spring bean注入异常

1. Overview

In this article, we are discussing the Spring org.springframework.beans.factory.NoSuchBeanDefinitionException – this is a common exception thrown by the BeanFactory when trying to resolve a bean that simply isn’t defined in the Spring Context.

We’ll illustrate the possible causes for this problem and the available solutions.

And of course, exceptions happen when you least expect them; have a look at the full list of exceptions and solutions in Spring.

2. Cause: No qualifying bean of type […] found for dependency

The most common cause of this exception is simply trying to inject a bean that isn’t defined. For example – BeanB is wiring in a collaborator – BeanA: 

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@Component public class BeanA {        @Autowired      private BeanB dependency;      //... }

Now, if the dependency – BeanB – is not defined in the Spring Context, the bootstrap process will fail with the no such bean definition exception:

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org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [org.baeldung.packageB.BeanB]    found for dependency: expected at least 1 bean which qualifies as    autowire candidate for this dependency. Dependency annotations:    {@org.springframework.beans.factory.annotation.Autowired(required= true )}

The reason is clearly indicated by Spring: “expected at least 1 bean which qualifies as autowire candidate for this dependency“

One reason BeanB may not exist in the context – if beans are picked up automatically by classpath scanning, and if BeanB is correctly annotated as a bean (@Component, @Repository, @Service, @Controller, etc) – is that it may be defined in a package that is not scanned by Spring:

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package org.baeldung.packageB; @Component public class BeanB { ...}

While the classpath scanning may be configured as follows:

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@Configuration @ComponentScan ( "org.baeldung.packageA" ) public class ContextWithJavaConfig {      ... }

If beans are not automatically scanned by instead defined manually, then BeanB is simply not defined in the current Spring Context.

3. Cause: No qualifying bean of type […] is defined

Another cause for the exception is the existence of two bean definitions in the context, instead of one. For example, if an interface – IBeanB is implemented by two beans – BeanB1 and BeanB2:

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@Component public class BeanB1 implements IBeanB {      // } @Component public class BeanB2 implements IBeanB {      // }

Now, if BeanA autowires this interface, Spring will not know which one of the two implementations to inject:

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@Component public class BeanA {        @Autowired      private IBeanB dependency;      ... }

And again, this will result in a NoSuchBeanDefinitionException being thrown by the BeanFactory:

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Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type    [org.baeldung.packageB.IBeanB] is defined: expected single matching bean but found 2: beanB1,beanB2

Similarly, Spring clearly indicates the reason for the wiring failure: “expected single matching bean but found 2”.

Notice, however, that in this case, the exact exception being thrown is not NoSuchBeanDefinitionException but a subclass – the NoUniqueBeanDefinitionException. This new exception has been introduced in Spring 3.2.1, for exactly this reason – to differentiate between the cause where no bean definition was found and this one – where several definitions are found in the context.

Before this change, the exception above was:

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Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [org.baeldung.packageB.IBeanB] is defined: expected single matching bean but found 2: beanB1,beanB2

One solution to this problem is to use the @Qualifier annotation to specify exactly the name of the bean we want to wire:

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@Component public class BeanA {        @Autowired      @Qualifier ( "beanB2" )      private IBeanB dependency;      ... }

Now Spring has enough information to make the decision of which bean to inject – BeanB1 or BeanB2 (the default name of BeanB2 is beanB2).

4. Cause: No Bean Named […] is defined

A NoSuchBeanDefinitionException may also be thrown when a bean that isn’t defined is requested by name from the Spring context:

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@Component public class BeanA implements InitializingBean {        @Autowired      private ApplicationContext context;        @Override      public void afterPropertiesSet() {          context.getBean( "someBeanName" );      } }

In this case, there is no bean definition for “someBeanName” – leading to the following exception:

1 2	Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'someBeanName' is defined

Again, Spring clearly and concisely indicates the reason for the failure: “No bean named X is defined“.

5. Cause: Proxied Beans

When a bean in the context is proxied using the JDK Dynamic Proxy mechanism, then the proxy will not extend the target bean (it will, however, implement the same interfaces).

Because of this, if the bean is injected by an interface, it will be correctly wired in. If however the bean is injected by the actual class, then Spring will not find a bean definition that matches the class – since the proxy does not actually extend the class.

A very common reason the bean may be proxied is the Spring transactional support – namely beans that are annotated with @Transactional.

For example, if ServiceA injects ServiceB, and both services are transactional, injecting by the class definition will not work:

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@Service @Transactional public class ServiceA implements IServiceA{        @Autowired      private ServiceB serviceB;      ... }   @Service @Transactional public class ServiceB implements IServiceB{      ... }

The same two services, this time correctly injecting by the interface, will be OK:

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@Service @Transactional public class ServiceA implements IServiceA{        @Autowired      private IServiceB serviceB;      ... }   @Service @Transactional public class ServiceB implements IServiceB{      ... }

6. Conclusion

This tutorial discussed examples of the possible causes for the common NoSuchBeanDefinitionException – with a focus on how to address these exceptions in practice.

The implementation of all these exceptions examples can be found in the GitHub project – this is an Eclipse based project, so it should be easy to import and run as it is.

Finally, the full list of exceptions and solutions in Spring might be a good resource to bookmark.