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Spring事务实现原理源码分析

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导读：本篇文章讲解 Spring事务实现原理源码分析，希望对大家有帮助，欢迎收藏，转发！站点地址：www.bmabk.com

Spring事务实现原理源码分析

原创2021-06-14 09:21·FastCoder

环境：spring5.3.3

先阅读：《Spring IOC容器对Bean实例化的过程详解源码分析》

目录：

Spring事务实现原理源码分析

 

1、开启事务

通过@
EnableTransactionManagement注解开启事务功能

@Configuration
@EnableTransactionManagement
public class AnnoConfig {
}    
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
    //...
}    

2、注册相应处理器及切面(Advisor)定义

查看
TransactionManagementConfigurationSelector类

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

  @Override
  protected String[] selectImports(AdviceMode adviceMode) {
    switch (adviceMode) {
      case PROXY:
        return new String[] {AutoProxyRegistrar.class.getName(),
          ProxyTransactionManagementConfiguration.class.getName()};
      case ASPECTJ:
        return new String[] {determineTransactionAspectClass()};
      default:
        return null;
    }
  }
  //...
}

AutoProxyRegistrar类主要用来注册一个BeanPostProcessor

public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
    private final Log logger = LogFactory.getLog(getClass());
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        boolean candidateFound = false;
        Set<String> annTypes = importingClassMetadata.getAnnotationTypes();
        for (String annType : annTypes) {
            AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
            if (candidate == null) {
                continue;
            }
            Object mode = candidate.get("mode");
            Object proxyTargetClass = candidate.get("proxyTargetClass");
            if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
                    Boolean.class == proxyTargetClass.getClass()) {
                candidateFound = true;
                if (mode == AdviceMode.PROXY) {
                    AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
                    if ((Boolean) proxyTargetClass) {
                        AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
                        return;
                    }
                }
            }
        }
        // ...
    }

}

AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);注册BeanPostProcessor

@Nullable
public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
    return registerAutoProxyCreatorIfNecessary(registry, null);
}

@Nullable
public static BeanDefinition registerAutoProxyCreatorIfNecessary(
    BeanDefinitionRegistry registry, @Nullable Object source) {

    return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
}

InfrastructureAdvisorAutoProxyCreator是一个实现了SmartInstantiationAwareBeanPostProcessor接口（该接口会在Bean实例化前和实例化后分别执行响应动作）的处理器。

注意：如果你的配置类中开启了AOP代理@EnableAspectJAutoProxy那么这个
InfrastructureAdvisorAutoProxyCreator是不会被注册的。

public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

    // 定义切面（切面定义也可以通过@Aspect）；该Bean在Spring IOC容器初始化时会对符合条件的Bean进行关联生成代理对象
    @Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor(
            TransactionAttributeSource transactionAttributeSource, TransactionInterceptor transactionInterceptor) {

        BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
        advisor.setTransactionAttributeSource(transactionAttributeSource);
        advisor.setAdvice(transactionInterceptor);
        if (this.enableTx != null) {
            advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
        }
        return advisor;
    }

    // 注解事务属性的解析
    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionAttributeSource transactionAttributeSource() {
        return new AnnotationTransactionAttributeSource();
    }

    // 定义事务通知拦截器
    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionInterceptor transactionInterceptor(TransactionAttributeSource transactionAttributeSource) {
        TransactionInterceptor interceptor = new TransactionInterceptor();
        interceptor.setTransactionAttributeSource(transactionAttributeSource);
        if (this.txManager != null) {
            interceptor.setTransactionManager(this.txManager);
        }
        return interceptor;
    }

}

3、代码执行流程

通知类TransactionInterceptor拦截器

3.1 执行通知类的invoke方法

当在调用业务方法（业务类已经被通过cglib生成了代理类）的时候，首先执行该通知类的invoke方法

public Object invoke(MethodInvocation invocation) throws Throwable {
    Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

    return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {
        @Override
        @Nullable
        public Object proceedWithInvocation() throws Throwable {
            return invocation.proceed();
        }
        @Override
        public Object getTarget() {
            return invocation.getThis();
        }
        @Override
        public Object[] getArguments() {
            return invocation.getArguments();
        }
    });
}

3.2 创建事务对象信息

进入invokeWithinTransaction方法

@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.
    TransactionAttributeSource tas = getTransactionAttributeSource();
    final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
    // 获取事务管理对象
    final TransactionManager tm = determineTransactionManager(txAttr);

    // ...

