MappingContextEvent source makes spring-data modules create circular dependencies

[Aloren]

MappingContextEvent contains source field. Consumers of this event use source field to understand which MappingContext created the event.
The standard approach in spring-data module to use the source would be:

class MyConsumer implements ApplicationListener<MappingContextEvent<?, ?>> {

    private final MyMappingContext expectedMappingContext;

    @Override
    public void onApplicationEvent(MappingContextEvent<?, ?> event) {
        if(event.wasEmittedBy(expectedMappingContext)) {
            // process this event
        }
}

Such API creates circular dependency.

Before Spring Boot 2.6 (probably changes regarding circular references were introduced in Spring to be exact, unfortunately I'm not aware which version) such circular dependencies worked, now it is disabled out of the box, is considered harmful and requires spring.main.allow-circular-references=true property.

This API is used in spring-data-couchbase, spring-data-mongodb and spring-data-aerospike for index creating. I've researched these implementations and such API makes developers create a total mess in code for dealing with circular dependency issue.

Useful links to understand how this is currently implemented and which issues are there:
spring-projects/spring-data-mongodb#2823
spring-projects/spring-data-couchbase#858
spring-projects/spring-data-couchbase#298
spring-projects/spring-data-couchbase@03ad369
https://github.com/spring-projects/spring-data-couchbase/blob/main/src/main/java/org/springframework/data/couchbase/core/CouchbaseTemplate.java#L182-L207
https://github.com/spring-projects/spring-data-mongodb/blob/main/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoTemplate.java#L232-L246

I've just faced the same issue in spring-data-aerospike with Spring Boot 2.6 and found that resolving this is not that straight forward, and each spring-data implementation has it's own workarounds. So I thought that maybe there should be some other approach to resolve such issue?

[mp911de]

Stack trace:

org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'myConsumer': Requested bean is currently in creation: Is there an unresolvable circular reference?
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:355) ~[spring-beans-5.3.17.jar:5.3.17]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:227) ~[spring-beans-5.3.17.jar:5.3.17]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.17.jar:5.3.17]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:213) ~[spring-beans-5.3.17.jar:5.3.17]
	at org.springframework.context.event.AbstractApplicationEventMulticaster.retrieveApplicationListeners(AbstractApplicationEventMulticaster.java:264) ~[spring-context-5.3.17.jar:5.3.17]
	at org.springframework.context.event.AbstractApplicationEventMulticaster.getApplicationListeners(AbstractApplicationEventMulticaster.java:221) ~[spring-context-5.3.17.jar:5.3.17]
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:138) ~[spring-context-5.3.17.jar:5.3.17]
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:421) ~[spring-context-5.3.17.jar:5.3.17]
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:378) ~[spring-context-5.3.17.jar:5.3.17]
	at org.springframework.data.mapping.context.AbstractMappingContext.addPersistentEntity(AbstractMappingContext.java:391) ~[spring-data-commons-2.6.3.jar:2.6.3]
	at org.springframework.data.mapping.context.AbstractMappingContext.addPersistentEntity(AbstractMappingContext.java:341) ~[spring-data-commons-2.6.3.jar:2.6.3]
	at java.base/java.lang.Iterable.forEach(Iterable.java:75) ~[na:na]
	at org.springframework.data.mapping.context.AbstractMappingContext.initialize(AbstractMappingContext.java:494) ~[spring-data-commons-2.6.3.jar:2.6.3]
	at org.springframework.data.mapping.context.AbstractMappingContext.afterPropertiesSet(AbstractMappingContext.java:486) ~[spring-data-commons-2.6.3.jar:2.6.3]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1863) ~[spring-beans-5.3.17.jar:5.3.17]

Debug output:

Description:

The dependencies of some of the beans in the application context form a cycle:

┌─────┐
|  myConsumer defined in file [/Users/mpaluch/playground/circular-beans/target/classes/com/example/circularbeans/MyConsumer.class]
↑     ↓
|  mongoMappingContext defined in class path resource [org/springframework/boot/autoconfigure/data/mongo/MongoDataConfiguration.class]
└─────┘

[Aloren]

We've overcome this issue right now the following way:

class MyConsumer implements ApplicationListener<MappingContextEvent<?, ?>> , SmartLifecycle {

private final ObjectProvider<MyMappingContext> mappingContext;
private final Set<MappingContextEvent> events = new HashSet<>();

    public void onApplicationEvent(MappingContextEvent<?, ?> event) {
        events.add(event);
    }

    @Override
    public void start() {
       events.forEach(event -> {
                    MyMappingContext mappingContext = getMappingContext();
                    if (event.wasEmittedBy(mappingContext)) {
                        //process event
                    }
        });
    }

    private MyMappingContext getMappingContext() {
        MyMappingContext mappingContext = this.mappingContext.getIfUnique();
        if(mappingContext == null) {
            throw new IllegalStateException("MyMappingContext bean is not available in context OR is not unique");
        }
        return mappingContext;
    }
}

[mp911de]

Can you provide us with a bit of further detail on what you're trying to achieve? The event-context relationship is indeed difficult because most events happen in the initialization phase where the context is not yet fully initialized.

On a related note, the index creation that is based on MappingContext (in e.g. MongoDB) isn't using events during MappingContext initialization, instead, the mapping context initializes and once the context is initialized, MongoTemplate iterates the initial entity set to create indexes. The event publication is only used for types that are discovered during runtime operations.

[spring-projects-issues]

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