聊聊spring的TransactionalEventListener

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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

doc

• 聊聊TransactionSynchronization的invokeAfterCompletion
• 聊聊spring的TransactionSynchronizationAdapter