    // 事务对象转换
    PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
    // 获取连接点完整的信息（标识符，完整包名+类名+方法）
    final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

    if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
        // Standard transaction demarcation with getTransaction and commit/rollback calls.
        // 如果需要，根据给定的TransactionAttribute创建事务对象
        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.
            // 调用下个拦截器（如果有）或者目标方法
            retVal = invocation.proceedWithInvocation();
        }
        catch (Throwable ex) {
            // target invocation exception
            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);
            }
        }
        
        commitTransactionAfterReturning(txInfo);
        return retVal;
    }
    // ...
}

相关重要注释已经在代码中注释了

进入
createTransactionIfNecessary方法

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) {
            @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");
            }
        }
    }
    return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

进入 tm.getTransaction(txAttr)代码的执行

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

    // Use defaults if no transaction definition given.
        TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

        Object transaction = doGetTransaction();
        boolean debugEnabled = logger.isDebugEnabled();

        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());
        }

        // 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'");
        }
        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 {
                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);
        }

3.2.1 从当前上下文中获取事务信息

先判断当前上下文中是否已存在事务。

继续计入doGetTransaction()方法的执行

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

这行代码获取Connection数据库连接对象

protected DataSource obtainDataSource() {
    DataSource dataSource = getDataSource();
    Assert.state(dataSource != null, "No DataSource set");
    return dataSource;
}

getResource方法

public static Object getResource(Object key) {
    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;
}

进入doGetResource方法

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

首次conHolder获取的为null。

3.2.2 正常流程执行创建事务对象

如果当前上下文中没有事务，将按照正常的流程继续执行。

判断当前事务的定义中判断事务的传播行为，如果是需要事务的将执行startTransaction方法中的如下代码

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 {
        return startTransaction(def, transaction, debugEnabled, suspendedResources);
    }
    catch (RuntimeException | Error ex) {
        resume(null, suspendedResources);
        throw ex;
    }
}
private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,
            boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {

    boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
    DefaultTransactionStatus status = newTransactionStatus(
        definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
    doBegin(transaction, definition);
    prepareSynchronization(status, definition);
    return status;
}

进入doBegin方法

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

    try {
        if (!txObject.hasConnectionHolder() ||
            txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
            // 从数据源获取一个连接对象
            Connection newCon = obtainDataSource().getConnection();
            if (logger.isDebugEnabled()) {
                logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
            }
            // 将Connection包装放到ConnectionHolder中并且设置到DataSourceTransactionObject里
            txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
        }

        txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
        con = txObject.getConnectionHolder().getConnection();
        // 下面代码设置事务隔离级别，只读，是否自动提交等。
        Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
        txObject.setPreviousIsolationLevel(previousIsolationLevel);
        txObject.setReadOnly(definition.isReadOnly());

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

        prepareTransactionalConnection(con, definition);
        txObject.getConnectionHolder().setTransactionActive(true);

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

        // Bind the connection holder to the thread.
        if (txObject.isNewConnectionHolder()) {
            // 将当前的连接对象（ConnectionHolder）绑定到当前的线程上（ThreadLocal）
            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);
    }
}
private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources");
public static void bindResource(Object key, Object value) throws IllegalStateException {
    Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
    Assert.notNull(value, "Value must not be null");
    Map<Object, Object> map = resources.get();
    // set ThreadLocal Map if none found
    if (map == null) {
        map = new HashMap<>();
        resources.set(map);
    }
    Object oldValue = map.put(actualKey, value);
    // Transparently suppress a ResourceHolder that was marked as void...
    if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
        oldValue = null;
    }
    if (oldValue != null) {
        throw new IllegalStateException("Already value [" + oldValue + "] for key [" +
                                        actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
    }
    if (logger.isTraceEnabled()) {
        logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" +
                     Thread.currentThread().getName() + "]");
    }
}

到这startTransaction方法就是进行了获取数据库连接对象Connection然后保存到ThreadLocal中，并且设置了当前连接对象相关事务的属性等。

