How Spring Resolves Bean Dependencies: Types, Collections, and @Lazy Proxies

1 Determine the Type

When Spring injects a property, it first checks the type of the injected attribute. The possible types are:

Optional

ObjectFactory

ObjectProvider

javax.inject.Provider

<code>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 {
            // If the property has @Lazy, a proxy will be created; otherwise null is returned
            Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, requestingBeanName);
            if (result == null) {
                result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
            }
            return result;
        }
    }
}
</code>

2 Find Dependency Objects

The doResolveDependency method performs the actual lookup.

<code>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();

        // 2.1 Resolve collection types
        Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
        if (multipleBeans != null) {
            return multipleBeans;
        }

        // 2.2 Find matching bean instances
        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);
    }
}
</code>

2.1 Determine Collection Types

Spring checks whether the required type is one of the following collection-like descriptors:

StreamDependencyDescriptor

Array

Collection

Map

<code>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()) {
        // array handling logic (omitted for brevity)
        return null;
    } else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
        // collection handling logic (omitted for brevity)
        return null;
    } else if (Map.class == type) {
        // map handling logic (omitted for brevity)
        return null;
    } else {
        return null;
    }
}
</code>

2.2 Find Candidate Beans

<code>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);
        }
    }

    if (result.isEmpty()) {
        boolean multiple = indicatesMultipleBeans(requiredType);
        DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
        for (String candidate : candidateNames) {
            if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)
                    && (!multiple || getAutowireCandidateResolver().hasQualifier(descriptor))) {
                addCandidateEntry(result, candidate, descriptor, requiredType);
            }
        }
        if (result.isEmpty() && !multiple) {
            for (String candidate : candidateNames) {
                if (isSelfReference(beanName, candidate) &&
                        (!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
                        isAutowireCandidate(candidate, fallbackDescriptor)) {
                    addCandidateEntry(result, candidate, descriptor, requiredType);
                }
            }
        }
    }
    return result;
}
</code>

In summary, Spring determines the injected property's type, resolves collection or single‑bean dependencies accordingly, and finally retrieves matching bean instances from the container.