@Service
public class DemoServiceImpl implements DemoService {@Autowiredprivate JdbcTemplate jdbcTemplate;@Autowiredprivate DemoServiceImpl self;@Transactional@Overridepublic void insertDB() {String sql = "INSERT INTO sys_user(`id`, `username`) VALUES (?, ?)";jdbcTemplate.update(sql, uuid(), "taven");try {// 内嵌事务将会回滚，而外部事务不会受到影响self.nested();} catch (Exception e) {e.printStackTrace();}}@Transactional(propagation = Propagation.NESTED)@Overridepublic void nested() {String sql = "INSERT INTO sys_user(`id`, `username`) VALUES (?, ?)";jdbcTemplate.update(sql, uuid(), "nested");throw new RuntimeException("rollback nested");}private String uuid() {return UUID.randomUUID().toString();}
}

// 伪代码
public Object invokeWithinTransaction() {// 开启事务connection.beginTransaction();try {// 反射执行方法Object result = invoke();// 提交事务connection.commit();return result;} catch(Exception e) {// 发生异常时回滚connection.rollback();throw e;} }

public interface PlatformTransactionManager extends TransactionManager {TransactionStatus getTransaction(@Nullable TransactionDefinition definition)throws TransactionException;void commit(TransactionStatus status) throws TransactionException;void rollback(TransactionStatus status) throws TransactionException;}

protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,final InvocationCallback invocation) throws Throwable {// If the transaction attribute is null, the method is non-transactional.TransactionAttributeSource tas = getTransactionAttributeSource();// 当前事务的属性 TransactionAttribute extends TransactionDefinitionfinal TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);// 事务属性中可以定义当前使用哪个事务管理器// 如果没有定义就去Spring上下文找到一个可用的 TransactionManagerfinal TransactionManager tm = determineTransactionManager(txAttr);// 省略了响应式事务的处理 ...PlatformTransactionManager ptm = asPlatformTransactionManager(tm);final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {// Standard transaction demarcation with getTransaction and commit/rollback calls.TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);Object retVal;try {// This is an around advice: Invoke the next interceptor in the chain.// This will normally result in a target object being invoked.// 如果有下一个拦截器则执行，最终会执行到目标方法，也就是我们的业务代码retVal = invocation.proceedWithInvocation();}catch (Throwable ex) {// target invocation exception// 当捕获到异常时完成当前事务 （提交或者回滚）completeTransactionAfterThrowing(txInfo, ex);throw ex;}finally {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);}}// 根据事务的状态提交或者回滚commitTransactionAfterReturning(txInfo);return retVal;}// 省略了编程式事务的处理 ...
}

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) {txAttr = new DelegatingTransactionAttribute(txAttr) {@Overridepublic String getName() {return joinpointIdentification;}};}TransactionStatus status = null;if (txAttr != null) {if (tm != null) {// 通过事务管理器开启事务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);
}

public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)throws TransactionException {// Use defaults if no transaction definition given.TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());// 获取当前事务，该方法有继承 AbstractPlatformTransactionManager 的子类自行实现Object transaction = doGetTransaction();boolean debugEnabled = logger.isDebugEnabled();// 如果目前存在事务if (isExistingTransaction(transaction)) {// Existing transaction found -> check propagation behavior to find out how to behave.return handleExistingTransaction(def, transaction, debugEnabled);}// Check definition settings for new transaction.if (def.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {throw new InvalidTimeoutException("Invalid transaction timeout", def.getTimeout());}// 传播类型PROPAGATION_MANDATORY, 要求当前必须有事务// No existing transaction found -> check propagation behavior to find out how to proceed.if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {throw new IllegalTransactionStateException("No existing transaction found for transaction marked with propagation 'mandatory'");}// PROPAGATION_REQUIRED, PROPAGATION_REQUIRES_NEW, PROPAGATION_NESTED 不存在事务时创建事务else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {SuspendedResourcesHolder suspendedResources = suspend(null);if (debugEnabled) {logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);}try {// 开启事务return startTransaction(def, transaction, debugEnabled, suspendedResources);}catch (RuntimeException | Error ex) {resume(null, suspendedResources);throw ex;}}else {// Create "empty" transaction: no actual transaction, but potentially synchronization.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);return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);}
}

