关于java:浅入浅出-Spring-事务传播实现原理

本文和大家一起刨析 Spring 事务的相干源码，篇幅较长，代码片段较多，倡议应用电脑浏览

本文指标

• 了解 Spring 事务管理外围接口
• 了解 Spring 事务管理的外围逻辑
• 了解事务的流传类型及其实现原理

版本

SpringBoot 2.3.3.RELEASE

什么是事务的流传？

Spring 除了封装了事务管制之外，还形象出了 事务的流传 这个概念，事务的流传并不是关系型数据库所定义的，而是 Spring 在封装事务时做的加强扩大，能够通过@Transactional 指定事务的流传，具体类型如下

举个栗子

以嵌套事务为例

@Service
public class DemoServiceImpl implements DemoService {
    @Autowired
    private JdbcTemplate jdbcTemplate;

    @Autowired
    private DemoServiceImpl self;

    @Transactional
    @Override
    public 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，而内部事务不会受到影响
            self.nested();} catch (Exception e) {e.printStackTrace();
        }
    }

    @Transactional(propagation = Propagation.NESTED)
    @Override
    public 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();}
}

上述代码中，nested()办法标记了事务流传类型为嵌套，如果 nested() 中抛出异样仅会回滚 nested() 办法中的 sql，不会影响到 insertDB() 办法中曾经执行的 sql

留神：service 调用外部办法时，如果间接应用 this 调用，事务不会失效。因而应用 this 调用相当于跳过了内部的代理类，所以 AOP 不会失效，无奈应用事务

思考

家喻户晓，Spring 事务是通过 AOP 实现的，如果是咱们本人写一个 AOP 管制事务，该怎么做呢？

// 伪代码
public Object invokeWithinTransaction() {
    // 开启事务
    connection.beginTransaction();
    try {
        // 反射执行办法
        Object result = invoke();
        // 提交事务
        connection.commit();
        return result;
    } catch(Exception e) {
        // 产生异样时回滚
        connection.rollback();
        throw e;
    } 
    
}

在这个根底上，咱们来思考一下如果是咱们本人做的话，事务的流传该如何实现

以 PROPAGATION_REQUIRED 为例，这个仿佛很简略，咱们判断一下以后是否有事务（能够思考应用 ThreadLocal 存储已存在的事务对象），如果有事务，那么就不开启新的事务。反之，没有事务，咱们就创立新的事务

如果事务是由以后切面开启的，则提交 / 回滚事务，反之不做解决

那么事务流传中形容的挂起（暂停）以后事务，和内嵌事务是如何实现的？

<!–

这里提前剧透一下，内嵌事务是应用关系型数据库的 savepoint 实现的
–>

源码动手

要浏览事务流传相干的源码，咱们先来理解下 Spring 事务管理的外围接口与类

1. TransactionDefinition

该接口定义了事务的所有属性（隔离级别，流传类型，超时工夫等等），咱们日常开发中常常应用的 @Transactional 其实最终会被转化为 TransactionDefinition

1. TransactionStatus

事务的状态，以最罕用的实现 DefaultTransactionStatus 为例，该类存储了以后的事务对象，savepoint，以后挂起的事务，是否实现，是否仅回滚等等

1. TransactionManager

这是一个空接口，间接继承他的 interface 有 PlatformTransactionManager（咱们平时用的就是这个，默认的实现类 DataSourceTransactionManager）以及
ReactiveTransactionManager（响应式事务管理器，因为不是本文重点，咱们不多说）

从上述两个接口来看，TransactionManager 的次要作用

• 通过 TransactionDefinition 开启一个事务，返回 TransactionStatus
• 通过 TransactionStatus 提交、回滚事务（理论开启事务的 Connection 通常存储在 TransactionStatus 中）

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

    
    void commit(TransactionStatus status) throws TransactionException;

    
    void rollback(TransactionStatus status) throws TransactionException;

