BeanDefinition 实例化过程

BeanDefinition 实例化过程

AbstractBeanFactory#doGetBean

AbstractBeanFactory#
    /**
     *  根据 bean 名称读取 bean
     *
     * @param name  bean 名称
     * @param requiredType  返回类型
     * @param args  参数
     * @param typeCheckOnly 只执行类型检测
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
        // 获取规范名称【将别名转换为规范名称】
        final String beanName = transformedBeanName(name);
        Object bean;

        // 1）尝试从单例缓存中读取【手动注册的单例】
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            if (logger.isTraceEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }
        // 2）创建单例
        else {
            // 此多例 bean 是否在创建过程中
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 检查 parentBeanFactory 中是否存在 Bean 定义
            final BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                final String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
                else if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else if (requiredType != null) {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
                else {
                    return (T) parentBeanFactory.getBean(nameToLookup);
                }
            }

            // 如果无需类型检查，则标记指定的 bean 已经创建
            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 保证当前 bean 依赖的 bean 已经初始化
                final String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (final String dep : dependsOn) {
                        // 是否有循环依赖
                        if (isDependent(beanName, dep)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                        }
                        // 注册依赖
                        registerDependentBean(dep, beanName);
                        try {
                            // 实例化依赖的 bean
                            getBean(dep);
                        }
                        catch (final NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                // 1）创建 bean 单例
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, () -> {
                        try {
                            // AbstractAutowireCapableBeanFactory
                            return createBean(beanName, mbd, args);
                        }
                        catch (final BeansException ex) {
                            destroySingleton(beanName);
                            throw ex;
                        }
                    });
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }
                // 2）创建多例 bean 实例
                else if (mbd.isPrototype()) {
                    Object prototypeInstance = null;
                    try {
                        beforePrototypeCreation(beanName);
                        prototypeInstance = createBean(beanName, mbd, args);
                    }
                    finally {
                        afterPrototypeCreation(beanName);
                    }
                    bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                }
                // 3）创建指定作用域的 bean
                else {
                    final String scopeName = mbd.getScope();
                    final Scope scope = scopes.get(scopeName);
                    if (scope == null) {
                        throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                    }
                    try {
                        final Object scopedInstance = scope.get(beanName, () -> {
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                afterPrototypeCreation(beanName);
                            }
                        });
                        bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    }
                    catch (final IllegalStateException ex) {
                        throw new BeanCreationException(beanName,
                                "Scope '" + scopeName + "' is not active for the current thread; consider " +
                                        "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                                        ex);
                    }
                }
            }
            catch (final BeansException ex) {
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
        }

        // 请求类型是否匹配目标实例
        if (requiredType != null && !requiredType.isInstance(bean)) {
            try {
                // 执行类型转换
                final T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
                if (convertedBean == null) {
                    throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
                }
                return convertedBean;
            }
            catch (final TypeMismatchException ex) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Failed to convert bean '" + name + "' to required type '" +
                            ClassUtils.getQualifiedName(requiredType) + "'", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        // 返回 bean
        return (T) bean;
    }

DefaultSingletonBeanRegistry#
    /** 单例对象缓存: bean 名称映射到 bean 实例 */
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

    /** 单例工厂缓存： bean 名称映射到 ObjectFactory，用于处理循环依赖 */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

    /** 早期单例对象缓存: bean 名称映射到 bean 实例，用于处理循环依赖，此 bean 还未初始化 */
    private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

    /** 已经注册的单例 bean 名称，按照注册顺序写入 */
    private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

    /** 当前正在创建的 bean 的名称，用于处理循环依赖 */
    private final Set<String> singletonsCurrentlyInCreation =
            Collections.newSetFromMap(new ConcurrentHashMap<>(16));

    @Override
    @Nullable
    public Object getSingleton(String beanName) {
        return getSingleton(beanName, true);
    }

