开启新事物

2） commit

public final void commit(TransactionStatus status) throws TransactionException {
	if (status.isCompleted()) {// completed=true,说明之前已经提交或回滚
		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()) {// rollbackOnly=true,请求回滚
		if (defStatus.isDebug()) {
			logger.debug("Transactional code has requested rollback");
		}
		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;
	}
	processCommit(defStatus);
}

处理提交

private void processCommit(DefaultTransactionStatus status) throws TransactionException {
	try {
		boolean beforeCompletionInvoked = false;
		try {
			boolean unexpectedRollback = false;
			// 为提交做准备，空方法，交给子类去实现
			prepareForCommit(status);			
			// newSynchronization=true，执行TransactionSynchronization子类的beforeCommit方法，提供给我们扩展实现，actualTransactionActive有东西的话，提交Connection、Session
			triggerBeforeCommit(status);
			// newSynchronization=true，执行TransactionSynchronization子类的beforeCompletion方法，提供给我们扩展实现，不被其它资源引用，（referenceCount=0）的话，解绑本地线程的resources，holderActive=false,关闭Connection、Session
			triggerBeforeCompletion(status);
			beforeCompletionInvoked = true;
			if (status.hasSavepoint()) {
				if (status.isDebug()) {
					logger.debug("Releasing transaction savepoint");
				}
				unexpectedRollback = status.isGlobalRollbackOnly();
				// releaseSavepoint,设置savepoint=null
				status.releaseHeldSavepoint();
			}
			else if (status.isNewTransaction()) {// newTransaction=true
				if (status.isDebug()) {
					logger.debug("Initiating transaction commit");
				}
				unexpectedRollback = status.isGlobalRollbackOnly();// 是否需要回滚
				doCommit(status);// 执行commit方法
			}
			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 {
			// newSynchronization=true的话，在提交之后触发，空方法，交给子类去实现
			triggerAfterCommit(status);
		}
		finally {
			// newSynchronization=true，清除当前线程的synchronizations，TransactionSynchronization子类的afterCompletion方法，给我们提供的扩展
			triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
		}
	}
	finally {
		// 设置TransactionStatus的completed=true，可能需要清除本地线程信息,重置connection,关闭连接，恢复挂起资源
		cleanupAfterCompletion(status);
	}
}

关键方法1：doCommit

提交事务

protected void doCommit(DefaultTransactionStatus status) {
    DataSourceTransactionManager.DataSourceTransactionObject txObject = (DataSourceTransactionManager.DataSourceTransactionObject)status.getTransaction();
    // 获取Connection对象
    Connection con = txObject.getConnectionHolder().getConnection();
    if (status.isDebug()) {
        this.logger.debug("Committing JDBC transaction on Connection [" + con + "]");
    }
    try {
    	// 执行提交
        con.commit();
    } catch (SQLException var5) {
        throw new TransactionSystemException("Could not commit JDBC transaction", var5);
    }
}

关键方法2：cleanupAfterCompletion

• 设置TransactionStatus的completed=true
• newSynchronization=true的话，清除当前线程的事务信息，仅保留事务资源ConnectionHolder
• newTransaction=true的话，移除线程中的ConnectionHolder，重置connection，清空ConnectionHolder和ResourceHolder事务状态（newConnectionHolder=true，移除当前线程的connection，关闭连接）
• suspendedResources有的话，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

private void cleanupAfterCompletion(DefaultTransactionStatus status) {
	status.setCompleted();// 设置completed=true
	if (status.isNewSynchronization()) {
		// 清除当前线程的事务信息，仅保留事务资源ConnectionHolder
		TransactionSynchronizationManager.clear();
	}
	if (status.isNewTransaction()) {
		// 移除当前线程的ConnectionHolder，重置connection
		doCleanupAfterCompletion(status.getTransaction());
	}
	if (status.getSuspendedResources() != null) {// 有挂起资源
		if (status.isDebug()) {
			logger.debug("Resuming suspended transaction after completion of inner transaction");
		}
		Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
		// 恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization
		resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
	}
}

