@Autowired通过源码进行原理详解

BeanPostPrecessors Invoke Procedure(执行流程)

1.InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation(Class<?> beanClass, String beanName);
invoke before doCreateBean(),if the method returns not null,the return Object will as the Bean, avoid invoke doCreateBean() ;

2.SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors(Class<?> beanClass, String beanName);
used to determine to use which Constructor to instance Bean;

3.MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
the Bean gets instantiated, modify the RootBeanDefinition ,such as adding PropertyValue etc. before populate Bean;

4.SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference(Object bean, String beanName);
invoke before populate Bean,used for resolve Circular Reference;

5.InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation(Object bean, String beanName);
invoke in populate Bean, but before fill in values，return false will break the opreation for filling values in Bean;

6.InstantiationAwareBeanPostProcessor.postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName);
invoke in populate Bean, but before fill in values，modify the PropertyValues which will be apply for the Bean,
return null will break the opreation for filling values in Bean;

7.BeanPostProcessor.postProcessBeforeInitialization(Object bean, String beanName);
invoked before invokeInitMethods, modify the finally Bean instance

8.BeanPostProcessor.postProcessAfterInitialization(Object bean, String beanName);
invoked after invokeInitMethods, modify the finally Bean instance

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
        implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
        //此方法的含义是解析Bean类里构造函数上的@Autowired注解，如果有合适的标识了@Autowired的构造函数,
        //则在实例化此Bean时会使用此构造函数，@Import和@ComponentScan得到的Bean会如此解析，
        //@Bean标识方法生成的Bean不会如此
        public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
            throws BeanCreationException {

        // Let's check for lookup methods here..
        if (!this.lookupMethodsChecked.contains(beanName)) {
            try {
                ReflectionUtils.doWithMethods(beanClass, new ReflectionUtils.MethodCallback() {
                    @Override
                    public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
                        Lookup lookup = method.getAnnotation(Lookup.class);
                        if (lookup != null) {
                            LookupOverride override = new LookupOverride(method, lookup.value());
                            try {
                                RootBeanDefinition mbd = (RootBeanDefinition) beanFactory.getMergedBeanDefinition(beanName);
                                mbd.getMethodOverrides().addOverride(override);
                            }
                            catch (NoSuchBeanDefinitionException ex) {
                                throw new BeanCreationException(beanName,
                                        "Cannot apply @Lookup to beans without corresponding bean definition");
                            }
                        }
                    }
                });
            }
            catch (IllegalStateException ex) {
                throw new BeanCreationException(beanName, "Lookup method resolution failed", ex);
            }
            catch (NoClassDefFoundError err) {
                throw new BeanCreationException(beanName, "Failed to introspect bean class [" + beanClass.getName() +
                        "] for lookup method metadata: could not find class that it depends on", err);
            }
            this.lookupMethodsChecked.add(beanName);
        }

        // Quick check on the concurrent map first, with minimal locking.
        Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
        if (candidateConstructors == null) {
            // Fully synchronized resolution now...
            synchronized (this.candidateConstructorsCache) {
                //对每个类的构造函数只解析一次，解析完会存储结果，以备下次服用
                candidateConstructors = this.candidateConstructorsCache.get(beanClass);
                if (candidateConstructors == null) {
                    Constructor<?>[] rawCandidates;
                    try {
                        rawCandidates = beanClass.getDeclaredConstructors();
                    }
                    catch (Throwable ex) {
                        throw new BeanCreationException(beanName,
                                "Resolution of declared constructors on bean Class [" + beanClass.getName() +
                                "] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
                    }
                    List<Constructor<?>> candidates = new ArrayList<Constructor<?>>(rawCandidates.length);
                    Constructor<?> requiredConstructor = null;
                    Constructor<?> defaultConstructor = null;
                    for (Constructor<?> candidate : rawCandidates) {
                        //查看构造函数上是否标识@Autowired注解，获取注解的属性
                        AnnotationAttributes ann = findAutowiredAnnotation(candidate);
                        if (ann == null) {
                            Class<?> userClass = ClassUtils.getUserClass(beanClass);
                            if (userClass != beanClass) {
                                try {
                                    Constructor<?> superCtor =
                                            userClass.getDeclaredConstructor(candidate.getParameterTypes());
                                    //如果此构造函数是重载了父类的构造函数，则寻找父类的构造函数，查看是否标识@Autowired注解，获取注解的属性
                                    ann = findAutowiredAnnotation(superCtor);
                                }
                                catch (NoSuchMethodException ex) {
                                    // Simply proceed, no equivalent superclass constructor found...
                                }
                            }
                        }
                        //当构造函数上存在@Autowired注解
                        if (ann != null) {
                            //@Autowired默认的required = true,当有一个required = true的@Autowired标识的构造函数时，
                            //不能有其他的构造函数再标识@Autowired，否则会报错
                            if (requiredConstructor != null) {
                                throw new BeanCreationException(beanName,
                                        "Invalid autowire-marked constructor: " + candidate +
                                        ". Found constructor with 'required' Autowired annotation already: " +
                                        requiredConstructor);
                            }
                            //@Autowired的required的属性值，true / false
                            boolean required = determineRequiredStatus(ann);
                            if (required) {
                                if (!candidates.isEmpty()) {
                                    throw new BeanCreationException(beanName,
                                            "Invalid autowire-marked constructors: " + candidates +
                                            ". Found constructor with 'required' Autowired annotation: " +
                                            candidate);
                                }
                                requiredConstructor = candidate;
                            }
                            candidates.add(candidate);
                        } 
                        else if (candidate.getParameterTypes().length == 0) {
                            defaultConstructor = candidate;
                        }
                    }
                    if (!candidates.isEmpty()) {
                        // Add default constructor to list of optional constructors, as fallback.
                        ////当没有构造函数标识@Autowired时，设置默认的构造函数作为额外选择
                        if (requiredConstructor == null) {
                            if (defaultConstructor != null) {
                                candidates.add(defaultConstructor);
                            }  //如果没有默认的无参构造函数，且有@Autowired（required = false）的构造函数，则发出警告信
                            else if (candidates.size() == 1 && logger.isWarnEnabled()) {
                                logger.warn("Inconsistent constructor declaration on bean with name '" + beanName +
                                        "': single autowire-marked constructor flagged as optional - " +
                                        "this constructor is effectively required since there is no " +
                                        "default constructor to fall back to: " + candidates.get(0));
                            }
                        }
                        candidateConstructors = candidates.toArray(new Constructor<?>[candidates.size()]);
                    }  //若只有一个构造函数，且没标识@Autowired，其参数长度>0，将其作为候选者
                    else if (rawCandidates.length == 1 && rawCandidates[0].getParameterTypes().length > 0) {
                        candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
                    } //没有合适的构造函数
                    else {
                        candidateConstructors = new Constructor<?>[0];
                    }
                    this.candidateConstructorsCache.put(beanClass, candidateConstructors);
                }
            }
        }
        return (candidateConstructors.length > 0 ? candidateConstructors : null);
    }

}

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);
    }

}

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;
    }
}

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);
            }
        }
    }
}

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
            }
        }
    }

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);
        }
    }

}

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;
        }
  }

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));
        }
    }

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);
        }
    }

}

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;
    }

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;
        }
    }