@Override
protected Object doGetTransaction() {DataSourceTransactionObject txObject = new DataSourceTransactionObject();txObject.setSavepointAllowed(isNestedTransactionAllowed());ConnectionHolder conHolder =(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());txObject.setConnectionHolder(conHolder, false);return txObject;
}
​
@Override
protected boolean isExistingTransaction(Object transaction) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}

private static final ThreadLocal<Map<Object, Object>> resources =new NamedThreadLocal<>("Transactional resources");@Nullable
// TransactionSynchronizationManager::getResource
public static Object getResource(Object key) {// DataSourceTransactionManager 调用该方法时，以数据源作为key// TransactionSynchronizationUtils::unwrapResourceIfNecessary 如果key为包装类，则获取被包装的对象// 我们可以忽略该逻辑Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);Object value = doGetResource(actualKey);if (value != null && logger.isTraceEnabled()) {logger.trace("Retrieved value [" + value + "] for key [" + actualKey + "] bound to thread [" +Thread.currentThread().getName() + "]");}return value;
}/*** Actually check the value of the resource that is bound for the given key.*/
@Nullable
private static Object doGetResource(Object actualKey) {Map<Object, Object> map = resources.get();if (map == null) {return null;}Object value = map.get(actualKey);// Transparently remove ResourceHolder that was marked as void...if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) {map.remove(actualKey);// Remove entire ThreadLocal if empty...if (map.isEmpty()) {resources.remove();}value = null;}return value;
}

private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)throws TransactionException {// 如果事务的传播要求以非事务方式执行 抛出异常if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {throw new IllegalTransactionStateException("Existing transaction found for transaction marked with propagation 'never'");}// PROPAGATION_NOT_SUPPORTED 如果存在事务，则挂起当前事务，以非事务方式执行if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {if (debugEnabled) {logger.debug("Suspending current transaction");}// 挂起当前事务Object suspendedResources = suspend(transaction);boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);// 构建一个无事务的TransactionStatusreturn prepareTransactionStatus(definition, null, false, newSynchronization, debugEnabled, suspendedResources);}// PROPAGATION_REQUIRES_NEW 如果存在事务，则挂起当前事务，新建一个事务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 {return startTransaction(definition, transaction, debugEnabled, suspendedResources);}catch (RuntimeException | Error beginEx) {resumeAfterBeginException(transaction, suspendedResources, beginEx);throw beginEx;}}// PROPAGATION_NESTED 内嵌事务，就是我们开头举得例子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() + "]");}// 非JTA事务管理器都是通过savePoint实现的内嵌事务// savePoint：关系型数据库中事务可以创建还原点，并且可以回滚到还原点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.DefaultTransactionStatus status =prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);// 创建还原点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.return startTransaction(definition, transaction, debugEnabled, null);}}// 如果执行到这一步传播类型一定是，PROPAGATION_SUPPORTS 或者 PROPAGATION_REQUIRED// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.if (debugEnabled) {logger.debug("Participating in existing transaction");}// 校验目前方法中的事务定义和已存在的事务定义是否一致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);// 构建一个TransactionStatus，但不开启事务return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}

// DataSourceTransactionManager::doSuspend
@Override
protected Object doSuspend(Object transaction) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;txObject.setConnectionHolder(null);return TransactionSynchronizationManager.unbindResource(obtainDataSource());
}

protected static final class TransactionInfo {@Nullableprivate final PlatformTransactionManager transactionManager;@Nullableprivate final TransactionAttribute transactionAttribute;private final String joinpointIdentification;@Nullableprivate TransactionStatus transactionStatus;@Nullableprivate TransactionInfo oldTransactionInfo;// ...
}