Spring IOC原理源码解析(@Autowired原理详解：标识属性与方法)（二）

Love The Way You Lie 2022-06-06 09:51 133阅读 0赞

源码推荐看这篇博客的时候打开Spring源码，一边看源码，一边看博客上代码的关键处的注释，这样能更好的理解Spring IOC的流程及内部实现和使用方法。如果你对IOC的原理有些了解，则这些注释能帮你更深入的理解其实现方式。

Spring容器在每个Bean实例化之后，调用AutowiredAnnotationBeanPostProcessor的postProcessMergedBeanDefinition方法:

**代码块1**

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware { 
            @Override
        public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
            if (beanType != null) {  //搜索每个Bean内@Autowired注解的信息
                InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
                metadata.checkConfigMembers(beanDefinition);
            }
        }
    
        private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, PropertyValues pvs) {
            // Fall back to class name as cache key, for backwards compatibility with custom callers.
            String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
            // Quick check on the concurrent map first, with minimal locking.
            InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
            if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                synchronized (this.injectionMetadataCache) {
                    metadata = this.injectionMetadataCache.get(cacheKey);
                    if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                        if (metadata != null) {
                            metadata.clear(pvs);
                        }
                        try {  //解析@Autowired注解的信息，生成元数据，缓存起来
                            metadata = buildAutowiringMetadata(clazz);
                            this.injectionMetadataCache.put(cacheKey, metadata);
                        }
                        catch (NoClassDefFoundError err) {
                            throw new IllegalStateException("Failed to introspect bean class [" + clazz.getName() +
                                    "] for autowiring metadata: could not find class that it depends on", err);
                        }
                    }
                }
            }
            return metadata;
        }
    
        private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
            LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<InjectionMetadata.InjectedElement>();
            Class<?> targetClass = clazz;
    
            do {
                final LinkedList<InjectionMetadata.InjectedElement> currElements =
                        new LinkedList<InjectionMetadata.InjectedElement>();
    
                ReflectionUtils.doWithLocalFields(targetClass, new ReflectionUtils.FieldCallback() {
                    @Override
                    public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
                        //如果类内的属性上有@Autowired注解，则用工具类获取注解信息
                        AnnotationAttributes ann = findAutowiredAnnotation(field);
                        if (ann != null) {  //@Autowired注解不支持静态方法
                            if (Modifier.isStatic(field.getModifiers())) {
                                if (logger.isWarnEnabled()) {
                                    logger.warn("Autowired annotation is not supported on static fields: " + field);
                                }
                                return;
                            }  //获取@Autowired注解的required的属性值，如果true，但注入失败会抛出异常，false则不会
                            boolean required = determineRequiredStatus(ann);
                            currElements.add(new **AutowiredFieldElement**(field, required));
                        }
                    }
                });
    
                ReflectionUtils.doWithLocalMethods(targetClass, new ReflectionUtils.MethodCallback() {
                    @Override
                    public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
                        Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
                        if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
                            return;
                        }  //如果方法上有@Autowired注解，则获取注解信息
                        AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
                        if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
                            //@Autowired不支持静态方法
                            if (Modifier.isStatic(method.getModifiers())) {
                                if (logger.isWarnEnabled()) {
                                    logger.warn("Autowired annotation is not supported on static methods: " + method);
                                }
                                return;
                            }
                            //@Autowired注解标识在方法上的目的就是将容器内的Bean注入到方法的参数中，没有参数就违背了初衷
                            if (method.getParameterTypes().length == 0) {
                                if (logger.isWarnEnabled()) {
                                    logger.warn("Autowired annotation should only be used on methods with parameters: " +
                                            method);
                                }
                            }
                            boolean required = determineRequiredStatus(ann);
                            PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                            currElements.add(new **AutowiredMethodElement**(method, required, pd));
                        }
                    }
                });
    