}

1. TransactionInterceptor

事务拦截器，事务 AOP 的外围类（反对响应式事务，编程式事务，以及咱们罕用的规范事务），因为篇幅起因，本文只探讨规范事务的相干实现

上面咱们从事务逻辑的入口 TransactionInterceptor 动手，来看下 Spring 事务管理的外围逻辑以及事务流传的实现

TransactionInterceptor

TransactionInterceptor 实现了 MethodInvocation（这是实现 AOP 的一种形式），其外围逻辑在父类TransactionAspectSupport 中，办法地位：TransactionInterceptor::invokeWithinTransaction

    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();
        // 以后事务的属性 TransactionAttribute extends TransactionDefinition
        final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
        // 事务属性中能够定义以后应用哪个事务管理器
        // 如果没有定义就去 Spring 上下文找到一个可用的 TransactionManager
        final TransactionManager tm = determineTransactionManager(txAttr);

        // 省略了响应式事务的解决 ...
        
        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，最终会执行到指标办法，也就是咱们的业务代码
                retVal = invocation.proceedWithInvocation();}
            catch (Throwable ex) {
                // target invocation exception
                // 当捕捉到异样时实现以后事务（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);
                }
            }
            // 依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解决 ...
    }

这里代码很多，依据正文的地位，咱们能够把外围逻辑梳理进去

1. 获取以后事务属性，事务管理器（以注解事务为例，这些都能够通过 @Transactional 来定义）
2. createTransactionIfNecessary，判断是否有必要创立事务
3. invocation.proceedWithInvocation 执行拦截器链，最终会执行到指标办法
4. completeTransactionAfterThrowing当抛出异样后，实现这个事务，提交或者回滚，并抛出这个异样
5. commitTransactionAfterReturning 从办法命名来看，这个办法会提交事务。

然而深刻源码中会发现，该办法中也蕴含回滚逻辑，具体行为会依据以后 TransactionStatus 的一些状态来决定（也就是说，咱们也能够通过设置以后 TransactionStatus，来管制事务回滚，并不一定只能通过抛出异样），详见AbstractPlatformTransactionManager::commit

<!–

业务代码中能够通过 TransactionAspectSupport.currentTransactionStatus() 获取以后 TransactionStatus
–>

咱们持续，来看看 createTransactionIfNecessary 做了什么

TransactionAspectSupport::createTransactionIfNecessary

    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) {
                // 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);
    }

createTransactionIfNecessary 中的外围逻辑

1. 通过 PlatformTransactionManager（事务管理器）开启事务
2. prepareTransactionInfo 筹备事务信息，这个具体做了什么咱们稍后再讲

持续来看PlatformTransactionManager::getTransaction，该办法只有一个实现 AbstractPlatformTransactionManager::getTransaction

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

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

        // 获取以后事务，该办法有继承 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());
        }

        // 流传类型 PROPAGATION_MANDATORY, 要求以后必须有事务
        // 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 不存在事务时创立事务
        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);
        }
    }

代码很多，重点关注正文局部即可

1. doGetTransaction获取以后事务
2. 如果存在事务，则调用 handleExistingTransaction 解决，这个咱们稍后会讲到

接下来，会依据事务的流传决定是否开启事务

1. 如果事务流传类型为PROPAGATION_MANDATORY，且不存在事务，则抛出异样
2. 如果流传类型为 PROPAGATION_REQUIRED, PROPAGATION_REQUIRES_NEW, PROPAGATION_NESTED，且以后不存在事务，则调用 startTransaction 创立事务
3. 当不满足 3、4 时，例如 PROPAGATION_NOT_SUPPORTED，此时会执行 事务同步，然而不会创立真正的事务

Spring 事务同步 在之前一篇博客中有讲到，传送门????https://www.jianshu.com/p/788…

Spring 如何治理以后的事务

接下来讲讲下面提到的doGetTransaction、handleExistingTransaction，这两个办法是由不同的 TransactionManager 自行实现的

咱们以 SpringBoot 默认的 TransactionManager，DataSourceTransactionManager 为例

    @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());
    }

联合 AbstractPlatformTransactionManager::getTransaction 一起来看，doGetTransaction 其实获取的是以后的 Connection。
判断以后是否存在事务，是判断 DataSourceTransactionObject 对象中是否蕴含 connection，以及 connection 是否开启了事务。

