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本文主要研究一下spring的TransactionalEventListener

TransactionalEventListener

org/springframework/transaction/event/TransactionalEventListener.java

/**
 * An {@link EventListener} that is invoked according to a {@link TransactionPhase}.
 *
 * <p>If the event is not published within an active transaction, the event is discarded
 * unless the {@link #fallbackExecution} flag is explicitly set. If a transaction is
 * running, the event is processed according to its {@code TransactionPhase}.
 *
 * <p>Adding {@link org.springframework.core.annotation.Order @Order} to your annotated
 * method allows you to prioritize that listener amongst other listeners running before
 * or after transaction completion.
 *
 * @author Stephane Nicoll
 * @author Sam Brannen
 * @since 4.2
 */
@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@EventListener
public @interface TransactionalEventListener {

	/**
	 * Phase to bind the handling of an event to.
	 * <p>The default phase is {@link TransactionPhase#AFTER_COMMIT}.
	 * <p>If no transaction is in progress, the event is not processed at
	 * all unless {@link #fallbackExecution} has been enabled explicitly.
	 */
	TransactionPhase phase() default TransactionPhase.AFTER_COMMIT;

	/**
	 * Whether the event should be processed if no transaction is running.
	 */
	boolean fallbackExecution() default false;

	/**
	 * Alias for {@link #classes}.
	 */
	@AliasFor(annotation = EventListener.class, attribute = "classes")
	Class<?>[] value() default {};

	/**
	 * The event classes that this listener handles.
	 * <p>If this attribute is specified with a single value, the annotated
	 * method may optionally accept a single parameter. However, if this
	 * attribute is specified with multiple values, the annotated method
	 * must <em>not</em> declare any parameters.
	 */
	@AliasFor(annotation = EventListener.class, attribute = "classes")
	Class<?>[] classes() default {};

	/**
	 * Spring Expression Language (SpEL) attribute used for making the event
	 * handling conditional.
	 * <p>The default is {@code ""}, meaning the event is always handled.
	 * @see EventListener#condition
	 */
	String condition() default "";

}

TransactionalEventListener是EventListener的事务感知版本，默认的是TransactionPhase是AFTER_COMMIT

ApplicationListenerMethodTransactionalAdapter

org/springframework/transaction/event/ApplicationListenerMethodTransactionalAdapter.java

/**
 * {@link GenericApplicationListener} adapter that delegates the processing of
 * an event to a {@link TransactionalEventListener} annotated method. Supports
 * the exact same features as any regular {@link EventListener} annotated method
 * but is aware of the transactional context of the event publisher.
 *
 * <p>Processing of {@link TransactionalEventListener} is enabled automatically
 * when Spring's transaction management is enabled. For other cases, registering
 * a bean of type {@link TransactionalEventListenerFactory} is required.
 *
 * @author Stephane Nicoll
 * @author Juergen Hoeller
 * @since 4.2
 * @see ApplicationListenerMethodAdapter
 * @see TransactionalEventListener
 */
class ApplicationListenerMethodTransactionalAdapter extends ApplicationListenerMethodAdapter {

	private final TransactionalEventListener annotation;


	public ApplicationListenerMethodTransactionalAdapter(String beanName, Class<?> targetClass, Method method) {
		super(beanName, targetClass, method);
		TransactionalEventListener ann = AnnotatedElementUtils.findMergedAnnotation(method, TransactionalEventListener.class);
		if (ann == null) {
			throw new IllegalStateException("No TransactionalEventListener annotation found on method: " + method);
		}
		this.annotation = ann;
	}


	@Override
	public void onApplicationEvent(ApplicationEvent event) {
		if (TransactionSynchronizationManager.isSynchronizationActive()
				&& TransactionSynchronizationManager.isActualTransactionActive()) {
			TransactionSynchronization transactionSynchronization = createTransactionSynchronization(event);
			TransactionSynchronizationManager.registerSynchronization(transactionSynchronization);
		}
		else if (this.annotation.fallbackExecution()) {
			if (this.annotation.phase() == TransactionPhase.AFTER_ROLLBACK && logger.isWarnEnabled()) {
				logger.warn("Processing " + event + " as a fallback execution on AFTER_ROLLBACK phase");
			}
			processEvent(event);
		}
		else {
			// No transactional event execution at all
			if (logger.isDebugEnabled()) {
				logger.debug("No transaction is active - skipping " + event);
			}
		}
	}

	private TransactionSynchronization createTransactionSynchronization(ApplicationEvent event) {
		return new TransactionSynchronizationEventAdapter(this, event, this.annotation.phase());
	}


	private static class TransactionSynchronizationEventAdapter extends TransactionSynchronizationAdapter {

		private final ApplicationListenerMethodAdapter listener;

		private final ApplicationEvent event;

		private final TransactionPhase phase;

		public TransactionSynchronizationEventAdapter(ApplicationListenerMethodAdapter listener,
				ApplicationEvent event, TransactionPhase phase) {

			this.listener = listener;
			this.event = event;
			this.phase = phase;
		}

		@Override
		public int getOrder() {
			return this.listener.getOrder();
		}

		@Override
		public void beforeCommit(boolean readOnly) {
			if (this.phase == TransactionPhase.BEFORE_COMMIT) {
				processEvent();
			}
		}

		@Override
		public void afterCompletion(int status) {
			if (this.phase == TransactionPhase.AFTER_COMMIT && status == STATUS_COMMITTED) {
				processEvent();
			}
			else if (this.phase == TransactionPhase.AFTER_ROLLBACK && status == STATUS_ROLLED_BACK) {
				processEvent();
			}
			else if (this.phase == TransactionPhase.AFTER_COMPLETION) {
				processEvent();
			}
		}

		protected void processEvent() {
			this.listener.processEvent(this.event);
		}
	}

