Spring详解Spring中Bean的加载

Posted 2020-09-08

之前写过bean的解析，这篇来讲讲bean的加载，加载要比bean的解析复杂些，该文之前在小编原文中有发表过，要看原文的可以直接点击原文查看，从之前的例子开始，Spring中加载一个bean的方式：

TestBean bean = factory.getBean("testBean");

来看看getBean(String name)方法源码，

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}

该getBean(String name)方法位于AbstractBeanFactory抽象类中，AbstractBeanFactory与XmlBeanFactory类关系可以看下图：

接下去跟进doGetBean()方法源码：

protected <T> T doGetBean(
        final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
        throws BeansException {
    // 提取beanName
    final String beanName = transformedBeanName(name);
    Object bean;

    // 试着从缓存或实例工厂中获取
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.debug("Returning eagerly cached instance of singleton bean ‘" + beanName +
                        "‘ that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.debug("Returning cached instance of singleton bean ‘" + beanName + "‘");
            }
        }
        // 返回实例，有时存在如BeanFactory这样情况时并不是直接返回实例本身而是返回指定方法返回的实例
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we‘re already creating this bean instance:
        // We‘re assumably within a circular reference.
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        // 如果在所有已经加载的类中没有beanName则会尝试从parentBeanFactory中检测
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            // 到BeanFactory查找
            if (args != null) {
                // Delegation to parent with explicit args.
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }
        // 如果不是只做类型检查则是创建bean
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            // 将存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition，如果BeanName是子Bean的话会合并父类的相关属性
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            String[] dependsOn = mbd.getDependsOn();
            // 如果存在依赖的话要递归实例化依赖的bean
            if (dependsOn != null) {
                for (String dependsOnBean : dependsOn) {
                    if (isDependent(beanName, dependsOnBean)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between ‘" + beanName + "‘ and ‘" + dependsOnBean + "‘");
                    }
                    registerDependentBean(dependsOnBean, beanName);
                    getBean(dependsOnBean);
                }
            }

            // Create bean instance.
            // 依赖的bean实例化完后就可以实例化mbd了
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                    @Override
                    public Object getObject() throws BeansException {
                        try {
                            return createBean(beanName, mbd, args);
                        }
                        catch (BeansException ex) {
                            // Explicitly remove instance from singleton cache: It might have been put there
                            // eagerly by the creation process, to allow for circular reference resolution.
                            // Also remove any beans that received a temporary reference to the bean.
                            destroySingleton(beanName);
                            throw ex;
                        }
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            else if (mbd.isPrototype()) {
                // It‘s a prototype -> create a new instance.
                Object prototypeInstance = null;
                try {
                    beforePrototypeCreation(beanName);
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    afterPrototypeCreation(beanName);
                }
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }

            else {
                // 在对应的scope上实例化bean
                String scopeName = mbd.getScope();
                final Scope scope = this.scopes.get(scopeName);
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope ‘" + scopeName + "‘");
                }
                try {
                    Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                afterPrototypeCreation(beanName);
                            }
                        }
                    });
                    bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                }
                catch (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 (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // 检查需要的类型是否符合实例bean的实际类型
    if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
        try {
            return getTypeConverter().convertIfNecessary(bean, requiredType);
        }
        catch (TypeMismatchException ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Failed to convert bean ‘" + name + "‘ to required type [" +
                        ClassUtils.getQualifiedName(requiredType) + "]", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

整个bean加载的过程步骤相对繁琐，主要步骤有以下几点：

1. 转换beanName
   要知道平时开发中传入的参数name可能只是别名，也可能是FactoryBean，所以需要进行解析转换，一般会进行以下解析：
   （1）消除修饰符，比如name="&test"，会去除&使name="test"；
   （2）取alias表示的最后的beanName，比如别名test01指向名称为test02的bean则返回test02。

2. 从缓存中加载实例
   实例在Spring的同一个容器中只会被创建一次，后面再想获取该bean时，就会尝试从缓存中获取；如果获取不到的话再从singletonFactories中加载。

3. 实例化bean
   缓存中记录的bean一般只是最原始的bean状态，这时就需要对bean进行实例化。如果得到的是bean的原始状态，但又要对bean进行处理，这时真正需要的是工厂bean中定义的factory-method方法中返回的bean，上面源码中的getObjectForBeanInstance就是来完成这个工作的。

4. 检测parentBeanFacotory
   从源码可以看出如果缓存中没有数据会转到父类工厂去加载，源码中的!containsBeanDefinition(beanName)就是检测如果当前加载的xml配置文件中不包含beanName所对应的配置，就只能到parentBeanFacotory去尝试加载bean。

5. 存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition之前的文章介绍过XML配置文件中读取到的bean信息是存储在GernericBeanDefinition中的，但Bean的后续处理是针对于RootBeanDefinition的，所以需要转换后才能进行后续操作。

