How Spring Resolves Bean Dependencies: Types, Collections, and Providers

1 Determine the Injection Type

Spring first checks the type of the property being injected. It distinguishes among:

Optional

ObjectFactory

ObjectProvider

javax.inject.Provider

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
    private static Class<?> javaxInjectProviderClass;
    static {
        try {
            javaxInjectProviderClass = ClassUtils.forName("javax.inject.Provider", DefaultListableBeanFactory.class.getClassLoader());
        } catch (ClassNotFoundException ex) {
            javaxInjectProviderClass = null;
        }
    }
    public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
            @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
        if (Optional.class == descriptor.getDependencyType()) {
            return createOptionalDependency(descriptor, requestingBeanName);
        } else if (ObjectFactory.class == descriptor.getDependencyType() || ObjectProvider.class == descriptor.getDependencyType()) {
            return new DependencyObjectProvider(descriptor, requestingBeanName);
        } else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
            return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
        } else {
            // Handle @Lazy annotation and create a proxy if necessary
            Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, requestingBeanName);
            if (result == null) {
                result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
            }
            return result;
        }
    }
}

2 Find the Dependency Object

The core method doResolveDependency resolves the actual bean instance, handling shortcuts, multiple candidates, and type conversion.

public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
        @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
    InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
    try {
        Object shortcut = descriptor.resolveShortcut(this);
        if (shortcut != null) {
            return shortcut;
        }
        Class<?> type = descriptor.getDependencyType();
        // Resolve collections, arrays, maps, etc.
        Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
        if (multipleBeans != null) {
            return multipleBeans;
        }
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
        if (matchingBeans.isEmpty()) {
            if (isRequired(descriptor)) {
                raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
            }
            return null;
        }
        String autowiredBeanName;
        Object instanceCandidate;
        if (matchingBeans.size() > 1) {
            autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
            if (autowiredBeanName == null) {
                if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
                    return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
                } else {
                    return null;
                }
            }
            instanceCandidate = matchingBeans.get(autowiredBeanName);
        } else {
            Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
            autowiredBeanName = entry.getKey();
            instanceCandidate = entry.getValue();
        }
        if (autowiredBeanNames != null) {
            autowiredBeanNames.add(autowiredBeanName);
        }
        if (instanceCandidate instanceof Class) {
            instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
        }
        Object result = instanceCandidate;
        if (result instanceof NullBean) {
            if (isRequired(descriptor)) {
                raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
            }
            result = null;
        }
        if (!ClassUtils.isAssignableValue(type, result)) {
            throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
        }
        return result;
    } finally {
        ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
    }
}

2.1 Determine Collection Types

Spring checks whether the dependency type is one of the following collection forms:

StreamDependencyDescriptor

Array

Collection

Map

private Object resolveMultipleBeans(DependencyDescriptor descriptor, @Nullable String beanName,
        @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) {
    Class<?> type = descriptor.getDependencyType();
    if (descriptor instanceof StreamDependencyDescriptor) {
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
        if (autowiredBeanNames != null) {
            autowiredBeanNames.addAll(matchingBeans.keySet());
        }
        Stream<Object> stream = matchingBeans.keySet().stream()
                .map(name -> descriptor.resolveCandidate(name, type, this))
                .filter(bean -> !(bean instanceof NullBean));
        if (((StreamDependencyDescriptor) descriptor).isOrdered()) {
            stream = stream.sorted(adaptOrderComparator(matchingBeans));
        }
        return stream;
    } else if (type.isArray()) {
        // Handle array injection
        // ... (array conversion logic) ...
    } else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
        // Handle collection injection
        // ... (collection conversion logic) ...
    } else if (Map.class == type) {
        // Handle map injection
        // ... (map conversion logic) ...
    } else {
        return null;
    }
}

2.2 Find Candidate Beans

The method findAutowireCandidates retrieves beans matching the required type, considering resolvable dependencies, self‑references, and qualifier annotations.

protected Map<String, Object> findAutowireCandidates(@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
    String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this, requiredType, true, descriptor.isEager());
    Map<String, Object> result = CollectionUtils.newLinkedHashMap(candidateNames.length);
    // Check resolvableDependencies (e.g., HttpServletRequest)
    for (Map.Entry<Class<?>, Object> entry : this.resolvableDependencies.entrySet()) {
        Class<?> autowiringType = entry.getKey();
        if (autowiringType.isAssignableFrom(requiredType)) {
            Object autowiringValue = AutowireUtils.resolveAutowiringValue(entry.getValue(), requiredType);
            if (requiredType.isInstance(autowiringValue)) {
                result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
                break;
            }
        }
    }
    for (String candidate : candidateNames) {
        if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
            addCandidateEntry(result, candidate, descriptor, requiredType);
        }
    }
    // Fallback handling omitted for brevity
    return result;
}

This is how Spring determines the data type of an injected property and performs the appropriate resolution steps.

Finished!