What Causes a NoSuchBeanDefinitionException in Spring and How Can I Resolve It?

What is a NoSuchBeanDefinitionException and how do I fix it?

In Spring, a NoSuchBeanDefinitionException is thrown when the BeanFactory cannot locate a bean definition for a specified bean name. This can occur due to several reasons:

• Missing Bean Registration: The bean has not been registered with the BeanFactory. Register the bean using @Bean annotations, XML configuration, or bean factory APIs.
• Duplicate Bean Name: Multiple beans with the same name are registered in the application context. Use @Qualifier or the name attribute to differentiate between them or rename the beans.
• Incorrect Bean Name: The bean name used to reference the bean is incorrect. Verify the spelling and casing of the bean name.
• Bean Name Case Sensitivity: Bean names are case-sensitive in Spring. Ensure the bean name matches the case used during registration.
• Profile Mismatch: If bean registration is profile-specific, ensure the active Spring profile matches the profile where the bean is registered. Configure profiles using @Profile annotations or environment properties.
• AOP Proxy Interference: Spring uses proxies for certain operations (e.g., async execution, transaction management). Ensure the bean is not proxied differently than expected.
• Spring MVC Context Hierarchy: In Spring MVC applications, beans registered in the root context cannot be accessed by beans in the DispatcherServlet context. Move the beans to the appropriate context.

Collection Injection:

Spring handles bean injection for collections (e.g., List, Map) differently. If @Resource is used to inject a collection, it will look up beans by name. However, for @Autowired, Spring will attempt to locate and inject beans of the specified collection type. To specify a specific bean for injection, use @Qualifier or the name property. For @Bean methods using collections, use Spring Expression Language (SpEL) to refer to the bean by name.

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