                elements.addAll(0, currElements);
                //在解析完Bean的类型之后，递归的解析其父类，将所有的@Autowired的属性和方法收集起来，
                //且类的层级越高其属性会被越优先注入
                **targetClass = targetClass.getSuperclass()**;
            }
            while (targetClass != null && targetClass != Object.class);
    
            return new InjectionMetadata(clazz, elements);
        }
    
    }

在将@Autowired注解的信息解析成元数据之后，缓存起来，以备复用，每一个类只解析一次。

在BeanPostProcessor的postProcessMergedBeanDefinition（）方法执行之后，下一步就是执行postProcessPropertyValues（）方法，AutowiredAnnotationBeanPostProcessor的postProcessPropertyValues（）方法就是从Spring IOC容器从找到合适的Bean，注入属性。找到合适的Bean，注入@Autowired标识方法的参数，然后执行此方法。

**代码块2**

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware { 
    
        @Override
        public PropertyValues postProcessPropertyValues(
                PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
            //从缓存中找到此类的@Autowired元数据，尝试注入。没有@Autowired则会略过
            InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
            try {
                metadata.inject(bean, beanName, pvs);
            }
            catch (BeanCreationException ex) {
                throw ex;
            }
            catch (Throwable ex) {
                throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
            }
            return pvs;
        }
    }

InjectionMetadata，持有待注入的元数据信息，执行inject（）方法，开始注入属性或方法参数。

**代码块3**

public class InjectionMetadata { 
    
        private static final Log logger = LogFactory.getLog(InjectionMetadata.class);
    
        private final Class<?> targetClass;
        //待注入的属性，是一个InjectedElement集合，按类等层级排列，父类的@Autowired排前
        private final Collection<InjectedElement> injectedElements;
    
        private volatile Set<InjectedElement> checkedElements;
    
    
        public InjectionMetadata(Class<?> targetClass, Collection<InjectedElement> elements) {
            this.targetClass = targetClass;
            this.injectedElements = elements;
        }
    
        public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
            Collection<InjectedElement> elementsToIterate =
                    (this.checkedElements != null ? this.checkedElements : this.injectedElements);
            if (!elementsToIterate.isEmpty()) {
                boolean debug = logger.isDebugEnabled();
                for (InjectedElement element : elementsToIterate) {
                    if (debug) {
                        logger.debug("Processing injected element of bean '" + beanName + "': " + element);
                    }
                    //解析@Autowired注解生成的元数据类有AutowiredFieldElement，AutowiredMethodElement
                    //这两个类继承InjectionMetadata .InjectedElement，各自实现了inject方法。
                    //这两个类是AutowiredAnnotationBeanPostProcessor的私有内部类
                    element.inject(target, beanName, pvs);
                }
            }
        }
    }

下面先看属性的注入:

**代码块4**

private class AutowiredFieldElement extends InjectionMetadata.InjectedElement { 
    
            private final boolean required;
    
            private volatile boolean cached = false;
    
            private volatile Object cachedFieldValue;
    
            public AutowiredFieldElement(Field field, boolean required) {
                super(field, null);
                this.required = required;
            }
    
            @Override
            protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
                Field field = (Field) this.member;
                Object value;
                if (this.cached) {
                    value = resolvedCachedArgument(beanName, this.cachedFieldValue);
                }
                else {
                    DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
                    desc.setContainingClass(bean.getClass());
                    Set<String> autowiredBeanNames = new LinkedHashSet<String>(1);
                    TypeConverter typeConverter = beanFactory.getTypeConverter();
                    try {
                        //如果@Autowired标识的属性有一个合适的待注入对象，则缓存这个Bean的名称，
                        //如果再次生成这个Bean时，就不需要重新按类型去搜索Spring容器，直接获取这个缓存Bean的名称
                        value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
                    }
                    catch (BeansException ex) {
                        throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
                    }
                    synchronized (this) {
                        if (!this.cached) {
                            if (value != null || this.required) {
                                this.cachedFieldValue = desc;
                                //注册Bean依赖
                                registerDependentBeans(beanName, autowiredBeanNames);
                                if (autowiredBeanNames.size() == 1) {
                                    String autowiredBeanName = autowiredBeanNames.iterator().next();
                                    if (beanFactory.containsBean(autowiredBeanName)) {
                                        //@Autowired标识属性类型和Bean的类型要匹配，因此Array,Collection,Map类型的属性不支持缓存属性Bean名称
                                        if (beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                                            this.cachedFieldValue = new ShortcutDependencyDescriptor(
                                                    desc, autowiredBeanName, field.getType());
                                        }
                                    }
                                }
                            }
                            else {
                                this.cachedFieldValue = null;
                            }
                            this.cached = true;
                        }
                    }
                }
                if (value != null) {
                    ReflectionUtils.makeAccessible(field);
                    field.set(bean, value);  //通过反射为属性赋值
                }
            }
        }

