Spring 事务原理总结七

今天是二零二四年二月十八，农历正月初九。同时今天也是农历新年假期后的第一个工作日。我的内心既兴奋，又担忧，更急躁。兴奋是因为假期后的第一个工作日工作轻松；担忧是因为经过了这么长时间，我依旧没搞明白Spring事务，总觉得有些东西没搞懂，却又不知道是哪里，所以只能在这里徘徊不前；急躁是因为许多事情都要用钱去解决，自己那微薄的收入根本就是杯水车薪，而自己又不知道用哪种方式去筹集足够的资金来解决问题，所以只能整日急不可耐，而问题依旧。到底该怎么办呢？∙∙∙∙∙∙

针对第二种心理，我觉得没有其他办法，只能继续梳理，但也不能因此而放弃后面知识点的梳理。今天就让我们来聊一下Spring事务的几个面试题吧！我想围绕下面这样几个问题展开：

1. 《Spring事务原理总结四》这篇文章中提到的将TransactionInfo绑定到当前线程的意义是什么呢？
2. Spring是如何解决事务嵌套的？

首先回忆一下前面几篇文章的主要内容：《Spring事务原理总结一》主要梳理了事务的基本概念及特征，同时也梳理了Spring事务的基本用法；《Spring事务原理总结二》则主要梳理了Spring框架解析及注册事务代理的流程；《Spring事务原理总结三》则主要梳理了Spring事务的一些核心组件及其继承结构；《Spring事务原理总结四》梳理了Spring事务的执行流程；《Spring事务原理总结五》主要梳理并回答了前一篇文章中遗留的几个问题；《Spring事务原理总结六》主要梳理了Spring事务的异常回滚流程。接着让我们试着利用这些文章梳理的内容来回答这两个问题。

首先看第一个问题，将TransactionInfo对象绑定到当前线程的操作，在《Spring事务原理总结四》这篇文章中有提到，如下图所示：

当时只是说了这些代码位于TransactionAspectSupport类中，不过并没有深究其意义。现在让我们细化一下这个说法，然后在梳理完第二个问题后来回答这个问题。TransactionInfos是TransactionAspectSupport类中的最终静态内部类（该类内部定义了bindToThread()方法和restoreThreadLocalStatus()方法。所以截图中说前者位于TransactionAspectSupport类的说法也是对的，因为其所在的类位于TransactionAspectSupport类中），其源码如下所示：

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;
 
    public TransactionInfo(@Nullable PlatformTransactionManager transactionManager,
          @Nullable TransactionAttribute transactionAttribute, String joinpointIdentification) {
 
       this.transactionManager = transactionManager;
       this.transactionAttribute = transactionAttribute;
       this.joinpointIdentification = joinpointIdentification;
    }
 
    public PlatformTransactionManager getTransactionManager() {
       Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");
       return this.transactionManager;
    }
 
    @Nullable
    public TransactionAttribute getTransactionAttribute() {
       return this.transactionAttribute;
    }
 
    /**
     * Return a String representation of this joinpoint (usually a Method call)
     * for use in logging.
     */
    public String getJoinpointIdentification() {
       return this.joinpointIdentification;
    }
 
    public void newTransactionStatus(@Nullable TransactionStatus status) {
       this.transactionStatus = status;
    }
 
    @Nullable
    public TransactionStatus getTransactionStatus() {
       return this.transactionStatus;
    }
 
    /**
     * Return whether a transaction was created by this aspect,
     * or whether we just have a placeholder to keep ThreadLocal stack integrity.
     */
    public boolean hasTransaction() {
       return (this.transactionStatus != null);
    }
 
    private void bindToThread() {
       // Expose current TransactionStatus, preserving any existing TransactionStatus
       // for restoration after this transaction is complete.
       this.oldTransactionInfo = transactionInfoHolder.get();
       transactionInfoHolder.set(this);
    }
 
    private void restoreThreadLocalStatus() {
       // Use stack to restore old transaction TransactionInfo.
       // Will be null if none was set.
       transactionInfoHolder.set(this.oldTransactionInfo);
    }
 
    @Override
    public String toString() {
       return (this.transactionAttribute != null ? this.transactionAttribute.toString() : "No transaction");
    }
}

这个类上有bindToThread()方法和restoreThreadLocalStatus()两个方法，它们的主要作用就是更改当前线程持有的ThreadLocal上的TransactionInfo值。

下面让我们来看一下第二个问题，要回答这个问题，需要从TransactionInterceptor的invoke(MethodInvocation)方法看起，这个方法的代码如下所示：

@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);
    final TransactionManager tm = determineTransactionManager(txAttr);
 
    // ……
// 这里删除了一些无用代码
 
    PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
    final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
 
    if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager cpptm)) {
       // 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.
          retVal = invocation.proceedWithInvocation();
       }
       catch (Throwable ex) {
          // target invocation exception
          completeTransactionAfterThrowing(txInfo, ex);
          throw ex;
       }
       finally {
          cleanupTransactionInfo(txInfo);
       }
 
       if (retVal != null && txAttr != null) {
          TransactionStatus status = txInfo.getTransactionStatus();
          if (status != null) {
             if (retVal instanceof Future future && future.isDone()) {
                try {
                   future.get();
                }
                catch (ExecutionException ex) {
                   if (txAttr.rollbackOn(ex.getCause())) {
                      status.setRollbackOnly();
                   }
                }
                catch (InterruptedException ex) {
                   Thread.currentThread().interrupt();
                }
             }
             else if (vavrPresent && VavrDelegate.isVavrTry(retVal)) {
                // Set rollback-only in case of Vavr failure matching our rollback rules...
                retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
             }
          }
       }
 
       commitTransactionAfterReturning(txInfo);
       return retVal;
    }
 
    else {
       Object result;
       final ThrowableHolder throwableHolder = new ThrowableHolder();
 
       // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
       try {
          result = cpptm.execute(txAttr, status -> {
             TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
             try {
                Object retVal = invocation.proceedWithInvocation();
                if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
                   // Set rollback-only in case of Vavr failure matching our rollback rules...
                   retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
                }
                return retVal;
             }
             catch (Throwable ex) {
                if (txAttr.rollbackOn(ex)) {
                   // A RuntimeException: will lead to a rollback.
                   if (ex instanceof RuntimeException runtimeException) {
                      throw runtimeException;
                   }
                   else {
                      throw new ThrowableHolderException(ex);
                   }
                }
                else {
                   // A normal return value: will lead to a commit.
                   throwableHolder.throwable = ex;
                   return null;
                }
             }
             finally {
                cleanupTransactionInfo(txInfo);
             }
          });
       }
       catch (ThrowableHolderException ex) {
          throw ex.getCause();
       }
       catch (TransactionSystemException ex2) {
          if (throwableHolder.throwable != null) {
             logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
             ex2.initApplicationException(throwableHolder.throwable);
          }
          throw ex2;
       }
       catch (Throwable ex2) {
          if (throwableHolder.throwable != null) {
             logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
          }
          throw ex2;
       }
 
       // Check result state: It might indicate a Throwable to rethrow.
       if (throwableHolder.throwable != null) {
          throw throwableHolder.throwable;
       }
       return result;
    }
}

当调用事务代理对象时，程序会经过判断后，走进if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager cpptm))分支。首先让我们考虑一下非事务嵌套的操作场景，if分支中的retVal = invocation.proceedWithInvocation();就表示业务代码执行完成了，后面就是释放资源（即调用cleanupTransactionInfo(txInfo)方法）、提交事务（则是指调用commitTransactionAfterReturning(txInfo)方法）等一系列常规操作。这样整个事务和业务处理逻辑就执行完成了。接着再来考虑一下事务嵌套的处理场景（即需要事务的业务处理代码中调用了另外一个需要事务的业务代码），这个时候第一次进入if分支的程序执行完retVal = invocation.proceedWithInvocation();后，会再次进入到if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager cpptm))分支，这个时候我们需要关注if分支中的TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification)这句，先来看一下这个被调用的方法（createTransactionIfNecessary()）及其关联方法（prepareTransactionInfo()）的源码：

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);
}
// 
protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
       @Nullable TransactionAttribute txAttr, String joinpointIdentification,
       @Nullable TransactionStatus status) {
 
    TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
    if (txAttr != null) {
       // We need a transaction for this method...
       if (logger.isTraceEnabled()) {
          logger.trace("Getting transaction for [" + txInfo.getJoinpointIdentification() + "]");
       }
       // The transaction manager will flag an error if an incompatible tx already exists.
       txInfo.newTransactionStatus(status);
    }
    else {
       // The TransactionInfo.hasTransaction() method will return false. We created it only
       // to preserve the integrity of the ThreadLocal stack maintained in this class.
       if (logger.isTraceEnabled()) {
          logger.trace("No need to create transaction for [" + joinpointIdentification +
                "]: This method is not transactional.");
       }
    }
 
    // We always bind the TransactionInfo to the thread, even if we didn't create
    // a new transaction here. This guarantees that the TransactionInfo stack
    // will be managed correctly even if no transaction was created by this aspect.
    txInfo.bindToThread();
    return txInfo;
}

这时我们主要关注prepareTransactionInfo()方法中的txInfo.bindToThread()，关于这个被调用的方法的详情可以看参看前面的源码。这里再贴一下图片：

从图中不难看出，这个方法会先从当前线程的ThreadLocal中拿出一个TransactionInfo对象，并将其赋值给TransactionInfo的oldTransactionInfo属性，然后将新的TransactionInfo对象重新赋值到当前线程的ThreadLocal中。接着就是就是继续调用retVal = invocation.proceedWithInvocation()，然后调用cleanupTransactionInfo(txInfo)方法，最后再调用commitTransactionAfterReturning(txInfo)方法。这里我们再啰嗦一下cleanupTransactionInfo ()方法，其源码如下图所示：

protected void cleanupTransactionInfo(@Nullable TransactionInfo txInfo) {
    if (txInfo != null) {
       txInfo.restoreThreadLocalStatus();
    }
}

该方法最终调用的是TransactionInfo类中的restoreThreadLocalStatus()方法，其源码如下图所示：

说白了就是将当前TransactionInfo对象中的oldTransactionInfo对象重新赋值到当前线程的ThreadLocal对象中，这个对象就是嵌套事务的上一层事务。至此我们就可以用自己的语言来回答前面的两个问题了！

总的来看，将TransactionInfo绑定到当前线程的主要目的就是解决嵌套事务的。Spring解决嵌套的主要思路就是先将当前的TransactionInfo对象绑定到当前线程，当当前TransactionInfo对应的业务处理代码调用其他事务代理时，会将当前线程中保存的TransactionInfo对象赋值给新的TransactionInfo对象的oldTransactionInfo属性，然后将新的TransactionInfo对象重新绑定到当前线程的ThreadLocal上，这样就实现了前后两个事务的关联，并完成对事务传播行为的处理。下面这幅图展示了出现事务嵌套情况时的处理逻辑：