6. 初始化依赖的bean
   这里应该比较好理解，就是bean中可能依赖了其他bean属性，在初始化bean之前会先初始化这个bean所依赖的bean属性。

7. 创建bean
   Spring容器根据不同scope创建bean实例。
   整个流程就是如此，下面会讲解一些重要步骤的源码。

上面有提到，单例在Spring中的同一容器中只会被创建一次，后面再获取bean的话会直接从缓存中获取，这里是尝试加载，先从缓存中加载，再次就是从singletonFactories中加载；因为在bean中可能会在依赖注入，要避免循环依赖，Spring创建bean时会不等bean创建完成就会将创建该bean的ObjectFactory提前曝光加入到缓存中，但下一个bean创建时要依赖上个bean的话，就直接使用ObjectFacotry。

@Override
public Object getSingleton(String beanName) {
    return getSingleton(beanName, true);  // true表示允许早期依赖
}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 尝试从缓存获取实例
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            // 若该bean正在加载则不处理
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    // 存入到缓存中
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
/* 这两个方法在DefaultSingletonBeanRegistry类中 */

源码可以看出这个方法先尝试从singletonObjects中获取实例，如果获取不到值就从earlySingletonObject中去获取，如果再获取不到的话则到singletonFactories里获取beanName对应的ObjectFactory，再调用这个ObjectFactory的getObject来创建bean，并放到earlySingletonObject中，并且从singletonFactories里remove掉这个ObjectFactory。这里有几个存储bean的不同map：
-singletonObjects：保存BeanName和创建bean实例之间的关系。
-singletonFactories：保存BeanName和创建bean实例的工厂之间的关系。
-earlySingletonObject：保存BeanName和创建bean实例之间的关系，与-singletonObjects不同的是当一个单例bean被放到里面后，那当bean在创建过程中，就可以通过getBean方法获取到，可以用来检测循环引用。
-registeredSingletons：保存当前所有已注册的bean。

如果上面缓存中不存在已经加载的单例bean就要重新开始bean的加载过程了，Spring中使用getSingleton重载方法实现bean的加载过程。

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "‘beanName‘ must not be null");
    synchronized (this.singletonObjects) {
        // 先检查bean是否已经加载
        Object singletonObject = this.singletonObjects.get(beanName);
        // 如果空才进行singleton的bean的初始化
        if (singletonObject == null) {
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                        "Singleton bean creation not allowed while the 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 + "‘");
            }
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<Exception>();
            }
            try {
                // 初始化bean
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                // Has the singleton object implicitly appeared in the meantime ->
                // if yes, proceed with it since the exception indicates that state.
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                // 存入缓存
                addSingleton(beanName, singletonObject);
            }
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }
}

这段代码使用了回调方法，使程序可以在单例创建的前后做一些准备及处理操作，真正的获取单例bean的方法其实并不是在这个方法实现的，而是在ObjectFactory类型的实例singletonFactory中实现的。

下面准备创建bean

看看createBean()方法源码（该方法在AbstractAutowireCapableBeanFactory类中）：

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
        throws BeanCreationException {

    if (logger.isDebugEnabled()) {
        logger.debug("Creating instance of bean ‘" + beanName + "‘");
    }
    // 锁定class，根据设置的class属性或根据className来解析Class
    resolveBeanClass(mbd, beanName);

    // 验证和准备覆盖的方法
    try {
        mbd.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
                beanName, "Validation of method overrides failed", ex);
    }

    try {
        // 用BeanPostProcessors返回代理来替代真正的实例
        Object bean = resolveBeforeInstantiation(beanName, mbd);
        if (bean != null) {
            return bean;
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                "BeanPostProcessor before instantiation of bean failed", ex);
    }

    Object beanInstance = doCreateBean(beanName, mbd, args);
    if (logger.isDebugEnabled()) {
        logger.debug("Finished creating instance of bean ‘" + beanName + "‘");
    }
    return beanInstance;
}

从createBean()方法源码可以看出主要做了以下操作：
1. 根据设置的class属性或根据className来解析Class；
2. 对覆盖进行标记并验证，在Spring配置中是存在lookup-mothod和replace-method的，这两个配置的加载其实就是将配置统一存放在BeanDefinition中的methodOverrides属性里，这个方法的操作也就是针对于这两个配置的；
3. 应用初始化前的后处理器，最后创建bean。
在createBean()方法里执行完resolveBeforeInstantiation方法后，如果创建了代理且不为空的话就直接返回，否则需要进行常规bean的创建，这个创建过程是在doCreateBean中完成的，跟进源码：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 根据指定bean使用相应策略创建实例
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

    // Allow post-processors to modify the merged bean definition.
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            mbd.postProcessed = true;
        }
    }

    // 检测循环依赖，是否需要提早曝光
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
            isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        if (logger.isDebugEnabled()) {
            logger.debug("Eagerly caching bean ‘" + beanName +
                    "‘ to allow for resolving potential circular references");
        }
        // 避免循环依赖，可以在bean初始化完成前将创建实例的ObjectFactory加入工厂
        addSingletonFactory(beanName, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                // 对bean再次依赖引用
                // AOP也是在这里将advice动态织入bean中，若没有则直接返回bean，不做处理
                return getEarlyBeanReference(beanName, mbd, bean);
            }
        });
    }