接下来就需要Spring IOC容器根据DependencyDescriptor依赖描述去寻找容器中合适的Bean:

**代码块5**

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable { 
    
        public Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName,
                Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
    
            descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
            if (javaUtilOptionalClass == descriptor.getDependencyType()) {
                return new OptionalDependencyFactory().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 Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
            }
            else {
                Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
                        descriptor, requestingBeanName);
                if (result == null) {
                    //大部分情况下均是执行此步骤，解析依赖
                    result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
                }
                return result;
            }
        }
    
        public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
                Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
    
            InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
            try { //如果是快捷方式的依赖描述，则直接通过候选者的名称来择取Bean，返回
                Object shortcut = descriptor.resolveShortcut(this);
                if (shortcut != null) {
                    return shortcut;
                }
    
                Class<?> type = descriptor.getDependencyType();
                Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
                if (value != null) {
                    if (value instanceof String) {
                        String strVal = resolveEmbeddedValue((String) value);
                        BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
                        value = evaluateBeanDefinitionString(strVal, bd);
                    }
                    TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
                    return (descriptor.getField() != null ?
                            converter.convertIfNecessary(value, type, descriptor.getField()) :
                            converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
                }
                //如果标识@Autowired注解的属性是容器类型，Array,Collection,Map,则在这个方法中解析，
                //如果类型是容器类型，且返回了合适的Bean,则解析成功，代码详解请看此段代码的下一段代码
                Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
                if (multipleBeans != null) { //如果确实时容器类型的属性，则直接返回
                    return multipleBeans;
                }
                //非容器类型属性，按类型去IOC容器内择取所有类型匹配的候选者
                Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
                if (matchingBeans.isEmpty()) { //如果没有合适的Bean，但注解的required = true，则抛出异常，若required = false,则注入失败。
                    if (descriptor.isRequired()) {
                        raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
                    }
                    return null;
                }
    
                String autowiredBeanName;
                Object instanceCandidate;
    
                if (matchingBeans.size() > 1) {
                    //非容器属性，但是有多个候选者，此时需要从中选出最优的那个,代码详解看代码块9
                    autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
                    if (autowiredBeanName == null) {
                        if (descriptor.isRequired() || !indicatesMultipleBeans(type)) {
                            return descriptor.resolveNotUnique(type, matchingBeans);
                        }
                        else {
                            // In case of an optional Collection/Map, silently ignore a non-unique case:
                            // possibly it was meant to be an empty collection of multiple regular beans
                            // (before 4.3 in particular when we didn't even look for collection beans).
                            return null;
                        }
                    }
                    instanceCandidate = matchingBeans.get(autowiredBeanName);
                }
                else {
                    // We have exactly one match.
                    Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
                    autowiredBeanName = entry.getKey();
                    instanceCandidate = entry.getValue();
                }
    
                if (autowiredBeanNames != null) {
                    autowiredBeanNames.add(autowiredBeanName);
                }
                return (instanceCandidate instanceof Class ?
                        descriptor.resolveCandidate(autowiredBeanName, type, this) : instanceCandidate);
            }
            finally {
                ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
            }
        }
    
