Spring Boot - Injection of resource dependencies failed, was actually of type 'com.sun.proxy.$Proxy

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I’ve encountered a rather tricky bug while injecting a dependency in a Spring Boot application:

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Error creating bean with name 'be.solidsyntax.ConsumingService': 
Injection of resourcedependencies failed; 
nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException: 
Bean named 'dependency' is expected to be of type 'be.solidsyntax.Dependency'
 but was actually of type 'com.sun.proxy.$Proxy121'
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor
.postProcessProperties(CommonAnnotationBeanPostProcessor.java:324)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory
.populateBean(AbstractAutowireCapableBeanFactory.java:1411)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory
.autowireBeanProperties(AbstractAutowireCapableBeanFactory.java:391)
at org.springframework.test.context.support.DependencyInjectionTestExecutionListener
.injectDependencies(DependencyInjectionTestExecutionListener.java:119)
at org.springframework.test.context.support.DependencyInjectionTestExecutionListener
.prepareTestInstance(DependencyInjectionTestExecutionListener.java:83)
at org.springframework.boot.test.autoconfigure
.SpringBootDependencyInjectionTestExecutionListener
.prepareTestInstance(SpringBootDependencyInjectionTestExecutionListener.java:44)

In most cases this exception is caused by injecting a bean which was enhanced by a Spring aspect into a field referencing an implementation instead on an interface. This was not the case in my setup.

The setup

We have an Interface:

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package be.solidsyntax;
public interface Dependency {
}

A service implementing the interface:

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package be.solidsyntax;
import org.springframework.stereotype.Service;
@Service("dependency")
public class DependencyImpl implements Dependency {
}

And a service injecting the interface as a dependency:

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package be.solidsyntax;
import org.springframework.stereotype.Service;
import javax.annotation.Resource;
@Service
public class ConsumingService {
    @Resource
    private Dependency dependency;
    public ConsumingService() {
    }
}

At no point are we trying to inject a concrete class (DependencyImpl) instead on an interface (Dependency), so this couldn’t have been the problem.

The solution

It turns out the problem lies with the @Resource annotation. There are some (subtle) differences between injecting dependencies with @Resource and @Autowired.
To quote the Spring reference guide:

@Resource is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process. @Autowired has rather different semantics: After selecting candidate beans by type, the specified String qualifier value is considered within those type-selected candidates only.

In short:
@Resource tries to inject a bean named ‘dependency’, which in this setup is of type DependencyImpl
@Autowired tries to inject a bean with type ‘Dependency’, of which we have one: DependencyImpl

Our fixed turns out to be rather simple for this case, replace @Resource with @Autowired:

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package be.solidsyntax;
import org.springframework.stereotype.Service;
import javax.annotation.Resource;
@Service
public class ConsumingService {
    @Autowired
    private Dependency dependency;
    public ConsumingService() {
    }
}