What does the spring annotation @ConditionalOnMissingBean do?

Question

I am trying to start a springboot application where this annotation has been used. When I try to start the application it gives me the following error:

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Answer 1

The @ConditionalOnMissingBean annotation is used to load a bean only if a given bean is missing:

@Bean
@ConditionalOnMissingBean(SomeBean.class)
public SomeBean otherBean(){
    return new SomeBean();
}

The above bean will get loaded by Spring only if there is no other bean of this type present in the context. On the other hand, if there is already a bean of the type SomeBean present in the application context, the above bean will not be created.

Some use cases where this annotation comes in handy are:

• Specifying a fallback bean which gets only loaded as a backup if there is no bean of the same type present (for example: using an in-memory database if there is no real database configured)
• Specifying a default bean which allows being overridden in the case that a more specific bean of the same type is present in the context (for example: using a default authentication mechanism unless someone decides to replace it with his own custom authentication)

Reference: https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-developing-auto-configuration.html

Answer 2

The @ConditionalOnMissingBean annotation is a spring conditional annotation for registering beans only when they are not already in the application context.

See the documentation: https://docs.spring.io/spring-boot/docs/2.0.0.RELEASE/api/org/springframework/boot/autoconfigure/condition/ConditionalOnMissingBean.html

Answer 3

We use @ConditionalOnMissingBean if we want to include a bean only if a specified bean is not present. For ex.

Let's configure a transactionManager bean that will only be loaded if a bean of type JpaTransactionManager is not already defined:

@Bean
@ConditionalOnMissingBean(type = "JpaTransactionManager")
JpaTransactionManager transactionManager(EntityManagerFactory entityManagerFactory) {
    JpaTransactionManager transactionManager = new JpaTransactionManager();
    transactionManager.setEntityManagerFactory(entityManagerFactory);
    return transactionManager;
}

To understand more consider this scenario as well.

Let's say in my project I configured a bean videoDecoderService

@Bean
@ConditionalOnMissingBean(VideoDecoderService.class)
public videoDecoderService videoDecoderService(){
return new VideoDecoderService;
}

What it will do is whoever is using my project would be able to override the videoDecoderService with the videoDecoderService of their own. If they are not writing their own videoDecoderService then this default one will be provided.

Answer 4

You should not skip the part of the documentation that clearly says:

The condition can only match the bean definitions that have been processed by the application context so far and, as such, it is strongly recommended to use this condition on auto-configuration classes only.

It goes on later to say:

If a candidate bean may be created by another auto-configuration, make sure that the one using this condition runs after

In rather simple words it might mean that the @Bean brought in by the some other auto-configuration should take precedence over the one that you defined. For that to work, you need to properly set-up the order of those configurations via @AutoConfigureBefore.