    /**
     *  根据 bean 的名称获取单例
     */
    @Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        // 此单例已经实例化
        Object singletonObject = singletonObjects.get(beanName);
        // 当前 beanName 在创建过程中
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (singletonObjects) {
                // 从 earlySingletonObjects 中读取 bean
                singletonObject = earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    // 如果允许循环引用，则从 singletonFactories 中读取 ObjectFactory 并创建实例
                    final ObjectFactory<?> singletonFactory = singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        // 将新建的单例加入到 earlySingletonObjects 中
                        singletonObject = singletonFactory.getObject();
                        earlySingletonObjects.put(beanName, singletonObject);
                        singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }

    public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "Bean name must not be null");
        synchronized (singletonObjects) {
            Object singletonObject = singletonObjects.get(beanName);
            if (singletonObject == null) {
                if (singletonsCurrentlyInDestruction) {
                    throw new BeanCreationNotAllowedException(beanName,
                            "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                            "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
                }
                // 1）将 beanName 写入 singletonsCurrentlyInCreation
                beforeSingletonCreation(beanName);
                boolean newSingleton = false;
                final boolean recordSuppressedExceptions = suppressedExceptions == null;
                if (recordSuppressedExceptions) {
                    suppressedExceptions = new LinkedHashSet<>();
                }
                try {
                    // 2）创建实例
                    singletonObject = singletonFactory.getObject();
                    newSingleton = true;
                }
                catch (final IllegalStateException ex) {
                    // Has the singleton object implicitly appeared in the meantime ->
                    // if yes, proceed with it since the exception indicates that state.
                    singletonObject = singletonObjects.get(beanName);
                    if (singletonObject == null) {
                        throw ex;
                    }
                }
                catch (final BeanCreationException ex) {
                    if (recordSuppressedExceptions) {
                        for (final Exception suppressedException : suppressedExceptions) {
                            ex.addRelatedCause(suppressedException);
                        }
                    }
                    throw ex;
                }
                finally {
                    if (recordSuppressedExceptions) {
                        suppressedExceptions = null;
                    }
                    // 将 beanName 从 singletonsCurrentlyInCreation 中移除
                    afterSingletonCreation(beanName);
                }
                if (newSingleton) {
                    /**
                     * 将单例写入 singletonObjects 和 registeredSingletons，
                     * 并从为解决循环依赖而使用的 earlySingletonObjects 和 singletonFactories 中移除
                     */
                    addSingleton(beanName, singletonObject);
                }
            }
            return singletonObject;
        }
    }

/**
 *  默认创建 bean 的抽象 Bean 工厂，具有 RootBeanDefinition 指定的所有功能。
 *  提供 bean 创建、属性填充、自动注入、初始化、处理运行时 bean 引用、调用初始化方法等。
 *  支持基于构造函数的注入、基于属性名称的注入、基于属性类型的注入。
 */
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
    /**
     *  创建 Bean 的核心方法
     *  1）实例化
     *  2）依赖注入属性
     *  3）执行 bean 的后处理
     */
    @Override
    protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {
        if (logger.isTraceEnabled()) {
            logger.trace("Creating instance of bean '" + beanName + "'");
        }
        RootBeanDefinition mbdToUse = mbd;

        // 1）保证 bean 类型已经完成解析
        final Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        //  2）准备方法覆盖
        try {
            mbdToUse.prepareMethodOverrides();
        }
        catch (final BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                    beanName, "Validation of method overrides failed", ex);
        }

        try {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            final Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
            // 如果创建成功，则返回
            if (bean != null) {
                return bean;
            }
        }
        catch (final Throwable ex) {
            throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                    "BeanPostProcessor before instantiation of bean failed", ex);
        }

        try {
            // 创建 bean 实例
            final Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            if (logger.isTraceEnabled()) {
                logger.trace("Finished creating instance of bean '" + beanName + "'");
            }
            return beanInstance;
        }
        catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
            // A previously detected exception with proper bean creation context already,
            // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
            throw ex;
        }
        catch (final Throwable ex) {
            throw new BeanCreationException(
                    mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
        }
    }