// TransactionSynchronizationManager.java
public static void clear() {
	synchronizations.remove();
	currentTransactionName.remove();
	currentTransactionReadOnly.remove();
	currentTransactionIsolationLevel.remove();
	actualTransactionActive.remove();
}

protected void doCleanupAfterCompletion(Object transaction) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;

	// Remove the connection holder from the thread, if exposed.
	if (txObject.isNewConnectionHolder()) {
		// 移除当前线程的ConnectionHolder
		TransactionSynchronizationManager.unbindResource(obtainDataSource());
	}

	// Reset connection.
	Connection con = txObject.getConnectionHolder().getConnection();
	try {
		if (txObject.isMustRestoreAutoCommit()) {// mustRestoreAutoCommit=true,恢复自动提交
			con.setAutoCommit(true);
		}
		// 重置隔离级别和是否只读，con.setTransactionIsolation(上一个隔离级别);con.setReadOnly(false);
		DataSourceUtils.resetConnectionAfterTransaction(
				con, txObject.getPreviousIsolationLevel(), txObject.isReadOnly());
	}
	catch (Throwable ex) {
		logger.debug("Could not reset JDBC Connection after transaction", ex);
	}

	if (txObject.isNewConnectionHolder()) {
		if (logger.isDebugEnabled()) {
			logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
		}
		// 如果当前是事务连接，设置ConnectionHolder.currentConnection=null,否则关闭连接con.close()
		DataSourceUtils.releaseConnection(con, this.dataSource);
	}
	// 清空ConnectionHolder和ResourceHolder事务状态
	txObject.getConnectionHolder().clear();
}

public void clear() {
    this.synchronizedWithTransaction = false;
	this.rollbackOnly = false;
	this.deadline = null;
    this.transactionActive = false;
    this.savepointsSupported = null;
    this.savepointCounter = 0;
}

（1）实验

a、单事务

调用processCommit

• 执行提交/完成前置方法
• doCommit：提交事务，con.commit();
• 执行提交/完成后置方法
• cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息以及资源，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

b、多事务

外层方法默认是REQUIRED，表示有事务，没有事务的情况下就不用演示了，就是走我们的单事务逻辑，接下来我们还是分析以下代码，来看看不同传播行为的提交方式

@Transactional
public void m1(){
	forlanDao.update(xxx);
	forlanService.m2();
}

@Transactional(propagation = Propagation.REQUIRED)
// @Transactional(propagation = Propagation.NESTED)
// @Transactional(propagation = Propagation.REQUIRES_NEW)
// @Transactional(propagation = Propagation.NOT_SUPPORTED)
public void m2(){
	forlanDao.insert(xxx);
}

REQUIRED

REQUIRED，SUPPORTS，MANDATORY三者是一样的，都是有事务就加入当前事务，调用getTransaction方法得到的TransactionStatus是一样的。
先调用m2()方法的commitTransactionAfterReturning，执行过程如下：

• 调用processCommit
  • cleanupAfterCompletion：设置TransactionStatus的completed=true

再调用m1()方法的commitTransactionAfterReturning

• 调用processCommit
  • 执行提交/完成前置方法
  • doCommit：提交事务，con.commit();
  • 执行提交/完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

NESTED

先调用m2()方法的commitTransactionAfterReturning，执行过程如下：

• 调用processCommit
  • releaseHeldSavepoint：releaseSavepoint,设置savepoint=null
  • cleanupAfterCompletion：设置TransactionStatus的completed=true

再调用m1()方法的commitTransactionAfterReturning

• 调用processCommit
  • 执行提交/完成前置方法
  • doCommit：提交事务，con.commit();
  • 执行提交/完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

REQUIRES_NEW

先调用m2()方法的commitTransactionAfterReturning，执行过程如下：

• 调用processCommit
  • 执行提交/完成前置方法
  • doCommit：提交事务，con.commit();
  • 执行提交/完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