3.2.3 存储事务相关信息

回到
createTransactionIfNecessary方法接着执行最后的return中的方法

return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);

该方法的作用就是把事务管理器，事务属性，连接点信息，事务的状态等信息通过TransactionInfo对象保存到ThreadLocal中。

protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
                                                 @Nullable TransactionAttribute txAttr, String joinpointIdentification,
                                                 @Nullable TransactionStatus status) {

    TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
    // ...
    txInfo.bindToThread();
    return txInfo;
}

3.2.4 绑定当前事务信息到Thread

bindToThread方法

private static final ThreadLocal<TransactionInfo> transactionInfoHolder = new NamedThreadLocal<>("Current aspect-driven transaction");
private void bindToThread() {
    // Expose current TransactionStatus, preserving any existing TransactionStatus
    // for restoration after this transaction is complete.
    this.oldTransactionInfo = transactionInfoHolder.get();
    transactionInfoHolder.set(this);
}

4、拦截器链或实际业务方法执行

到这又回到invokeWithinTransaction方法继续向下执行

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

    if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
        try {
            retVal = invocation.proceedWithInvocation();
        }
        catch (Throwable ex) {
            completeTransactionAfterThrowing(txInfo, ex);
            throw ex;
        }
        finally {
            cleanupTransactionInfo(txInfo);
        }
        commitTransactionAfterReturning(txInfo);
        return retVal;
    }
    // ...
}

retVal = invocation.proceedWithInvocation()

这行代码核心了就是调用下一个拦截器（如果有），如果没有拦截器调用那将调用我们的业务方法了。

4.1 事务提交

// 提交事务
commitTransactionAfterReturning(txInfo);
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());
    }
}

进入commit方法

public final void commit(TransactionStatus status) throws TransactionException {
    // ...
    processCommit(defStatus);
}

进入processCommit方法

private void processCommit(DefaultTransactionStatus status) throws TransactionException {
    try {
        boolean beforeCompletionInvoked = false;

        try {
            // ...
            if (status.hasSavepoint()) {
                // ...
            }
            else if (status.isNewTransaction()) {
                if (status.isDebug()) {
                    logger.debug("Initiating transaction commit");
                }
                unexpectedRollback = status.isGlobalRollbackOnly();
                doCommit(status);
            }
            else if (isFailEarlyOnGlobalRollbackOnly()) {
                unexpectedRollback = status.isGlobalRollbackOnly();
            }

            // ...
        }
        // ...
        try {
            triggerAfterCommit(status);
        }
        finally {
            triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
        }

    }
    finally {
        cleanupAfterCompletion(status);
    }
}

进入doCommit方法

protected void doCommit(DefaultTransactionStatus status) {
    DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
    Connection con = txObject.getConnectionHolder().getConnection();
    if (status.isDebug()) {
        logger.debug("Committing JDBC transaction on Connection [" + con + "]");
    }
    try {
        con.commit();
    }
    catch (SQLException ex) {
        throw translateException("JDBC commit", ex);
    }
}

这里是真正事务提交的地方。

4.2 打扫战场

最后cleanupAfterCompletion方法会清空当前上下文中的相关信息。

private void cleanupAfterCompletion(DefaultTransactionStatus status) {
    status.setCompleted();
    if (status.isNewSynchronization()) {
        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);
        resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
    }
}

进入
TransactionSynchronizationManager.clear()方法

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

进入doCleanupAfterCompletion方法

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

    // Remove the connection holder from the thread, if exposed.
    if (txObject.isNewConnectionHolder()) {
        TransactionSynchronizationManager.unbindResource(obtainDataSource());
    }

    // Reset connection.
    Connection con = txObject.getConnectionHolder().getConnection();
    try {
        if (txObject.isMustRestoreAutoCommit()) {
            con.setAutoCommit(true);
        }
        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");
        }
        DataSourceUtils.releaseConnection(con, this.dataSource);
    }

    txObject.getConnectionHolder().clear();
}

该方法主要是恢复当前Connection连接对象的默认值（是否只读，隔离级别）；将当前的Connection对象归还到连接池中。

到此整个事务就执行完毕了。

给个关注+转发呗谢谢

公众：Springboot实战案例锦集

Spring事务实现原理源码分析

Springboot基础使用@Conditional多条件注册Bean

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