    /**
     *  创建 Bean 实例
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
            throws BeanCreationException {
        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        // 如果是单例
        if (mbd.isSingleton()) {
            // 如果是未完成 FactoryBean 实例
            instanceWrapper = factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            // 创建 BeanWrapper
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = instanceWrapper.getWrappedInstance();
        final Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            // 写入 bean Class
            mbd.resolvedTargetType = beanType;
        }

        // Allow post-processors to modify the merged bean definition.
        synchronized (mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                try {
                    applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                }
                catch (final Throwable ex) {
                    throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                            "Post-processing of merged bean definition failed", ex);
                }
                mbd.postProcessed = true;
            }
        }

        // 急切地缓存未创建完成的单例，以解决循环依赖问题
        final boolean earlySingletonExposure = mbd.isSingleton() && allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName);
        if (earlySingletonExposure) {
            if (logger.isTraceEnabled()) {
                logger.trace("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
            // 将此实例加入 singletonFactories
            addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        }

        // 初始化 bean 实例
        Object exposedObject = bean;
        try {
            // 依赖注入属性
            populateBean(beanName, mbd, instanceWrapper);
            // 触发 bean 实例的后处理
            exposedObject = initializeBean(beanName, exposedObject, mbd);
        }
        catch (final Throwable ex) {
            if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
                throw (BeanCreationException) ex;
            }
            else {
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
            }
        }

        if (earlySingletonExposure) {
            final Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    final String[] dependentBeans = getDependentBeans(beanName);
                    final Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                    for (final String dependentBean : dependentBeans) {
                        if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }
                    if (!actualDependentBeans.isEmpty()) {
                        throw new BeanCurrentlyInCreationException(beanName,
                                "Bean with name '" + beanName + "' has been injected into other beans [" +
                                        StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                                        "] in its raw version as part of a circular reference, but has eventually been " +
                                        "wrapped. This means that said other beans do not use the final version of the " +
                                        "bean. This is often the result of over-eager type matching - consider using " +
                                "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                    }
                }
            }
        }

        // Register bean as disposable.
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (final BeanDefinitionValidationException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }

    /**
     *  使用指定的实例化策略创建 bean 实例：
     *  工厂方法、有参构造函数、无参构造函数
     */
    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
        // 决定 bean 类型
        final Class<?> beanClass = resolveBeanClass(mbd, beanName);
        // 目标类型不是 public && 不允许通过非 public 方法创建实例，则抛出 BeanCreationException 异常
        if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
        }

        // 1）尝试通过实例生成器创建 bean
        final Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
        if (instanceSupplier != null) {
            return obtainFromSupplier(instanceSupplier, beanName);
        }

        // 2）尝试通过工厂方法创建 bean
        if (mbd.getFactoryMethodName() != null) {
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        // Shortcut when re-creating the same bean...
        boolean resolved = false;
        boolean autowireNecessary = false;
        if (args == null) {
            synchronized (mbd.constructorArgumentLock) {
                if (mbd.resolvedConstructorOrFactoryMethod != null) {
                    resolved = true;
                    autowireNecessary = mbd.constructorArgumentsResolved;
                }
            }
        }
        if (resolved) {
            if (autowireNecessary) {
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                return instantiateBean(beanName, mbd);
            }
        }

        // 3）尝试通过自动注入的构造函数创建 bean 实例
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        // 4）尝试通过首选构造函数创建
        ctors = mbd.getPreferredConstructors();
        if (ctors != null) {
            return autowireConstructor(beanName, mbd, ctors, null);
        }

        // 5）使用无参构造函数创建实例
        return instantiateBean(beanName, mbd);
    }

    protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
        try {
            Object beanInstance;
            final BeanFactory parent = this;
            if (System.getSecurityManager() != null) {
                beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
                getInstantiationStrategy().instantiate(mbd, beanName, parent),
                getAccessControlContext());
            }
            else {
                // 使用指定的实例化策略创建 bean 实例，一般通过 BeanUtils.instantiateClass(constructorToUse) 方法创建
                beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
            }
            final BeanWrapper bw = new BeanWrapperImpl(beanInstance);
            initBeanWrapper(bw);
            return bw;
        }
        catch (final Throwable ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
        }
    }

    protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
        for (final BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof MergedBeanDefinitionPostProcessor) {
                final MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
                bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
            }
        }
    }