}

ApplicationListenerMethodTransactionalAdapter继承了ApplicationListenerMethodAdapter，它的构造器会找到指定方法的TransactionalEventListener信息；其onApplicationEvent方法在有事务的时候会创建并注册transactionSynchronization到当前事务，没有事务若允许fallbackExecution也会执行processEvent TransactionSynchronizationEventAdapter只是覆盖了beforeCommit及afterCompletion两个方法，在afterCompletion方法中根据status的值与phase的值的匹配关系决定是否执行processEvent

TransactionalEventListenerFactory

org/springframework/transaction/event/TransactionalEventListenerFactory.java

/**
 * {@link EventListenerFactory} implementation that handles {@link TransactionalEventListener}
 * annotated methods.
 *
 * @author Stephane Nicoll
 * @since 4.2
 */
public class TransactionalEventListenerFactory implements EventListenerFactory, Ordered {

	private int order = 50;


	public void setOrder(int order) {
		this.order = order;
	}

	@Override
	public int getOrder() {
		return this.order;
	}


	@Override
	public boolean supportsMethod(Method method) {
		return AnnotatedElementUtils.hasAnnotation(method, TransactionalEventListener.class);
	}

	@Override
	public ApplicationListener<?> createApplicationListener(String beanName, Class<?> type, Method method) {
		return new ApplicationListenerMethodTransactionalAdapter(beanName, type, method);
	}

}

TransactionalEventListenerFactory用于创建ApplicationListenerMethodTransactionalAdapter

EventListenerMethodProcessor

org/springframework/context/event/EventListenerMethodProcessor.java

public class EventListenerMethodProcessor
		implements SmartInitializingSingleton, ApplicationContextAware, BeanFactoryPostProcessor {

	//......

	@Override
	public void afterSingletonsInstantiated() {
		ConfigurableListableBeanFactory beanFactory = this.beanFactory;
		Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set");
		String[] beanNames = beanFactory.getBeanNamesForType(Object.class);
		for (String beanName : beanNames) {
			if (!ScopedProxyUtils.isScopedTarget(beanName)) {
				Class<?> type = null;
				try {
					type = AutoProxyUtils.determineTargetClass(beanFactory, beanName);
				}
				catch (Throwable ex) {
					// An unresolvable bean type, probably from a lazy bean - let's ignore it.
					if (logger.isDebugEnabled()) {
						logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
					}
				}
				if (type != null) {
					if (ScopedObject.class.isAssignableFrom(type)) {
						try {
							Class<?> targetClass = AutoProxyUtils.determineTargetClass(
									beanFactory, ScopedProxyUtils.getTargetBeanName(beanName));
							if (targetClass != null) {
								type = targetClass;
							}
						}
						catch (Throwable ex) {
							// An invalid scoped proxy arrangement - let's ignore it.
							if (logger.isDebugEnabled()) {
								logger.debug("Could not resolve target bean for scoped proxy '" + beanName + "'", ex);
							}
						}
					}
					try {
						processBean(beanName, type);
					}
					catch (Throwable ex) {
						throw new BeanInitializationException("Failed to process @EventListener " +
								"annotation on bean with name '" + beanName + "'", ex);
					}
				}
			}
		}
	}

	private void processBean(final String beanName, final Class<?> targetType) {
		if (!this.nonAnnotatedClasses.contains(targetType) &&
				AnnotationUtils.isCandidateClass(targetType, EventListener.class) &&
				!isSpringContainerClass(targetType)) {

			Map<Method, EventListener> annotatedMethods = null;
			try {
				annotatedMethods = MethodIntrospector.selectMethods(targetType,
						(MethodIntrospector.MetadataLookup<EventListener>) method ->
								AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class));
			}
			catch (Throwable ex) {
				// An unresolvable type in a method signature, probably from a lazy bean - let's ignore it.
				if (logger.isDebugEnabled()) {
					logger.debug("Could not resolve methods for bean with name '" + beanName + "'", ex);
				}
			}

			if (CollectionUtils.isEmpty(annotatedMethods)) {
				this.nonAnnotatedClasses.add(targetType);
				if (logger.isTraceEnabled()) {
					logger.trace("No @EventListener annotations found on bean class: " + targetType.getName());
				}
			}
			else {
				// Non-empty set of methods
				ConfigurableApplicationContext context = this.applicationContext;
				Assert.state(context != null, "No ApplicationContext set");
				List<EventListenerFactory> factories = this.eventListenerFactories;
				Assert.state(factories != null, "EventListenerFactory List not initialized");
				for (Method method : annotatedMethods.keySet()) {
					for (EventListenerFactory factory : factories) {
						if (factory.supportsMethod(method)) {
							Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
							ApplicationListener<?> applicationListener =
									factory.createApplicationListener(beanName, targetType, methodToUse);
							if (applicationListener instanceof ApplicationListenerMethodAdapter) {
								((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
							}
							context.addApplicationListener(applicationListener);
							break;
						}
					}
				}
				if (logger.isDebugEnabled()) {
					logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
							beanName + "': " + annotatedMethods);
				}
			}
		}
	}

	//......
}		

EventListenerMethodProcessor实现了SmartInitializingSingleton接口，其afterSingletonsInstantiated方法先确定type，然后执行processBean，该方法会先收集annotatedMethods，然后遍历该方法，在遍历factories针对支持该方法的factory执行createApplicationListener，添加到context中

小结

TransactionalEventListener是EventListener的事务感知版本，默认的是TransactionPhase是AFTER_COMMIT，TransactionSynchronizationEventAdapter只是覆盖了beforeCommit及afterCompletion两个方法，在afterCompletion方法中根据status的值与phase的值的匹配关系决定是否执行processEvent，因而这里抛出的异常会被捕获并log下来