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Spring事务融入(REQUIRED)具体实现步骤解析

news 来源：原创 2025/5/11 7:04:00

Spring事务融入(REQUIRED传播行为)是Spring事务管理中最核心的机制，下面我将深入剖析其具体实现步骤和关键代码逻辑。

1. 整体流程概览

事务融入(REQUIRED)的核心逻辑是：

检查当前线程是否存在事务
存在则融入(加入)该事务
不存在则创建新事务

2. 详细实现步骤

步骤1：事务获取入口

从@Transactional注解或编程式事务调用开始，最终会进入PlatformTransactionManager的实现类：

// AbstractPlatformTransactionManager.java
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) {// 获取或创建事务对象Object transaction = doGetTransaction();// 检查是否存在事务if (isExistingTransaction(transaction)) {// 存在事务则处理融入逻辑return handleExistingTransaction(def, transaction, debugEnabled);}// 不存在事务则创建新事务return startTransaction(def, transaction, debugEnabled, suspendedResources);
}

步骤2：判断是否存在事务

以DataSourceTransactionManager为例：

// DataSourceTransactionManager.java
@Override
protected boolean isExistingTransaction(Object transaction) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;// 关键判断：连接持有者不为null且连接已开启事务return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}

步骤3：处理已存在事务(融入核心)

// AbstractPlatformTransactionManager.java
private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled) {// 1. 处理NEVER传播行为if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {throw new IllegalTransactionStateException(...);}// 2. 处理NOT_SUPPORTED传播行为(挂起现有事务)if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {Object suspendedResources = suspend(transaction);return prepareTransactionStatus(...);}// 3. 处理REQUIRES_NEW传播行为(挂起并新建)if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {SuspendedResourcesHolder suspendedResources = suspend(transaction);return startTransaction(definition, transaction, debugEnabled, suspendedResources);}// 4. 处理NESTED传播行为(创建保存点)if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {if (!isNestedTransactionAllowed()) {throw new NestedTransactionNotSupportedException(...);}Object savepoint = createSavepoint(transaction);return prepareTransactionStatus(...);}// 5. 默认处理REQUIRED/SUPPORTS/MANDATORY(融入现有事务)if (debugEnabled) {logger.debug("Participating in existing transaction");}// 关键点：设置"新事务"标志为false，表示融入现有事务return prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
}

步骤4：准备事务状态(融入关键)

// AbstractPlatformTransactionManager.java
protected final DefaultTransactionStatus prepareTransactionStatus(TransactionDefinition definition, Object transaction, boolean newTransaction, boolean newSynchronization,boolean debug, @Nullable Object suspendedResources) {// 创建事务状态对象DefaultTransactionStatus status = newTransactionStatus(definition, transaction, newTransaction, newSynchronization,debug, suspendedResources);// 初始化事务同步(重要!)prepareSynchronization(status, definition);return status;
}

步骤5：事务同步准备

// AbstractPlatformTransactionManager.java
protected void prepareSynchronization(DefaultTransactionStatus status,TransactionDefinition definition) {if (status.isNewSynchronization()) {// 初始化新事务的同步器TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(definition.getIsolationLevel());TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());// 初始化同步回调列表TransactionSynchronizationManager.initSynchronizations();}
}

3. 关键实现技术

3.1 线程绑定机制

Spring通过TransactionSynchronizationManager维护线程绑定资源：

// TransactionSynchronizationManager.java
private static final ThreadLocal<Map<Object, Object>> resources =new NamedThreadLocal<>("Transactional resources");public static void bindResource(Object key, Object value) {Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);Map<Object, Object> map = resources.get();if (map == null) {map = new HashMap<>();resources.set(map);}Object oldValue = map.put(actualKey, value);// 检查是否重复绑定if (oldValue != null) {throw new IllegalStateException(...);}
}

3.2 连接资源管理

DataSourceTransactionManager对Connection的管理：

// DataSourceTransactionManager.java
@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;Connection con = null;try {// 获取或创建连接if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {Connection newCon = obtainDataSource().getConnection();txObject.setConnectionHolder(new ConnectionHolder(newCon), true);}// 设置事务属性con = txObject.getConnectionHolder().getConnection();Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);// 设置自动提交为falseif (con.getAutoCommit()) {con.setAutoCommit(false);}// 绑定到当前线程bindResource(obtainDataSource(), txObject.getConnectionHolder());}catch (Throwable ex) {// 异常处理...}
}

4. 事务融入的典型场景

场景1：ServiceA调用ServiceB

// ServiceA
@Transactional(propagation = Propagation.REQUIRED)
public void methodA() {// 操作1serviceB.methodB(); // 融入同一事务// 操作2
}// ServiceB
@Transactional(propagation = Propagation.REQUIRED)
public void methodB() {// 操作3
}

执行流程：

methodA开始时创建新事务
调用methodB时检测到已有事务，直接融入
所有操作在同一个物理事务中执行

场景2：跨数据源事务融入

// 主数据源事务
@Transactional("primaryTransactionManager")
public void primaryOperation() {// 操作主库secondaryOperation(); // 调用次数据源操作
}// 次数据源事务
@Transactional("secondaryTransactionManager", propagation = Propagation.REQUIRED)
public void secondaryOperation() {// 操作从库
}

关键点：