    /**
     *  获取对指定bean的早期访问的引用，主要用于解决循环依赖
     */
    protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
        Object exposedObject = bean;
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (final BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                    final SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                    exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
                }
            }
        }
        return exposedObject;
    }

    /**
     *  使用从 BeanDefinition 中读取的属性值填充 bean 属性
     */
    @SuppressWarnings("deprecation")  // for postProcessPropertyValues
    protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
        if (bw == null) {
            if (mbd.hasPropertyValues()) {
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
            }
            else {
                // Skip property population phase for null instance.
                return;
            }
        }

        /**
         *  给 InstantiationAwareBeanPostProcessor 修改依赖注入流程的机会，
         *  可以实现属性注入。
         */
        boolean continueWithPropertyPopulation = true;

        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (final BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    final InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        continueWithPropertyPopulation = false;
                        break;
                    }
                }
            }
        }

        // 如果已经完成了字段的注入，则直接返回
        if (!continueWithPropertyPopulation) {
            return;
        }

        // 读取 PropertyValues
        PropertyValues pvs = mbd.hasPropertyValues() ? mbd.getPropertyValues() : null;
        if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
            final MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
            // Add property values based on autowire by name if applicable.
            if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
                autowireByName(beanName, mbd, bw, newPvs);
            }
            // Add property values based on autowire by type if applicable.
            if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
                autowireByType(beanName, mbd, bw, newPvs);
            }
            pvs = newPvs;
        }

        final boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        final boolean needsDepCheck = mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE;

        PropertyDescriptor[] filteredPds = null;
        if (hasInstAwareBpps) {
            if (pvs == null) {
                pvs = mbd.getPropertyValues();
            }
            for (final BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    final InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    // 尝试执行部分属性的依赖注入【注解切面】
                    PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
                    if (pvsToUse == null) {
                        if (filteredPds == null) {
                            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                        }
                        pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvsToUse == null) {
                            return;
                        }
                    }
                    pvs = pvsToUse;
                }
            }
        }
        if (needsDepCheck) {
            if (filteredPds == null) {
                filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            }
            checkDependencies(beanName, mbd, filteredPds, pvs);
        }

        // 执行剩余属性的依赖注入
        if (pvs != null) {
            applyPropertyValues(beanName, mbd, bw, pvs);
        }
    }

    protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
        // 没有属性需要执行依赖注入，直接返回
        if (pvs.isEmpty()) {
            return;
        }

        if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) {
            ((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
        }

        MutablePropertyValues mpvs = null;
        List<PropertyValue> original;

        if (pvs instanceof MutablePropertyValues) {
            mpvs = (MutablePropertyValues) pvs;
            if (mpvs.isConverted()) {
                // Shortcut: use the pre-converted values as-is.
                try {
                    bw.setPropertyValues(mpvs);
                    return;
                }
                catch (final BeansException ex) {
                    throw new BeanCreationException(
                            mbd.getResourceDescription(), beanName, "Error setting property values", ex);
                }
            }
            original = mpvs.getPropertyValueList();
        }
        else {
            // 读取待注入的属性列表
            original = Arrays.asList(pvs.getPropertyValues());
        }

        // 读取自定义类型转换器
        TypeConverter converter = getCustomTypeConverter();
        if (converter == null) {
            converter = bw;
        }

        // BeanDefinition 值解析器
        final BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);

