今天这篇文章，我想梳理一下Spring事务用到的几个核心组件。这些核心组件是我们理解Spring事务原理的基础。通过它们我们可以体会学习一下Spring设计者设计Spring事务时的基本思路。这些组件是：TransactionInfo、TransactionStatus、TransactionManager、TransactionAttribute、TransactionAttributeSource、ReactiveTransactionSupport等等。
首先让我们一起看一下TransactionInfo类。它是一个位于TransactionAspectSupport类中的内部类（这个类是上一节介绍的TransactionInterceptor的父类，TransactionInterceptor是一个增强类，用于对那些需要事务的方法进行增强）。TransactionInfo类的源码如下：
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");
}
}
通过源码不难看出TransactionInfo是一个最终类，也没有继承或实现其他接口或类，其中的属性有PlatformTransactionManager、TransactionAttribute、TransactionStatus、TransactionInfo（通过其变量名可以知道其代表的是老事务。还记得《Spring事务原理总结一》这篇文章中提到的事务传播属性吗？其中提到的REQUIRES_NEW可能就是通过这个属性来实现的）等。
然后让我们再一起看一下TransactionStatus接口，其又继承了SavepointManager、Flushable、TransactionExecution等接口。下面是该接口的源码：
public interface TransactionStatus extends TransactionExecution, SavepointManager, Flushable {
/**
* Return whether this transaction internally carries a savepoint,
* that is, has been created as nested transaction based on a savepoint.
* <p>This method is mainly here for diagnostic purposes, alongside
* {@link #isNewTransaction()}. For programmatic handling of custom
* savepoints, use the operations provided by {@link SavepointManager}.
* <p>The default implementation returns {@code false}.
* @see #isNewTransaction()
* @see #createSavepoint()
* @see #rollbackToSavepoint(Object)
* @see #releaseSavepoint(Object)
*/
default boolean hasSavepoint() {
return false;
}
/**
* Flush the underlying session to the datastore, if applicable:
* for example, all affected Hibernate/JPA sessions.
* <p>This is effectively just a hint and may be a no-op if the underlying
* transaction manager does not have a flush concept. A flush signal may
* get applied to the primary resource or to transaction synchronizations,
* depending on the underlying resource.
* <p>The default implementation is empty, considering flush as a no-op.
*/
@Override
default void flush() {
}
}
下图则是TransactionStatus接口的继承体系：
接着来让我们来看一下TransactionAttribute接口，该位于org.springframework.transaction.interceptor包中，先来看一下其源码：
public interface TransactionAttribute extends TransactionDefinition {
/**
* Return a qualifier value associated with this transaction attribute.
* <p>This may be used for choosing a corresponding transaction manager
* to process this specific transaction.
* @since 3.0
*/
@Nullable
String getQualifier();
/**
* Return labels associated with this transaction attribute.
* <p>This may be used for applying specific transactional behavior
* or follow a purely descriptive nature.
* @since 5.3
*/
Collection<String> getLabels();
/**
* Should we roll back on the given exception?
* @param ex the exception to evaluate
* @return whether to perform a rollback or not
*/
boolean rollbackOn(Throwable ex);
}
下图是TransactionAttribute这个接口的继承体系（这里面我们中断关注RuleBasedTransactionAttribute这个类，第一次跟踪事务的执行代码时实际类型为RuleBasedTransactionAttribute）：
最后再来看一下PlatformTransactionManager这个接口，该接口继承了TransactionManager接口，其有4个实现类，分别为AbstractPlatformTransactionManager、DataSourceTransactionManager、JdbcTransactionManager、JtaTransactionManager。个人理解它是一个管理接口。先看一下它的继承体系：
至此我们梳理了Spring事务涉及到的一些基本组件及其体系结构（这只是个人粗浅的认知，如果不全，还望各位大佬指正）。至于这些组件的作用，我们将在下一篇博客中逐一梳理。