再调用m1()方法的commitTransactionAfterReturning

• 调用processCommit
  • 执行提交/完成前置方法
  • doCommit：提交事务，con.commit();
  • 执行提交/完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

NOT_SUPPORTED

先调用m2()方法的commitTransactionAfterReturning，执行过程如下：

• 调用processCommit
  • 执行提交/完成前置方法
  • 执行提交/完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

再调用m1()方法的commitTransactionAfterReturning

• 调用processCommit
  • 执行提交/完成前置方法
  • doCommit：提交事务，con.commit();
  • 执行提交/完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

（2）总结

• transaction有没有值，表示是否以事务方式执行
• newConnectionHolder=true，表示是否以新事务方式执行
• mustRestoreAutoCommit=true，前面关闭了自动提交，需要恢复自动提交
• suspendedResources表示是否有挂起资源，如果是加入当前事务，就不需要挂起
• newTransaction=true，提交事务，con.commit()；重置connection，清空ConnectionHolder和ResourceHolder事务状态（newConnectionHolder=true，移除当前线程的connection，关闭连接）
• newSynchronization=true，执行提交/完成前置方法以及后置方法，清除当前线程的事务信息，可以理解为后面还有一个新事务要执行，所以需要事务同步

3）rollback

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");
	}
	DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
	processRollback(defStatus, false);
}

执行回滚

private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
	try {
		boolean unexpectedRollback = unexpected;
		try {
			// newSynchronization=true，执行TransactionSynchronization子类的beforeCompletion方法，提供给我们扩展实现，不被其它资源引用，（referenceCount=0）的话，解绑本地线程的resources，holderActive=false,关闭Connection、Session
			triggerBeforeCompletion(status);
			if (status.hasSavepoint()) {
				if (status.isDebug()) {
					logger.debug("Rolling back transaction to savepoint");
				}
				status.rollbackToHeldSavepoint();
			}
			else if (status.isNewTransaction()) {
				if (status.isDebug()) {
					logger.debug("Initiating transaction rollback");
				}
				// 事务回滚，con.rollback();
				doRollback(status);
			}
			else {
				// Participating in larger transaction
				if (status.hasTransaction()) {
					if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
						if (status.isDebug()) {
							logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
						}
						// 设置ResourceHolder的rollbackOnly=true
						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;
		}
		// newSynchronization=true，清除当前线程的synchronizations，TransactionSynchronization子类的afterCompletion方法，给我们提供的扩展
		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 {
		// 设置TransactionStatus的completed=true，可能需要清除本地线程信息,重置connection,关闭连接，恢复挂起资源
		cleanupAfterCompletion(status);
	}
}

（1）实验

a、单事务

调用processRollback

• 执行前置完成方法
• doRollback：事务回滚，con.rollback();
• 执行后置完成方法
• cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息以及资源，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

b、多事务

外层方法默认是REQUIRED，表示有事务，没有事务的情况下就不用演示了，就是走我们的单事务逻辑，接下来我们还是分析以下代码，来看看不同传播行为的回滚方式

@Transactional
public void m1(){
	forlanDao.update(xxx);
	forlanService.m2();
}

@Transactional(propagation = Propagation.REQUIRED)
// @Transactional(propagation = Propagation.NESTED)
// @Transactional(propagation = Propagation.REQUIRES_NEW)
// @Transactional(propagation = Propagation.NOT_SUPPORTED)
public void m2(){
	forlanDao.insert(xxx);
	int i = 1/0;
}

REQUIRED

REQUIRED，SUPPORTS，MANDATORY三者是一样的，都是有事务就加入当前事务，调用getTransaction方法得到的TransactionStatus是一样的。
先调用m2()方法的completeTransactionAfterThrowing，执行过程如下：

• 调用processRollback
  • doSetRollbackOnly： 设置ResourceHolder的rollbackOnly=true
  • cleanupAfterCompletion：设置TransactionStatus的completed=true