        // 创建一个深度拷贝，解析值引用
        final List<PropertyValue> deepCopy = new ArrayList<>(original.size());
        boolean resolveNecessary = false;
        for (final PropertyValue pv : original) {
            // 已经完成转换
            if (pv.isConverted()) {
                deepCopy.add(pv);
            }
            else {
                // 读取属性名称
                final String propertyName = pv.getName();
                // 读取元素值
                final Object originalValue = pv.getValue();
                // 解析值
                final Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
                Object convertedValue = resolvedValue;
                final boolean convertible = bw.isWritableProperty(propertyName) &&
                        !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
                // 如果需要转换，则执行类型转换
                if (convertible) {
                    convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
                }
                // 写入 BeanDefinition 中避免再次转换
                if (resolvedValue == originalValue) {
                    if (convertible) {
                        pv.setConvertedValue(convertedValue);
                    }
                    deepCopy.add(pv);
                }
                else if (convertible && originalValue instanceof TypedStringValue &&
                        !((TypedStringValue) originalValue).isDynamic() &&
                        !(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
                    pv.setConvertedValue(convertedValue);
                    deepCopy.add(pv);
                }
                else {
                    // 创建 PropertyValue 并加入 deepCopy
                    resolveNecessary = true;
                    deepCopy.add(new PropertyValue(pv, convertedValue));
                }
            }
        }
        if (mpvs != null && !resolveNecessary) {
            mpvs.setConverted();
        }

        // 将深度拷贝对象注入 bean 的属性中
        try {
            bw.setPropertyValues(new MutablePropertyValues(deepCopy));
        }
        catch (final BeansException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Error setting property values", ex);
        }
    }

AbstractPropertyAccessor#
    @Override
    public void setPropertyValues(PropertyValues pvs) throws BeansException {
        setPropertyValues(pvs, false, false);
    }

    @Override
    public void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown) throws BeansException {
        setPropertyValues(pvs, ignoreUnknown, false);
    }

    @Override
    public void setPropertyValues(PropertyValues pvs, boolean ignoreUnknown, boolean ignoreInvalid)
            throws BeansException {
        List<PropertyAccessException> propertyAccessExceptions = null;
        final List<PropertyValue> propertyValues = pvs instanceof MutablePropertyValues? ((MutablePropertyValues) pvs).getPropertyValueList() : Arrays.asList(pvs.getPropertyValues());
        for (final PropertyValue pv : propertyValues) {
            try {
                // 设置的属性
                setPropertyValue(pv);
            } catch (final NotWritablePropertyException ex) {
                if (!ignoreUnknown) {
                    throw ex;
                }
                // Otherwise, just ignore it and continue...
            } catch (final NullValueInNestedPathException ex) {
                if (!ignoreInvalid) {
                    throw ex;
                }
                // Otherwise, just ignore it and continue...
            } catch (final PropertyAccessException ex) {
                if (propertyAccessExceptions == null) {
                    propertyAccessExceptions = new ArrayList<>();
                }
                propertyAccessExceptions.add(ex);
            }
        }

        // If we encountered individual exceptions, throw the composite exception.
        if (propertyAccessExceptions != null) {
            final PropertyAccessException[] paeArray = propertyAccessExceptions.toArray(new PropertyAccessException[0]);
            throw new PropertyBatchUpdateException(paeArray);
        }
    }

AbstractNestablePropertyAccessor#
    @Override
    public void setPropertyValue(PropertyValue pv) throws BeansException {
        PropertyTokenHolder tokens = (PropertyTokenHolder) pv.resolvedTokens;
        // 1）属性名称为解析
        if (tokens == null) {
            // 读取属性名称
            final String propertyName = pv.getName();
            AbstractNestablePropertyAccessor nestedPa;
            try {
                // 读取最内层的属性访问器，如果存在嵌套属性
                nestedPa = getPropertyAccessorForPropertyPath(propertyName);
            }
            catch (final NotReadablePropertyException ex) {
                throw new NotWritablePropertyException(getRootClass(), nestedPath + propertyName,
                        "Nested property in path '" + propertyName + "' does not exist", ex);
            }
            // 读取最内层的属性名称
            tokens = getPropertyNameTokens(getFinalPath(nestedPa, propertyName));
            if (nestedPa == this) {
                // 无嵌套属性
                pv.getOriginalPropertyValue().resolvedTokens = tokens;
            }
            // 通过属性访问器设置属性值
            nestedPa.setPropertyValue(tokens, pv);
        }
        // 2）属性名称已解析
        else {
            setPropertyValue(tokens, pv);
        }
    }