咱们持续来看下 TransactionSynchronizationManager.getResource(obtainDataSource()) 获取以后 connection 的逻辑

TransactionSynchronizationManager::getResource

    private static final ThreadLocal<Map<Object, Object>> resources =
            new NamedThreadLocal<>("Transactional resources");
    
    @Nullable
    // TransactionSynchronizationManager::getResource
    public static Object getResource(Object key) {
        // DataSourceTransactionManager 调用该办法时，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
        // 咱们能够疏忽该逻辑
        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;
    }

看到这里，咱们能明确 DataSourceTransactionManager 是如何治理线程之间的 Connection，ThreadLocal 中存储一个 Map，key 为数据源对象，value 为该数据源在以后线程的 Connection

DataSourceTransactionManager 在开启事务后，会调用 TransactionSynchronizationManager::bindResource 将指定数据源的 Connection 绑定到以后线程

AbstractPlatformTransactionManager::handleExistingTransaction

咱们持续回头看，如果存在事务的状况，如何解决

    private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)
            throws TransactionException {

        // 如果事务的流传要求以非事务形式执行 抛出异样
        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，则挂起以后事务，以非事务形式执行
        if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {if (debugEnabled) {logger.debug("Suspending current transaction");
            }
            // 挂起以后事务
            Object suspendedResources = suspend(transaction);
            boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
            // 构建一个无事务的 TransactionStatus
            return prepareTransactionStatus(definition, null, false, newSynchronization, debugEnabled, suspendedResources);
        }

        // PROPAGATION_REQUIRES_NEW 如果存在事务，则挂起以后事务，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，就是咱们结尾举得例子
        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能够创立还原点，并且能够回滚到还原点
            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);
                // 创立还原点
                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);
            }
        }

        // 如果执行到这一步流传类型肯定是，PROPAGATION_SUPPORTS 或者 PROPAGATION_REQUIRED
        // Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
        if (debugEnabled) {logger.debug("Participating in existing transaction");
        }
        
        // 校验目前办法中的事务定义和已存在的事务定义是否统一
        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);
    }

这里代码很多，逻辑看上述正文即可。这里终于看到了期待已久的 挂起事务和内嵌事务 了，咱们还是看一下 DataSourceTransactionManager 的实现

• 挂起事务：通过TransactionSynchronizationManager::unbindResource 依据数据源获取以后的 Connection，并在 resource 中移除该 Connection。之后会将该 Connection 存储到 TransactionStatus 对象中

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

在事务提交或者回滚后，调用 AbstractPlatformTransactionManager::cleanupAfterCompletion会将 TransactionStatus 中缓存的 Connection 从新绑定到 resource 中

• 内嵌事务：通过关系型数据库的 savePoint 实现，提交或回滚的时候会判断如果以后事务为 savePoint 则开释 savePoint 或者回滚到 savePoint，具体逻辑参考AbstractPlatformTransactionManager::processRollback 和 AbstractPlatformTransactionManager::processCommit

至此，事务的流传源码剖析完结

prepareTransactionInfo

上文留下了一个问题，prepareTransactionInfo 办法做了什么，咱们先来看下 TransactionInfo 的构造

    protected static final class TransactionInfo {

        @Nullable
        private final PlatformTransactionManager transactionManager;

        @Nullable
        private final TransactionAttribute transactionAttribute;

        private final String joinpointIdentification;

        @Nullable
        private TransactionStatus transactionStatus;

        @Nullable
        private TransactionInfo oldTransactionInfo;
        
        // ...
    }

该类在 Spring 中的作用，是为了外部传递对象。ThreadLocal 中存储了最新的 TransactionInfo，通过以后 TransactionInfo 能够找到他的 oldTransactionInfo。每次创立事务时会新建一个 TransactionInfo（无论有没有真正的事务被创立）存储到 ThreadLocal 中，在每次事务完结后，会将以后 ThreadLocal 中的 TransactionInfo 重置为 oldTransactionInfo，这样的构造造成了一个链表，使得 Spring 事务在逻辑上能够有限嵌套上来

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