再调用m1()方法的completeTransactionAfterThrowing

• 调用processRollback
  • 执行完成前置方法
  • doRollback：事务回滚，con.rollback();
  • 执行完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

NESTED

先调用m2()方法的completeTransactionAfterThrowing，执行过程如下：

• 调用processRollback
  • rollbackToHeldSavepoint：回滚到保存点，设置ResourceHolder的rollbackOnly=true，releaseSavepoint，设置savepoint=null
  • cleanupAfterCompletion：设置TransactionStatus的completed=true

再调用m1()方法的completeTransactionAfterThrowing

• 调用processRollback
  • 执行完成前置方法
  • doRollback：事务回滚，con.rollback();
  • 执行完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

特殊，如果我们在m1()方法中捕获了异常，这时候m1()方法怎么执行？

@Transactional
public void insert(ForlanA forlanA, ForlanB forlanB) {
	forlanADao.insert(forlanA);
	try {
		forlanBService.insert(forlanB);
	} catch (Exception e) {
		System.out.println("报错了");
	}
}

这时候m1()方法会调用commitTransactionAfterReturning，执行processCommit提交事务，会不会把我们的m2()方法执行的一起提交？
答案肯定是不会的，什么叫嵌套事务，内部回滚是不会影响外部的，前面我们说到，有保存点，会调用rollbackToHeldSavepoint，就是在这里面回滚到了保存点，我们看下源码

public void rollbackToHeldSavepoint() throws TransactionException {
	Object savepoint = getSavepoint();
	if (savepoint == null) {
		throw new TransactionUsageException(
				"Cannot roll back to savepoint - no savepoint associated with current transaction");
	}
	// 关键就在这，会进行事务回滚到保存点,使用的
	getSavepointManager().rollbackToSavepoint(savepoint);
	getSavepointManager().releaseSavepoint(savepoint);
	setSavepoint(null);
}

/**
 * This implementation rolls back to the given JDBC 3.0 Savepoint.
 * @see java.sql.Connection#rollback(java.sql.Savepoint)
 */
public void rollbackToSavepoint(Object savepoint) throws TransactionException {
	ConnectionHolder conHolder = getConnectionHolderForSavepoint();
	try {
		// 回滚到保存点，调用了Connection.rollback(savepoint)
		conHolder.getConnection().rollback((Savepoint) savepoint);
		conHolder.resetRollbackOnly();
	}
	catch (Throwable ex) {
		throw new TransactionSystemException("Could not roll back to JDBC savepoint", ex);
	}
}

REQUIRES_NEW

先调用m2()方法的completeTransactionAfterThrowing，执行过程如下：

• 调用processRollback
  • 执行完成前置方法
  • doRollback：事务回滚，con.rollback();
  • 执行完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

再调用m1()方法的completeTransactionAfterThrowing

• 调用processRollback
  • 执行完成前置方法
  • doRollback：事务回滚，con.rollback();
  • 执行完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态

假设父方法中报错了，此时m2()方法已经执行了commitTransactionAfterReturning， 事务已经提交，不会回滚

NOT_SUPPORTED

先调用m2()方法的completeTransactionAfterThrowing，执行过程如下：

• 调用processRollback
  • 执行完成前置方法
  • 执行完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

再调用m1()方法的completeTransactionAfterThrowing

• 调用processRollback
  • 执行完成前置方法
  • doRollback：事务回滚，con.rollback();
  • 执行完成后置方法
  • cleanupAfterCompletion：设置TransactionStatus的completed=true，清除当前线程的事务信息，移除线程中的ConnectionHolder，重置connection，关闭连接，清空ConnectionHolder和ResourceHolder事务状态，恢复挂起的资源，绑定到本地线程resources；恢复挂起的TransactionSynchronization