    /**
     *  递归导航以返回最内层属性的属性访问器
     */
    @SuppressWarnings("unchecked")  // avoid nested generic
    protected AbstractNestablePropertyAccessor getPropertyAccessorForPropertyPath(String propertyPath) {
        final int pos = PropertyAccessorUtils.getFirstNestedPropertySeparatorIndex(propertyPath);
        // Handle nested properties recursively.
        if (pos > -1) {
            final String nestedProperty = propertyPath.substring(0, pos);
            final String nestedPath = propertyPath.substring(pos + 1);
            final AbstractNestablePropertyAccessor nestedPa = getNestedPropertyAccessor(nestedProperty);
            return nestedPa.getPropertyAccessorForPropertyPath(nestedPath);
        }
        else {
            return this;
        }
    }

    /**
     *  将属性名称解析为  token
     */
    private PropertyTokenHolder getPropertyNameTokens(String propertyName) {
        String actualName = null;
        // 内嵌的属性列表
        final List<String> keys = new ArrayList<>(2);
        int searchIndex = 0;
        // user['address'][name]
        while (searchIndex != -1) {
            // 读取 [ 的起始索引
            final int keyStart = propertyName.indexOf(PROPERTY_KEY_PREFIX, searchIndex);
            searchIndex = -1;
            if (keyStart != -1) {
                // 读取紧跟 ] 的索引
                final int keyEnd = propertyName.indexOf(PROPERTY_KEY_SUFFIX, keyStart + PROPERTY_KEY_PREFIX.length());
                if (keyEnd != -1) {
                    // 读取第一个属性名称
                    if (actualName == null) {
                        actualName = propertyName.substring(0, keyStart);
                    }
                    // 读取 key，如果 key 被 ' 或 " 包含，则将符号去除
                    String key = propertyName.substring(keyStart + PROPERTY_KEY_PREFIX.length(), keyEnd);
                    if (key.length() > 1 && key.startsWith("'") && key.endsWith("'") ||
                            key.startsWith("\"") && key.endsWith("\"")) {
                        key = key.substring(1, key.length() - 1);
                    }
                    // 加入 key
                    keys.add(key);
                    // 更新 searchIndex
                    searchIndex = keyEnd + PROPERTY_KEY_SUFFIX.length();
                }
            }
        }
        // 写入属性名称
        final PropertyTokenHolder tokens = new PropertyTokenHolder(actualName != null ? actualName : propertyName);
        // 如果存在分层属性
        if (!keys.isEmpty()) {
            // 写入规范名称 [address][name]
            tokens.canonicalName += PROPERTY_KEY_PREFIX +
                    StringUtils.collectionToDelimitedString(keys, PROPERTY_KEY_SUFFIX + PROPERTY_KEY_PREFIX) +
                    PROPERTY_KEY_SUFFIX;
            tokens.keys = StringUtils.toStringArray(keys);
        }
        return tokens;
    }

    protected void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
        if (tokens.keys != null) {
            processKeyedProperty(tokens, pv);
        }
        else {
            // 设置本地属性
            processLocalProperty(tokens, pv);
        }
    }

    private void processLocalProperty(PropertyTokenHolder tokens, PropertyValue pv) {
        // 获取基于属性描述符的 BeanPropertyHandler
        final PropertyHandler ph = getLocalPropertyHandler(tokens.actualName);
        if (ph == null || !ph.isWritable()) {
            if (pv.isOptional()) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Ignoring optional value for property '" + tokens.actualName +
                            "' - property not found on bean class [" + getRootClass().getName() + "]");
                }
                return;
            }
            else {
                throw createNotWritablePropertyException(tokens.canonicalName);
            }
        }

