Solution to BeanCurrentlyInCreationException Error in Spring Boot

In Spring, BeanCurrentlyInCreationException is an exception that occurs when there's a circular dependency during bean creation, and Spring's dependency injection mechanism can't resolve it. This usually happens when using constructor-based injection. Let's dive deeper into the cause and resolution of this error. 

Cause

Consider the following scenario: 

• BeanA requires an instance of BeanB in its constructor. 
• BeanB, in turn, requires an instance of BeanA in its constructor. 

Now, when Spring tries to create an instance of BeanA, it realizes it first needs to create BeanB. However, to create BeanB, it needs BeanA, leading to a circular dependency. Since both beans are currently in the creation process and depend on each other, Spring throws the BeanCurrentlyInCreationException.

Does Using Setter or Field Injection solves the issue?

No. In modern versions of Spring, the container will detect circular dependencies regardless of whether you're using constructor or setter/field-based injection. 

If both beans (or a chain of beans) have setter or field-based injections for one another, it would still result in a circular dependency, and Spring would throw BeanCurrentlyInCreationException.

How to Resolve

1. Rethink the Design: 

This is the most recommended approach. If there's a circular dependency, it might be an indication that the design can be improved. Can you split the functionality of one of the beans or introduce an intermediary service or bean? 

2. Using the @Lazy Annotation: 

This is one of the most common ways to resolve such an issue. By marking one of the beans as @Lazy, you're telling Spring to delay its instantiation until it's actually needed.

@Component
public class BeanA {
    private BeanB beanB;

    @Autowired
    public void setBeanB(@Lazy BeanB beanB) {
        this.beanB = beanB;
    }
}

@Component
public class BeanB {
    private BeanA beanA;

    @Autowired
    public void setBeanA(BeanA beanA) {
        this.beanA = beanA;
    }
}

3. @PostConstruct Initialization: 

If the circular dependency arises due to some logic in the initialization of the beans, try to defer that logic using the @PostConstruct annotation. This ensures that both beans are fully constructed before the logic runs.

@Component
public class BeanA {
    @Autowired
    private BeanB beanB;

    @PostConstruct
    public void init() {
        // Operations that involve beanB
    }
}

4. Java Config for Bean Creation: 

Explicitly control the creation of beans using Java configuration to manage their dependencies.

@Configuration
public class BeanConfig {

    @Bean
    @Lazy
    public BeanA beanA() {
        return new BeanA();
    }

    @Bean
    public BeanB beanB(BeanA beanA) {
        BeanB beanB = new BeanB();
        beanB.setBeanA(beanA);
        return beanB;
    }
}

The important takeaway here is that while Spring provides mechanisms to handle circular dependencies, it's a design smell. Circular dependencies can make the code harder to maintain and test. Consider reevaluating the design and responsibilities of your components to see if the circularity can be eliminated.