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        // 填充bean，注入属性值，如果存在依赖于其他bean的属性，会递归初始化
        populateBean(beanName, mbd, instanceWrapper);
        if (exposedObject != null) {
            // 调用初始化方法
            exposedObject = initializeBean(beanName, exposedObject, mbd);
        }
    }
    catch (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) {
        Object earlySingletonReference = getSingleton(beanName, false);
        // earlySingletonReference只有在检测到有循环依赖的情况下才会不为空
        if (earlySingletonReference != null) {
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                for (String dependentBean : dependentBeans) {
                    // 检测依赖
                    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 {
        // 依据scopse注册bean
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

上面源码完成的操作可以概括为以下几点：
1. 开始是单例的话要先清除缓存；
2. 实例化bean，将BeanDefinition转换为BeanWrapper；
3. 使用MergedBeanDefinitionPostProcessor，Autowired注解就是通过此方法实现类型的预解析；
4. 解决循环依赖问题；
5. 填充属性，将属性填充到bean实例中；
6. 注册DisposableBean；
7. 创建完成并返回

接下来创建bean实例，看createBeanInstance()方法：

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
    // 解析class
    Class<?> beanClass = resolveBeanClass(mbd, beanName);

    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());
    }

    // 若工厂方法不为空则使用工厂方法初始化
    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;
            }
        }
    }
    // 如果已经解析过则使用解析好的构造方法不需要再次锁定
    if (resolved) {
        if (autowireNecessary) {
            // 构造方法自动注入
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            // 使用默认构造方法
            return instantiateBean(beanName, mbd);
        }
    }

    // 根据参数解析构造方法
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    // No special handling: simply use no-arg constructor.
    return instantiateBean(beanName, mbd);
}

可以看出如果在RootBeanDefinition中存在factoryMethodName属性，或者说配置文件中配置了factory-method，那么Spring会尝试使用instantiateUsingFactoryMethod(beanName, mbd, args)方法根据RootBeanDefinition中的配置生成bean实例。
再解析构造方法并进行实例化，Spring会根据参数及类型判断使用哪个构造方法再进行实例化。判断调用哪个构造方法的过程会采用缓存机制，如果已经解析过则不需要重复解析而是从RootBeanDefinition中的属性resolvedConstructorOrFactoryMethod缓存的值去取，不然则需要再次解析。

创建bean后接下来就进行属性注入，属性注入的操作在populateBean()方法中，跟进源码：

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
    PropertyValues pvs = mbd.getPropertyValues();

    if (bw == null) {
        if (!pvs.isEmpty()) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
        }
        else {
            // Skip property population phase for null instance.
            return;
        }
    }

    // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
    // state of the bean before properties are set. This can be used, for example,
    // to support styles of field injection.
    boolean continueWithPropertyPopulation = true;

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

    if (!continueWithPropertyPopulation) {
        return;
    }

    if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
        MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

        // Add property values based on autowire by name if applicable.
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
            autowireByName(beanName, mbd, bw, newPvs);
        }

        // Add property values based on autowire by type if applicable.
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
            autowireByType(beanName, mbd, bw, newPvs);
        }

        pvs = newPvs;
    }

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

    if (hasInstAwareBpps || needsDepCheck) {
        PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
        if (hasInstAwareBpps) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                    if (pvs == null) {
                        return;
                    }
                }
            }
        }
        if (needsDepCheck) {
            checkDependencies(beanName, mbd, filteredPds, pvs);
        }
    }

    applyPropertyValues(beanName, mbd, bw, pvs);
}

在populateBean方法的中的主要处理流程：
1. InstantiationAwareBeanPostProcessor处理器的postProcessAfterInstantiation方法控制程序是否继续填充属性；
2. 根据注入类型提取依赖的bean，并存入PropertyValues中；
3. 应用InstantiationAwareBeanPostProcessor处理器的postProcessPropertyValues方法对属性在填充前再次处理，主要还是验证属性；
4. 将所有PropertyValues中的属性填充到BeanWrapper中。

最后初始化bean

学过Spring的都知道bean配置时有一个init-method属性，这个属性的作用是在bean实例化前调用init-method指定的方法进行需要的实例化操作，现在就进入这个方法了；Spring中程序已经执行过bean的实例化，并且进行了属性的填充，而就在这时将会调用用户设定的初始化方法。

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                invokeAwareMethods(beanName, bean);
                return null;
            }
        }, getAccessControlContext());
    }
    else {
        // 特殊bean处理
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        invokeInitMethods(beanName, wrappedBean, mbd);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                (mbd != null ? mbd.getResourceDescription() : null),
                beanName, "Invocation of init method failed", ex);
    }

    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }
    return wrappedBean;
}

最后加载完Bean并执行完初始化操作后，一个bean的加载基本就结束了。