        // 1）旧值
        Object oldValue = null;
        try {
            // 2）读取原始值
            final Object originalValue = pv.getValue();
            Object valueToApply = originalValue;
            if (!Boolean.FALSE.equals(pv.conversionNecessary)) {
                if (pv.isConverted()) {
                    valueToApply = pv.getConvertedValue();
                }
                else {
                    if (isExtractOldValueForEditor() && ph.isReadable()) {
                        try {
                            oldValue = ph.getValue();
                        }
                        catch (Exception ex) {
                            if (ex instanceof PrivilegedActionException) {
                                ex = ((PrivilegedActionException) ex).getException();
                            }
                            if (logger.isDebugEnabled()) {
                                logger.debug("Could not read previous value of property '" +
                                        nestedPath + tokens.canonicalName + "'", ex);
                            }
                        }
                    }
                    // 如果必要，则执行转换
                    valueToApply = convertForProperty(
                            tokens.canonicalName, oldValue, originalValue, ph.toTypeDescriptor());
                }
                // 写入转换标识
                pv.getOriginalPropertyValue().conversionNecessary = valueToApply != originalValue;
            }
            // 通过 PropertyHandler 写入值
            ph.setValue(valueToApply);
        }
        catch (final TypeMismatchException ex) {
            throw ex;
        }
        catch (final InvocationTargetException ex) {
            final PropertyChangeEvent propertyChangeEvent = new PropertyChangeEvent(
                    getRootInstance(), nestedPath + tokens.canonicalName, oldValue, pv.getValue());
            if (ex.getTargetException() instanceof ClassCastException) {
                throw new TypeMismatchException(propertyChangeEvent, ph.getPropertyType(), ex.getTargetException());
            }
            else {
                Throwable cause = ex.getTargetException();
                if (cause instanceof UndeclaredThrowableException) {
                    // May happen e.g. with Groovy-generated methods
                    cause = cause.getCause();
                }
                throw new MethodInvocationException(propertyChangeEvent, cause);
            }
        }
        catch (final Exception ex) {
            final PropertyChangeEvent pce = new PropertyChangeEvent(
                    getRootInstance(), nestedPath + tokens.canonicalName, oldValue, pv.getValue());
            throw new MethodInvocationException(pce, ex);
        }
    }

BeanWrapperImpl#
    /**
     *  获取 BeanPropertyHandler
     */
    @Override
    @Nullable
    protected BeanPropertyHandler getLocalPropertyHandler(String propertyName) {
        final PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(propertyName);
        return pd != null ? new BeanPropertyHandler(pd) : null;
    }

    private class BeanPropertyHandler extends PropertyHandler {
        /**
         *  属性描述符
         */
        private final PropertyDescriptor pd;

        public BeanPropertyHandler(PropertyDescriptor pd) {
            super(pd.getPropertyType(), pd.getReadMethod() != null, pd.getWriteMethod() != null);
            this.pd = pd;
        }

        @Override
        public ResolvableType getResolvableType() {
            return ResolvableType.forMethodReturnType(pd.getReadMethod());
        }

        @Override
        public TypeDescriptor toTypeDescriptor() {
            return new TypeDescriptor(property(pd));
        }

        @Override
        @Nullable
        public TypeDescriptor nested(int level) {
            return TypeDescriptor.nested(property(pd), level);
        }

        /**
         *  读取值
         */
        @Override
        @Nullable
        public Object getValue() throws Exception {
            // 获取读方法
            final Method readMethod = pd.getReadMethod();
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    ReflectionUtils.makeAccessible(readMethod);
                    return null;
                });
                try {
                    return AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
                    readMethod.invoke(getWrappedInstance(), (Object[]) null), acc);
                }
                catch (final PrivilegedActionException pae) {
                    throw pae.getException();
                }
            }
            else {
                // 读取值
                ReflectionUtils.makeAccessible(readMethod);
                return readMethod.invoke(getWrappedInstance(), (Object[]) null);
            }
        }