（2）总结

• savepoint不为空，回滚到保存点（代码已经回滚），释放保存点，savepoint
• newTransaction=true，回滚事务，con.rollback()；重置connection，清空ConnectionHolder和ResourceHolder事务状态（newConnectionHolder=true，移除当前线程的connection，关闭连接）
• transaction不为空，设置ResourceHolder的rollbackOnly=true
• mustRestoreAutoCommit=true，前面关闭了自动提交，需要恢复自动提交
• suspendedResources表示是否有挂起资源，如果是加入当前事务，就不需要挂起
• newSynchronization=true，执行完成前置方法以及后置方法，清除当前线程的事务信息，可以理解为后面还有一个新事务要执行，所以需要事务同步

三、事务源码串联起飞

1、通过debug模式来看看怎么调用事务

调试代码，打断点

可以看到，调用了 TransactionInterceptor 的 invoke 方法，这里就是开始处理事务的起点

@Nullable
public Object invoke(MethodInvocation invocation) throws Throwable {
    // 获取目标类
    Class<?> targetClass = invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null;
    // 获取目标方法
    Method var10001 = invocation.getMethod();
    invocation.getClass();
    return this.invokeWithinTransaction(var10001, targetClass, invocation::proceed);
}

调用了 TransactionAspectSupport 的 invokeWithinTransaction 方法

protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
		final InvocationCallback invocation) throws Throwable {

	// 如果事务属性为空，则该方法是非事务性的
	TransactionAttributeSource tas = getTransactionAttributeSource();
	// 通过事务属性源对象拿到方法的事务属性信息：propagationBehavior,readOnly...
	final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
	// 获取事务管理器对象
	final TransactionManager tm = determineTransactionManager(txAttr);

	if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager) {
		// 响应式编程逻辑
		....
	}
	// 将事务管理器强制转为平台事务管理器
	PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
	// 获取连接点唯一标识：目前方法具体路径
	final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

	if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
		// 创建事务
		TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

		Object retVal;
		try {
			// 调用目标对象方法
			retVal = invocation.proceedWithInvocation();
		}
		catch (Throwable ex) {
			// target invocation exception
			completeTransactionAfterThrowing(txInfo, ex);
			throw ex;
		}
		finally {
			// 清除事务信息
			cleanupTransactionInfo(txInfo);
		}

		if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
			// Set rollback-only in case of Vavr failure matching our rollback rules...
			TransactionStatus status = txInfo.getTransactionStatus();
			if (status != null && txAttr != null) {
				retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
			}
		}
		// // 执行commit操作
		commitTransactionAfterReturning(txInfo);
		return retVal;
	}

	else {
		// CallbackPreferringPlatformTransactionManager的实例，才执行的逻辑
		...
	}
}

调用了 TransactionAspectSupport的createTransactionIfNecessary方法，方法里面又调用了AbstractPlatformTransactionManager的 getTransaction方法，这个就是我们前面分析过的核心方法了，到这里已经关联上了

protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
		@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {
	// If no name specified, apply method identification as transaction name.
	if (txAttr != null && txAttr.getName() == null) {
		// 设置targetDefinition和targetAttribute属性值，和TransactionAttribute的一样，设置joinpointIdentification
		txAttr = new DelegatingTransactionAttribute(txAttr) {
			@Override
			public String getName() {
				return joinpointIdentification;
			}
		};
	}
	TransactionStatus status = null;
	if (txAttr != null) {
		if (tm != null) {
			status = tm.getTransaction(txAttr);
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
						"] because no transaction manager has been configured");
			}
		}
	}
	// 创建TransactionInfo并填充属性，并绑定到transactionInfoHolder线程对象中
	return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

private static final ThreadLocal<TransactionInfo> transactionInfoHolder =
			new NamedThreadLocal<>("Current aspect-driven transaction");

TransactionAttribute继承TransactionDefinition，propagationBehavior、isolationLevel，readOnly等属性会因我们注解填写的不同而不同 TransactionStatus会根据是单事务还是双事务，设置不同的属性值

2、TransactionInterceptor作为入口，如何注入容器中的？

TransactionInterceptor 就是事务处理的 advice，AOP中的增强方法
1）我们点击开启注解事务的注解@EnableTransactionManagement，可以看到里面@的Import

2）进入到TransactionManagementConfigurationSelector类，可以看到里面注入了ProxyTransactionManagementConfiguration类

3）进入到ProxyTransactionManagementConfiguration类，可以看到里面把TransactionInterceptor设置给advisor

四、提炼

1、@Transactional怎么实现事务的？

@Transactional就是我们说的声明式事务，其实就是把编程式事务的公共代码抽取出来去AOP实现，要在方法前后增强，所以用环绕通知，出现异常，异常通知进行回滚

// 进行前置增强：获取数据库连接，关闭自动提交，绑定Map<数据源，数据库连接>到线程对象
// @Transactional的信息其实会被解析为TransactionDefinition，最终得到TransactionStatus对象
TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

Object retVal;
try {
	// 调用目标对象方法
	retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
	// 出现异常，执行rollback操纵
	completeTransactionAfterThrowing(txInfo, ex);
	throw ex;
}
finally {
	// 设置之前的事务消息到当前线程transactionInfoHolder.set(this.oldTransactionInfo)
	cleanupTransactionInfo(txInfo);
}

// 执行commit操作
commitTransactionAfterReturning(txInfo);
return retVal;

2、怎么实现事务传播行为的？

如果需要了解不同传播行为的效果，可以查看这里

对于不同的传播行为，前面创建事务的时候，我们得到了TransactionStatus对象，关键在于里面的值不同，代码会去实现不同的效果
NEVER的话，是不会得到TransactionStatus，直接就抛异常了
其它情况得到的对象内部如下：

• NOT_SUPPORTED：transaction=null，suspendedResources=挂起资源，savepoint=null，newTransaction=false，newSynchronization= true
• REQUIRED，SUPPORTS，MANDATORY：transaction={newConnectionHolder = false，mustRestoreAutoCommit = false}，suspendedResources=null，savepoint=null，newTransaction=false，newSynchronization= false
• NESTED：transaction={newConnectionHolder = false，mustRestoreAutoCommit = false}，suspendedResources=null，savepoint=保存点，newTransaction=false，newSynchronization= false
• REQUIRES_NEW：transaction={newConnectionHolder = true，mustRestoreAutoCommit = true}，suspendedResources=挂起资源，savepoint=null，newTransaction=true，newSynchronization= true

主要区别就在于上面这些值，我们来分析下是怎么提交和回滚的

1）NOT_SUPPORTED

• transaction为null，表示不以事务的方式运行，不存在事务提交或回滚
• newSynchronization= true，表示需要同步事务信息，suspendedResources存在挂起资源，说明后面会新起事务执行

2）REQUIRED，SUPPORTS，MANDATORY

这三者得到的TransactionStatus对象是一样的，执行情况也是一样，我们来分析下

• transaction不为null，表示以事务的方式执行
• newTransaction=false，说明不会提交事务，newSynchronization=false，表示不需要同步事务，mustRestoreAutoCommit = false，说明后面需要恢复自动提交，这就说明是加入外层事务一起提交或回滚，无论是内层方法还是外层方法，报错都会一起回滚，属于加入事务行为

3）NESTED

它存在一个特殊的值，那就是savepoint，其他值基本和REQUIRED的一样，说明加入外层事务一起提交或回滚，真的是这样？savepoint有什么用？
那就是出现异常时，内层事务会直接进行回滚，Connection.rollback(savepoint)，说明内外层是不同事务，但是正常执行的话，是等最终才一起提交的，这种属于嵌套事务，比较特殊的是，内层事务异常的话，外层事务捕获了，外层事务是正常提交的

4）REQUIRES_NEW

首先transaction不为null，表示以事务的方式执行，newConnectionHolder=true，说明是新起事务执行，相对于内外层是两个不同的事务