    }

Spring IOC容器解析Array,Collection,Map类型的属性注入:

**代码块6**

private Object resolveMultipleBeans(DependencyDescriptor descriptor, String beanName,
                Set<String> autowiredBeanNames, TypeConverter typeConverter) {
    
            Class<?> type = descriptor.getDependencyType();
            if (type.isArray()) {  //如果@Autowired标识的是数组类型的属性
                Class<?> componentType = type.getComponentType(); //获取数组的内容类型
                ResolvableType resolvableType = descriptor.getResolvableType();
                Class<?> resolvedArrayType = resolvableType.resolve();
                if (resolvedArrayType != null && resolvedArrayType != type) { 
                    type = resolvedArrayType;
                    componentType = resolvableType.getComponentType().resolve();
                }
                if (componentType == null) {
                    return null;
                } //通过类型去IOC容器内择取符合的Bean都是使用这个方法，很重要，此方法请看此段代码的下一段
                Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType,
                        new MultiElementDescriptor(descriptor)); //对依赖描述做了封装，区别普通类型的属性注入
                if (matchingBeans.isEmpty()) {
                    return null;
                }
                if (autowiredBeanNames != null) {
                    autowiredBeanNames.addAll(matchingBeans.keySet());
                }
                TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
                //将得到的Bean的候选者们转换为属性类型，如从set转换为Array,List等
                Object result = converter.convertIfNecessary(matchingBeans.values(), type);
                if (getDependencyComparator() != null && result instanceof Object[]) {
                    Arrays.sort((Object[]) result, adaptDependencyComparator(matchingBeans));
                }
                return result;
            }
            else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
                //获取Collection的泛型
                Class<?> elementType = descriptor.getResolvableType().asCollection().resolveGeneric();
                if (elementType == null) {
                    return null;
                }
                Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType,
                        new MultiElementDescriptor(descriptor));
                if (matchingBeans.isEmpty()) {
                    return null;
                }
                if (autowiredBeanNames != null) {
                    autowiredBeanNames.addAll(matchingBeans.keySet());
                }
                TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
                Object result = converter.convertIfNecessary(matchingBeans.values(), type);
                if (getDependencyComparator() != null && result instanceof List) {
                    Collections.sort((List<?>) result, adaptDependencyComparator(matchingBeans));
                }
                return result;
            }
            else if (Map.class == type) { 
                ResolvableType mapType = descriptor.getResolvableType().asMap();
                Class<?> keyType = mapType.resolveGeneric(0);
                if (String.class != keyType) {
                    return null;
                }
                Class<?> valueType = mapType.resolveGeneric(1);
                if (valueType == null) {
                    return null;
                }
                Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType,
                        new MultiElementDescriptor(descriptor));
                if (matchingBeans.isEmpty()) {
                    return null;
                }
                if (autowiredBeanNames != null) {
                    autowiredBeanNames.addAll(matchingBeans.keySet());
                }
                return matchingBeans;
            }
            else {
                return null;
            }
        }

Spring IOC容器通过类型到容器呢择取符合的Bean的方法:

**代码块7**

protected Map<String, Object> findAutowireCandidates(
                String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
            //Spring容IOC容器中获取所有的符合类型的Bean，包括非Singleton的scope,
            //也就是说request,session,prototype的均会被实例化，
            //request,session如果此时被实例化会报异常，因为这两个作用域的Bean实际是存储在HttpRequest中的，
            //此时还没有Http请求，如果是FactoryBean,则匹配getObject（）放回的类型
            String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                    this, requiredType, true, descriptor.isEager());
            Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
            //首先从容器自身注册了的依赖解析来匹配，Spring容器自身注册了很多Bean的依赖，
            //当使用者想要注入指定类型的Bean时，会优先从已注册的依赖内寻找匹配，这点很重要，代码详解请看此段下一段。
            for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
                if (autowiringType.isAssignableFrom(requiredType)) {
                    Object autowiringValue = this.resolvableDependencies.get(autowiringType);
                    autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
                    //如果注册的依赖Bean类型时指定类型的实例或是其父类，接口，则将其作为候选者，注册依赖的类型不会重复
                    if (requiredType.isInstance(autowiringValue)) {
                        result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
                        break;
                    }
                }
            }
            for (String candidate : candidateNames) {
                //@Autowried标识的属性不能和定义其的类的类型相同，就是类不能再注入相同类，会触发无限递归注入
                if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, 
                    addCandidateEntry(result, candidate, descriptor, requiredType);
                }
            }
            if (result.isEmpty() && !indicatesMultipleBeans(requiredType)) {
                // Consider fallback matches if the first pass failed to find anything...
                DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
                for (String candidate : candidateNames) {
                    if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)) {
                        addCandidateEntry(result, candidate, descriptor, requiredType);
                    }
                }
                if (result.isEmpty()) {
                    // Consider self references as a final pass...
                    // but in the case of a dependency collection, not the very same bean itself.
                    for (String candidate : candidateNames) {
                        if (isSelfReference(beanName, candidate) &&
                                (!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
                                isAutowireCandidate(candidate, fallbackDescriptor)) {
                            addCandidateEntry(result, candidate, descriptor, requiredType);
                        }
                    }
                }
            }
            return result;
        }
    
            /** * Add an entry to the candidate map: a bean instance if available or just the resolved * type, preventing early bean initialization ahead of primary candidate selection. */
        private void addCandidateEntry(Map<String, Object> candidates, String candidateName,
                DependencyDescriptor descriptor, Class<?> requiredType) {
            //当@Autowired标识的是容器类型的属性，生成的依赖描述类型是MultiElementDescriptor ，
            //因此所有的候选者均是合格的，所以会当场实例化他们。而如果属性的类型非容器，那么可能是多个候选者中挑一个，
            //此时实例化他们所有就不合适了，最终会把合格的那个实例化，如果没有合格的则不实例化，
            //提前实例化对Bean的很多方面有影响，比如AOP，EarlyReference等 */
            if (descriptor instanceof MultiElementDescriptor || containsSingleton(candidateName)) {
                candidates.put(candidateName, descriptor.resolveCandidate(candidateName, requiredType, this));
            }
            else {
                candidates.put(candidateName, getType(candidateName));
            }
        }

**如下代码所示，Spring在开始实例化Bean之前，已经注册了很多Bean的历依赖关系对，当@Autowired标识的属性类型是已经注册了的依赖类型，则将这些注册的对象作为候选者。这些对象并不一定是容器内的Bean，比如Http请求相关的对象，他们被注入的优先级比容器内Bean高。同时也忽略了一些依赖关系，也就是有些类型的Bean不允许被注入，这个实现方式稍后会看到**

**代码块8**

public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext, DisposableBean { 
    
        protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
            // Tell the internal bean factory to use the context's class loader etc.
            beanFactory.setBeanClassLoader(getClassLoader());
            beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
            beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
    
            // Configure the bean factory with context callbacks.
            beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
            beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
            beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
            beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
            beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
            beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
            beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
    
            // BeanFactory interface not registered as resolvable type in a plain factory.
            // MessageSource registered (and found for autowiring) as a bean.
            beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
            beanFactory.registerResolvableDependency(ResourceLoader.class, this);
            beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
            beanFactory.registerResolvableDependency(ApplicationContext.class, this);
        }
    }
    
    public abstract class WebApplicationContextUtils { 
    
            public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, ServletContext sc) {
            beanFactory.registerScope(WebApplicationContext.SCOPE_REQUEST, new RequestScope());
            beanFactory.registerScope(WebApplicationContext.SCOPE_SESSION, new SessionScope(false));
            beanFactory.registerScope(WebApplicationContext.SCOPE_GLOBAL_SESSION, new SessionScope(true));
            if (sc != null) {
                ServletContextScope appScope = new ServletContextScope(sc);
                beanFactory.registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
                // Register as ServletContext attribute, for ContextCleanupListener to detect it.
                sc.setAttribute(ServletContextScope.class.getName(), appScope);
            }
    
            beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
            beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
            beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
            beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
            if (jsfPresent) {
                FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
            }
        }
    
    }

普通属性输入，但是候选者有多个，如何从中挑选出最优解.

**代码块9**

protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {
            Class<?> requiredType = descriptor.getDependencyType();
            //根据@Primary注解来择取最优解
            String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
            if (primaryCandidate != null) {
                return primaryCandidate;
            }
            //根据@Order,@PriorityOrder，及实现Order接口的序号来择取最优解
            String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
            if (priorityCandidate != null) {
                return priorityCandidate;
            }
            // Fallback
            for (Map.Entry<String, Object> entry : candidates.entrySet()) {
                String candidateName = entry.getKey();
                Object beanInstance = entry.getValue();
    
                //如果通过以上两步都不能选择出最优解，则使用最基本的策略
                //首先如果这个类型已经由Spring注册过依赖关系对，则直接使用注册的对象，
                //候选者集合是LinkedHashMap,有序Map集合，容器注册的依赖对象位于LinkedHashMap的起始位置
                //如果没有注册过此类型的依赖关系，则根据属性的名称来匹配，、
                //如果属性名称和某个候选者的Bean名称或别名一致，那么直接将此Bean作为最优解
                if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
                        matchesBeanName(candidateName, descriptor.getDependencyName())) {
                    return candidateName;
                }
            }
            return null;
        }
    
    
        protected String determinePrimaryCandidate(Map<String, Object> candidates, Class<?> requiredType) {
            String primaryBeanName = null;
            for (Map.Entry<String, Object> entry : candidates.entrySet()) {
                String candidateBeanName = entry.getKey();
                Object beanInstance = entry.getValue();
                //候选者可以是父容器内的标识了@Primary的Bean，也可以是当前容器的。SpringMVC容器将Spring容器作为父容器
                if (isPrimary(candidateBeanName, beanInstance)) {
                    if (primaryBeanName != null) {
                        boolean candidateLocal = containsBeanDefinition(candidateBeanName);
                        boolean primaryLocal = containsBeanDefinition(primaryBeanName);
                        //此处确保同一个容器中同一个类型的多个Bean最多只有一个Bean标识了@Primary
                        if (candidateLocal && primaryLocal) {
                            throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
                                    "more than one 'primary' bean found among candidates: " + candidates.keySet());
                        }
                        //如果上一个@Primary的Bean是父容器的，则用当前容器的候选者覆盖之前的@Primary的Bean
                        else if (candidateLocal) {
                            primaryBeanName = candidateBeanName;
                        }
                    }
                    else {
                        primaryBeanName = candidateBeanName;
                    }
                }
            }
            return primaryBeanName;
        }
    
        protected String determineHighestPriorityCandidate(Map<String, Object> candidates, Class<?> requiredType) {
            String highestPriorityBeanName = null;
            Integer highestPriority = null;
            for (Map.Entry<String, Object> entry : candidates.entrySet()) {
                String candidateBeanName = entry.getKey();
                Object beanInstance = entry.getValue();
                Integer candidatePriority = getPriority(beanInstance);
                if (candidatePriority != null) {
                    if (highestPriorityBeanName != null) {
                        //不能同时存在两个最高优先级的序号
                        if (candidatePriority.equals(highestPriority)) {
                            throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
                                    "Multiple beans found with the same priority ('" + highestPriority +
                                    "') among candidates: " + candidates.keySet());
                        } //使用优先级序号最小的Bean作为最优解
                        else if (candidatePriority < highestPriority) {
                            highestPriorityBeanName = candidateBeanName;
                            highestPriority = candidatePriority;
                        }
                    }
                    else {
                        highestPriorityBeanName = candidateBeanName;
                        highestPriority = candidatePriority;
                    }
                }
            }
            return highestPriorityBeanName;
        }

@Autowired标识方法的解析过程和标识属性类似，代码详解如下:

private class AutowiredMethodElement extends InjectionMetadata.InjectedElement { 
    
            private final boolean required;
    
            private volatile boolean cached = false;
    
            private volatile Object[] cachedMethodArguments;
    
            public AutowiredMethodElement(Method method, boolean required, PropertyDescriptor pd) {
                super(method, pd);
                this.required = required;
            }
    
            @Override
            protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
                if (checkPropertySkipping(pvs)) {
                    return;
                }
                Method method = (Method) this.member;
                Object[] arguments;
                if (this.cached) { //如果已经解析过，且缓存了方法的参数注入，则不需要再次解析
                    // Shortcut for avoiding synchronization...
                    arguments = resolveCachedArguments(beanName);
                }
                else {
                    Class<?>[] paramTypes = method.getParameterTypes();
                    arguments = new Object[paramTypes.length];
                    DependencyDescriptor[] descriptors = new DependencyDescriptor[paramTypes.length];
                    Set<String> autowiredBeans = new LinkedHashSet<String>(paramTypes.length);
                    TypeConverter typeConverter = beanFactory.getTypeConverter();
                    for (int i = 0; i < arguments.length; i++) {
                        MethodParameter methodParam = new MethodParameter(method, i);
                        DependencyDescriptor currDesc = new DependencyDescriptor(methodParam, this.required);
                        currDesc.setContainingClass(bean.getClass());
                        descriptors[i] = currDesc;
                        try { //根据参数类型从Spring容器中寻找合适的Bean
                            Object arg = beanFactory.resolveDependency(currDesc, beanName, autowiredBeans, typeConverter);
                            //如果参数解析失败，但是required = false,则忽略执行此方法
                            if (arg == null && !this.required) {
                                arguments = null;
                                break;
                            }
                            arguments[i] = arg;
                        }
                        catch (BeansException ex) {
                            throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(methodParam), ex);
                        }
                    }
                    synchronized (this) {
                        if (!this.cached) {
                            if (arguments != null) {
                                this.cachedMethodArguments = new Object[paramTypes.length];
                                for (int i = 0; i < arguments.length; i++) {
                                    this.cachedMethodArguments[i] = descriptors[i];
                                }
                                registerDependentBeans(beanName, autowiredBeans);
                                if (autowiredBeans.size() == paramTypes.length) {
                                    Iterator<String> it = autowiredBeans.iterator();
                                    for (int i = 0; i < paramTypes.length; i++) {
                                        String autowiredBeanName = it.next();
                                        if (beanFactory.containsBean(autowiredBeanName)) {
                                            if (beanFactory.isTypeMatch(autowiredBeanName, paramTypes[i])) {
                                                this.cachedMethodArguments[i] = new ShortcutDependencyDescriptor(
                                                        descriptors[i], autowiredBeanName, paramTypes[i]);
                                            }
                                        }
                                    }
                                }
                            }
                            else {
                                this.cachedMethodArguments = null;
                            }
                            this.cached = true;
                        }
                    }
                }
                if (arguments != null) {
                    try {
                        ReflectionUtils.makeAccessible(method);
                        //参数解析完毕，执行此方法。@Autowired标识的方法执行的比@PostConstruct早
                        method.invoke(bean, arguments);  
                    }
                    catch (InvocationTargetException ex){
                        throw ex.getTargetException();
                    }
                }
            }
    
            private Object[] resolveCachedArguments(String beanName) {
                if (this.cachedMethodArguments == null) {
                    return null;
                }
                Object[] arguments = new Object[this.cachedMethodArguments.length];
                for (int i = 0; i < arguments.length; i++) {
                    arguments[i] = resolvedCachedArgument(beanName, this.cachedMethodArguments[i]);
                }
                return arguments;
            }
        }