        /**
         *  写入值
         */
        @Override
        public void setValue(final @Nullable Object value) throws Exception {
            // 获取写方法
            final Method writeMethod = pd instanceof GenericTypeAwarePropertyDescriptor ? ((GenericTypeAwarePropertyDescriptor) pd).getWriteMethodForActualAccess() :
                pd.getWriteMethod();
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    ReflectionUtils.makeAccessible(writeMethod);
                    return null;
                });
                try {
                    AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
                    writeMethod.invoke(getWrappedInstance(), value), acc);
                }
                catch (final PrivilegedActionException ex) {
                    throw ex.getException();
                }
            }
            else {
                // 写入值
                ReflectionUtils.makeAccessible(writeMethod);
                writeMethod.invoke(getWrappedInstance(), value);
            }
        }
    }

AbstractAutowireCapableBeanFactory#
    protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                invokeAwareMethods(beanName, bean);
                return null;
            }, getAccessControlContext());
        }
        else {
            // 1）执行 Aware 注入
            invokeAwareMethods(beanName, bean);
        }

        Object wrappedBean = bean;
        if (mbd == null || !mbd.isSynthetic()) {
            // 2）执行 BeanPostProcessor 的 postProcessBeforeInitialization 处理方法
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }

        try {
            // 3）执行初始化方法
            invokeInitMethods(beanName, wrappedBean, mbd);
        }
        catch (final Throwable ex) {
            throw new BeanCreationException(
                    mbd != null ? mbd.getResourceDescription() : null,
                            beanName, "Invocation of init method failed", ex);
        }
        if (mbd == null || !mbd.isSynthetic()) {
            // 4）执行 BeanPostProcessor 的 postProcessAfterInitialization 处理方法
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }

        // 返回创建完毕的 bean
        return wrappedBean;
    }

    private void invokeAwareMethods(final String beanName, final Object bean) {
        if (bean instanceof Aware) {
            // 注入 BeanName
            if (bean instanceof BeanNameAware) {
                ((BeanNameAware) bean).setBeanName(beanName);
            }
            // 注入 BeanClassLoader
            if (bean instanceof BeanClassLoaderAware) {
                final ClassLoader bcl = getBeanClassLoader();
                if (bcl != null) {
                    ((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
                }
            }
            // 注入 BeanFactory
            if (bean instanceof BeanFactoryAware) {
                ((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
            }
        }
    }

    /**
     *  执行 BeanPostProcessor#postProcessBeforeInitialization
     */
    @Override
    public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
            throws BeansException {
        Object result = existingBean;
        for (final BeanPostProcessor processor : getBeanPostProcessors()) {
            final Object current = processor.postProcessBeforeInitialization(result, beanName);
            if (current == null) {
                return result;
            }
            result = current;
        }
        return result;
    }

    /**
     *  执行 bean 实例最后一步处理
     */
    @Override
    public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
            throws BeansException {
        Object result = existingBean;
        for (final BeanPostProcessor processor : getBeanPostProcessors()) {
            final Object current = processor.postProcessAfterInitialization(result, beanName);
            if (current == null) {
                return result;
            }
            result = current;
        }
        return result;
    }

AbstractBeanFactory#
    /**
     *  返回实例本身或其创建的对象 FactoryBean
     */
    protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

        // bean 名称是否以 & 开头
        if (BeanFactoryUtils.isFactoryDereference(name)) {
            if (beanInstance instanceof NullBean) {
                return beanInstance;
            }
            if (!(beanInstance instanceof FactoryBean)) {
                throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
            }
        }

        /**
         *  如果不是 FactoryBean 则直接返回此对象
         */
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }

        Object object = null;
        if (mbd == null) {
            // 获取缓存的由 FactoryBean 创建的实例
            object = getCachedObjectForFactoryBean(beanName);
        }
        if (object == null) {
            final FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
            // Caches object obtained from FactoryBean if it is a singleton.
            if (mbd == null && containsBeanDefinition(beanName)) {
                mbd = getMergedLocalBeanDefinition(beanName);
            }
            final boolean synthetic = mbd != null && mbd.isSynthetic();
            // 从 FactoryBean 中获取 Bean 实例
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }