【Spring详解五】bean的加载

public class AppTest {
    @Test
    public void MyTestBeanTest() {
        BeanFactory bf = new XmlBeanFactory( new ClassPathResource("spring-config.xml"));
        MyTestBean myTestBean = (MyTestBean) bf.getBean("myTestBean");
    }
}

• boolean containsBean(String beanName) 判断工厂中是否包含给定名称的bean定义，若有则返回true
• Object getBean(String) 返回给定名称注册的bean实例。根据bean的配置情况，如果是singleton模式将返回一个共享实例，否则将返回一个新建的实例，如果没有找到指定bean，该方法可能会抛出异常
• Object getBean(String, Class) 返回以给定名称注册的bean实例，并转换为给定class类型
• Class getType(String name) 返回给定名称的bean的Class，如果没有找到指定的bean实例，则排除NoSuchBeanDefinitionException异常
• boolean isSingleton(String) 判断给定名称的bean定义是否为单例模式
• String[] getAliases(String name) 返回给定bean名称的所有别名

package org.springframework.beans.factory;  
import org.springframework.beans.BeansException;  
public interface BeanFactory {  
    String FACTORY_BEAN_PREFIX = "&";  
    Object getBean(String name) throws BeansException;  
    <T> T getBean(String name, Class<T> requiredType) throws BeansException;  
    <T> T getBean(Class<T> requiredType) throws BeansException;  
    Object getBean(String name, Object... args) throws BeansException;  
    boolean containsBean(String name);  
    boolean isSingleton(String name) throws NoSuchBeanDefinitionException;  
    boolean isPrototype(String name) throws NoSuchBeanDefinitionException;  
    boolean isTypeMatch(String name, Class<?> targetType) throws NoSuchBeanDefinitionException;  
    Class<?> getType(String name) throws NoSuchBeanDefinitionException;  
    String[] getAliases(String name);  
}

• BeanFactory：Spring IOC容器的鼻祖，是IOC容器的基础接口，所有的容器都是从它这里继承实现而来。可见其地位。BeanFactory提供了最基本的IOC容器的功能，即所有的容器至少需要实现的标准。
• XmlBeanFactory：实现了最基本的IOC容器的功能。 XMLBeanFactory 继承自DefaultListableBeanFactory 。
• DefaultListableBeanFactory实际包含了基本IOC容器所具有的所有重要功能，是一个完整的IOC容器。
• ApplicationContext包含BeanFactory的所有功能，通常建议比BeanFactory优先。

Resource resource = new FileSystemResource("beans.xml");
BeanFactory factory = new XmlBeanFactory(resource);

ClassPathResource resource = new ClassPathResource("beans.xml");
BeanFactory factory = new XmlBeanFactory(resource);

ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {"applicationContext.xml"});
BeanFactory factory = (BeanFactory) context;

package org.springframework.beans.factory;  
public interface FactoryBean<T> {  
    T getObject() throws Exception;  
    Class<?> getObjectType();  
    boolean isSingleton();  
}

• T getObject()：返回由FactoryBean创建的Bean实例，如果isSingleton()返回true，则该实例会放到Spring容器中单实例缓存池中；
• boolean isSingleton()：返回由FactoryBean创建的Bean实例的作用域是singleton还是prototype；
• Class getObjectType()：返回FactoryBean创建的Bean类型。

public class Car {  
    private int maxSpeed ;  
    private String brand ;  
    private double price ;  
    //get set 方法
}

import  org.springframework.beans.factory.FactoryBean;  
public  class CarFactoryBean implements  FactoryBean<Car> {  
    private String carInfo ;  
    public  Car getObject()  throws  Exception  {  
        Car car = new  Car();  
        String[] infos = carInfo.split(","); 
        car.setBrand(infos[0]);
        car.setMaxSpeed(Integer.valueOf(infos[1]));
        car.setPrice(Double.valueOf(infos[2]));
        return  car;  
    }  
    public  Class<Car> getObjectType(){  
        return Car.class ;  
    }  
    public boolean isSingleton(){  
        return false ;  
    }  
    public String getCarInfo(){  
        return  this.carInfo;  
    }  
  
    //接受逗号分割符设置属性信息  
    public void setCarInfo (String carInfo){  
        this.carInfo = carInfo;  
    }  
}

<bean d="car" class="com.wangjiafu.spring.CarFactoryBean" P:carInfo="大奔,600,1000000"/>

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

@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
        @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    //获取 beanName，这里是一个转换动作，将 name 转换为 beanName
    final String beanName = transformedBeanName(name);
    Object bean;
    /*
     *检查缓存中的实例工厂是否存在对应的实例
     *为何要优先使用这段代码呢？
     *因为在创建单例bean的时候会存在依赖注入的情况，而在创建依赖的时候为了避免循环依赖
     *spring创建bean的原则是在不等bean创建完就会将创建bean的objectFactory提前曝光，即将其加入到缓存中，一旦下个bean创建时依赖上个bean则直接使用objectFactory          
     *直接从缓存中或singletonFactories中获取objectFactory
     *就算没有循环依赖，只是单纯的依赖注入，如B依赖A，如果A已经初始化完成，B进行初始化时，需要递归调用getBean获取A，这是A已经在缓存里了，直接可以从这里取到
     */
    // Eagerly check singleton cache for manually registered singletons.
    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 + "'");
            }
        }
        //返回对应的实例，有些时候并不是直接返回实例，而是返回某些方法返回的实例
        //这里涉及到我们上面讲的FactoryBean，如果此Bean是FactoryBean的实现类，如果name前缀为"&",则直接返回此实现类的bean,如果没有前缀"&"，则需要调用此实现类的getObject方法，返回getObject里面真是的返回对象
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }
    else {
        //只有在单例的情况下才会解决循环依赖
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }
        //尝试从parentBeanFactory中查找bean
        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            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 {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }
        //如果不是仅仅做类型检查，则这里需要创建bean，并做记录
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }
        try {
            //将存储XML配置文件的GenericBeanDefinition转换为RootBeanDefinition，同时如果存在父bean的话则合并父bean的相关属性
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            //如果存在依赖则需要递归实例化依赖的bean
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dep : dependsOn) {
                    if (isDependent(beanName, dep)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    registerDependentBean(dep, beanName);
                    try {
                        getBean(dep);
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                    }
                }
            }

            // 单例模式
            // 实例化依赖的bean后对bean本身进行实例化
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, () -> {
                    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);
            }
            // 原型模式
            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);
            }
            // 从指定的 scope 下创建 bean
            else {
                String scopeName = mbd.getScope();
                final Scope scope = this.scopes.get(scopeName);
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                }
                try {
                    Object scopedInstance = scope.get(beanName, () -> {
                        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;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && !requiredType.isInstance(bean)) {
        try {
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        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;
}

1. 获取beanName ：这里传入的参数有可能是别名，也有可能是FactoryBean，所以需要进行一系列的解析，这些解析内容包括如下内容。
   1. 去除FactroyBean的修饰符，也就是如果name=“&aa”，那么会首先去除&而使name=“aa”。
   2. 去除alisa所表示的最终beanName，例如别名A指向名称为B的bean则返回B；若别名A指向别名B，别名B又指向名称为C的bean则返回C。
2. 缓存中获取单例 ：单例在Spring的同一个容器中只会被创建一次，后续在获取bean，就直接从单例缓存中获取了。当然这里也尝试加载，如果不成功则再次尝试从singletonFactories中加载。因为在创建单例bean的时候会在存在依赖注入的情况，而在创建依赖的时候为了避免循环依赖，在Spring创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光加入到缓存中，一旦下一个bean创建时候需要依赖上一个bean直接使用ObjectFactory。
3. 从bean的实例中获取对象 ：如果从缓存中得到了bean的原始状态，则需要对bean进行实例化。这里必须强调一下，缓存中记录的只是最原始的bean状态，并不一定是最终想要的bean。使用 getObjectForBeanInstance() 完成这个工作。
4. 原型模式的依赖检查 ：只有在单例情况下才会尝试解决循环依赖，如果存在A中有B的属性，B中有A的属性，那么当依赖注入的时候，就是产生当A还未创建完的时候因为对于B的创建再次返回创建A，造成循环依赖，也就是情况：isPrototypeCurrentlyInCreation(beanName) 判断为true。
5. 检测parentBeanFactory ：从代码上看，如果缓存没有数据的话直接转到父类工厂上去加载了，这是为什么呢？它是在检测如果加载的XML配置文件中不包含beanName所对应的配置，就只能到parentBeanFactory去尝试加载了，然后再去递归的调用 getBean() 方法。
6. 将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition ：因为从XML配置文件读取到的Bean的信息是存储在GenericBeanDefinition中的，但是所有的Bean后续处理都是针对RootBeanDefinition的，所以这里需要进行一个转换，转换的同时如果父类bean不为空的话，则会一并合并父类的属性。
7. 寻找依赖 ：因为bean的初始化过程中可能会用到某些属性，而某些属性可能是动态配置且配置成依赖于其他的bean，那么这个时候就有必要先加载依赖的bean，所以，在Spring加载顺序中在初始化某一个bean的时候首先会初始化这个bean所对应的依赖。
8. 针对不同Scope进行bean的创建 ：在Spring中存在不同的scope，其中默认的是singleton，但是还有注入prototype，request之类的，在这个步骤中，Spring会根据不同的配置进行不同的初始化策略。
9. 类型转换 ：程序到这里返回bean后基本结束了，通常对该方法的调用参数requireType是为空的，但是可能会存在这样的情况，返回的bean其实是一个String，但是requireType却传入Integer类型，那么这时候本步骤就会起作用了，它的功能就是将返回的bean转换为requireType所指定的类型。当然，String转换为Integer是最简单的一种转换，在Spring中提供了各种各样的转换器，用户也可以自己扩展转换器来满足自己的需求。

# AbstractBeanFactory
protected String transformedBeanName(String name) {
    return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}

# BeanFactoryUtils
String FACTORY_BEAN_PREFIX = "&"; //BeanFactory.FACTORY_BEAN_PREFIX
// 去除 FactoryBean 的修饰符
public static String transformedBeanName(String name) {
    Assert.notNull(name, "'name' must not be null");
    String beanName = name;
    while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
        beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
    }
    return beanName;
}

# SimpleAliasRegistry
// 转换 aliasName
public String canonicalName(String name) {
    String canonicalName = name;
    // Handle aliasing...
    String resolvedName;
    do {
        resolvedName = this.aliasMap.get(canonicalName);
        if (resolvedName != null) {
            canonicalName = resolvedName;
        }
    }
    while (resolvedName != null);
    return canonicalName;
}

• 去除 FactoryBean 的修饰符。如果 name 以 “&” 为前缀，那么会去掉该 “&”，例如，name = "&studentService"，则会是 name = "studentService"。
• 取指定的 alias 所表示的最终 beanName。主要是一个循环获取 beanName 的过程，例如别名 A 指向名称为 B 的 bean 则返回 B，若 别名 A 指向别名 B，别名 B 指向名称为 C 的 bean，则返回 C。

# DefaultSingletonBeanRegistry
@Override
@Nullable
public Object getSingleton(String beanName) {
    //参数true是允许早期依赖
    return getSingleton(beanName, true);
}
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    //检查缓存中是否存在实例,这里就是上面说的单纯的依赖注入，如B依赖A，如果A已经初始化完成，B进行初始化时，需要递归调用getBean获取A，这是A已经在缓存里了，直接可以从这里取到
    Object singletonObject = this.singletonObjects.get(beanName);
    //如果缓存为空且单例bean正在创建中，则锁定全局变量，为什么要判断bean在创建中呢？这里就是可以判断是否循环依赖了。
    //A依赖B，B也依赖A，A实例化的时候，发现依赖B，则递归去实例化B，B发现依赖A，则递归实例化A，此时会走到原点A的实例化，第一次A的实例化还没完成，只不过把实例化的对象加入到缓存中，但是状态还是正在创建中，由此回到原点发现A正在创建中，由此可以判断是循环依赖了
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            //如果此bean正在加载，则不处理
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                //当某些方法需要提前初始化的时候会直接调用addSingletonFactory把对应的ObjectFactory初始化策略存储在singletonFactory中
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    //使用预先设定的getObject方法
                    singletonObject = singletonFactory.getObject();
                    //记录在缓存中，注意earlySingletonObjects和singletonFactories是互斥的
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

1. 先尝试从singletonObjects里面获取实例，如果获取不到再从earlySingletonObjects里面获取，如果还获取不到，再尝试从singletonFactories里面获取beanName对应的ObjectFactory ；
2. 然后再调用这个 ObjectFactory#getObject() 方法创建bean,并放到earlySingletonObjects里面去，并且从singletonFactoryes里面remove掉这个ObjectFactory，而对于后续所有的内存操作都只为了循环依赖检测时候使用，即allowEarlyReference为true的时候才会使用。

• singletonObjects : 用于保存BeanName和创建bean实例之间的关系，beanName–>bean Instance

• earlySingletonObjects : 也是保存BeanName和bean实例之间的关系，与singletonObjects的不同之处在于，在这个Map里的Bean不是完整的，甚至还不能称之为“Bean”，只是一个Instance。即当一个单例bean被放到这里面后，那么当bean还在创建过程中，就可以通过getBean方法获取到了，其目的是用来检测循环引用。
• singletonFactories : 用于保存BeanName和创建bean的工厂之间的关系，banName–>ObjectFactory

• registeredSingletons : 用来保存当前所有已注册的bean。

1. 实例化 A，此时 A 还未完成 属性填充（AbstractAutowireCapableBeanFactory#populateBean）和 初始化方法（AbstractAutowireCapableBeanFactory#initializeBean）的执行，A 只是一个半成品。
2. 为 A 创建一个 Bean工厂，并放入到 singletonFactories 中（DefaultSingletonBeanRegistry#addSingletonFactory）。
3. 发现 A 需要注入 B 对象，但是一级、二级、三级缓存均未发现对象 B（DefaultSingletonBeanRegistry#getSingleton）。
4. 实例化 B，此时 B 还未完成属性填充和初始化方法的执行，B 只是一个半成品。
5. 为 B 创建一个 Bean工厂，并放入到 singletonFactories 中。
6. 发现 B 需要注入 A 对象，此时在一级、二级未发现对象A，但是在三级缓存中发现了对象 A，从三级缓存中得到对象 A，并将对象 A 放入二级缓存中，同时删除三级缓存中的对象 A。（注意，此时的 A还是一个半成品，并没有完成属性填充和执行初始化方法）
7. 将对象 A 注入到对象 B 中。
8. 对象 B 完成属性填充，执行初始化方法，并放入到一级缓存中，同时删除二级缓存中的对象 B。（此时对象 B 已经是一个成品）
9. 对象 A 得到对象B，将对象 B 注入到对象 A 中。（对象 A 得到的是一个完整的对象 B）
10. 对象 A完成属性填充，执行初始化方法，并放入到一级缓存中，同时删除二级缓存中的对象 A。

# AbstractBeanFactory
protected Object getObjectForBeanInstance(
        Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
    //如果指定的name是工厂相关的（以&开头的）
    if (BeanFactoryUtils.isFactoryDereference(name)) {
        //如果是NullBean则直接返回此bean
        if (beanInstance instanceof NullBean) {
            return beanInstance;
        }
        //如果不是FactoryBean类型，则验证不通过抛出异常
        if (!(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
        }
    }
    // Now we have the bean instance, which may be a normal bean or a FactoryBean.
    // If it's a FactoryBean, we use it to create a bean instance, unless the
    // caller actually wants a reference to the factory.
    //如果获取的beanInstance不是FactoryBean类型，则说明是普通的Bean，可直接返回
    //如果获取的beanInstance是FactoryBean类型,但是是以（以&开头的），也直接返回，此时返回的是FactoryBean的实例
    if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
        return beanInstance;
    }
    Object object = null;
    if (mbd == null) {
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        // Return bean instance from factory.
        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
        // Caches object obtained from FactoryBean if it is a singleton.
        if (mbd == null && containsBeanDefinition(beanName)) {
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        //到了这里说明获取的beanInstance是FactoryBean类型,但没有以"&"开头，此时就要返回factory内部getObject里面的对象了
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

• 对FactoryBean正确性的验证
• 对非FactoryBean不做任何处理
• 对bean进行转换
• 将从Factory中解析bean的工作委托给getObjectFromFactoryBean(factory, beanName, !synthetic)方法

# FactoryBeanRegistrySupport
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
    // 为单例模式且缓存中存在
    if (factory.isSingleton() && containsSingleton(beanName)) {

        synchronized (getSingletonMutex()) {
            // 从缓存中获取指定的 factoryBean
            Object object = this.factoryBeanObjectCache.get(beanName);

            if (object == null) {
                // 为空，则从 FactoryBean 中获取对象
                object = doGetObjectFromFactoryBean(factory, beanName);

                // 从缓存中获取
                Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
                if (alreadyThere != null) {
                    object = alreadyThere;
                }
                else {
                    // 需要后续处理
                    if (shouldPostProcess) {
                        // 若该 bean 处于创建中，则返回非处理对象，而不是存储它
                        if (isSingletonCurrentlyInCreation(beanName)) {
                            return object;
                        }
                        // 前置处理
                        beforeSingletonCreation(beanName);
                        try {
                            // 对从 FactoryBean 获取的对象进行后处理
                            // 生成的对象将暴露给bean引用
                            object = postProcessObjectFromFactoryBean(object, beanName);
                        }
                        catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                    "Post-processing of FactoryBean's singleton object failed", ex);
                        }
                        finally {
                            // 后置处理
                            afterSingletonCreation(beanName);
                        }
                    }
                    // 缓存
                    if (containsSingleton(beanName)) {
                        this.factoryBeanObjectCache.put(beanName, object);
                    }
                }
            }
            return object;
        }
    }
    else {
        // 非单例
        Object object = doGetObjectFromFactoryBean(factory, beanName);
        if (shouldPostProcess) {
            try {
                object = postProcessObjectFromFactoryBean(object, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
            }
        }
        return object;
    }
}

# DefaultSingletonBeanRegistry
public boolean isSingletonCurrentlyInCreation(String beanName) {
    return this.singletonsCurrentlyInCreation.contains(beanName);
}

# DefaultSingletonBeanRegistry
protected void beforeSingletonCreation(String beanName) {
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
    }
}

# DefaultSingletonBeanRegistry
protected void afterSingletonCreation(String beanName) {
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
        throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
    }
}

# FactoryBeanRegistrySupport
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
        throws BeanCreationException {

    Object object;
    try {
        //需要进行权限验证
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            //直接调用 FactoryBean#getObject 方法
            object = factory.getObject();
        }
    }
    catch (FactoryBeanNotInitializedException ex) {
        throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    }

    // Do not accept a null value for a FactoryBean that's not fully
    // initialized yet: Many FactoryBeans just return null then.
    if (object == null) {
        if (isSingletonCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(
                    beanName, "FactoryBean which is currently in creation returned null from getObject");
        }
        object = new NullBean();
    }
    return object;
}

# AbstractAutowireCapableBeanFactory
@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
    return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
        throws BeansException {
    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        Object current = beanProcessor.postProcessAfterInitialization(result, beanName);
        if (current == null) {
            return result;
        }
        result = current;
    }
    return result;
}

#DefaultSingletonBeanRegistry
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");

    // 全局加锁
    synchronized (this.singletonObjects) {
        // 从缓存中检查对应的bean是否已经加载过一遍
        // 因为 singleton 模式其实就是复用已经创建的 bean 所以这步骤必须检查
        Object singletonObject = this.singletonObjects.get(beanName);
        //  为空，开始加载过程
        if (singletonObject == null) {
            // 省略 部分代码

            // 加载前置处理
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            // 省略代码
            try {
                // 初始化 bean
                // 这个过程就是我上面讲的调用匿名内部类的方法，其实是调用 createBean() 方法
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            // 省略 catch 部分
            }
            finally {
                // 后置处理
                afterSingletonCreation(beanName);
            }
            // 加入缓存中
            if (newSingleton) {
                addSingleton(beanName, singletonObject);
            }
        }
        // 直接返回
        return singletonObject;
    }
}

1. 从缓存中检查对应的bean是否已经加载过一遍 ；
2. 如果没有加载，则记录beanName的正在加载状态 ；
3. 加载单例前记录加载状态。 可能你会觉得 beforeSingletonCreation(beanName) 方法是个空实现，里面没有任何逻辑，但其实这个函数中做了一个很重要的操作：记录加载状态，也就是通过 this.singletonsCurrentlyInCreation.add(beanName) 将当前正要创建的bean记录在缓存中，这样便可以对循环依赖进行检测。 在上节已经讲过。
4. 通过调用参数传入的 ObjectFactory#getObject() 方法实例化bean ；
5. 加载单例后的处理方法调用。 同步骤3的记录加载状态相似，当bean加载结束后需要调用 afterSingletonCreation(beanName) 移除缓存中对该bean的正在加载状态的记录 ；
6. 将结果记录至缓存并删除加载bean过程中所记录的各种辅助状态；
7. 返回处理结果

protected void addSingleton(String beanName, Object singletonObject) {
    synchronized (this.singletonObjects) {
        this.singletonObjects.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
        this.earlySingletonObjects.remove(beanName);
        this.registeredSingletons.add(beanName);
    }
}

if (mbd.isSingleton()) {
    sharedInstance = getSingleton(beanName, () -> {
        try {
            return createBean(beanName, mbd, args);
        }
        catch (BeansException ex) {
            destroySingleton(beanName);
            throw ex;
        }
    });
    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

# AbstractAutowireCapableBeanFactory
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
        throws BeanCreationException {

    if (logger.isDebugEnabled()) {
        logger.debug("Creating instance of bean '" + beanName + "'");
    }
    RootBeanDefinition mbdToUse = mbd;

    //锁定class，根据设置的class属性或者根据className来解析Class，确保此时的 bean 已经被解析了
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName); 
    // 如果获取的class 属性不为null，则克隆该 BeanDefinition
    // 主要是因为该动态解析的 class 无法保存到到共享的 BeanDefinition
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

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

    try {
        // 给 BeanPostProcessors 一个机会用来返回一个代理类而不是真正的类实例
        // AOP 的功能就是基于这个地方
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
        if (bean != null) {
            return bean;
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                "BeanPostProcessor before instantiation of bean failed", ex);
    }

    try {
        // 执行真正创建 bean 的过程
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isDebugEnabled()) {
            logger.debug("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }
    catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
    }
}

1. 根据设置的class属性或者根据className来解析Class ；
2. 对override属性进行标记及验证 ；
3. 应用初始化前的后处理器，解析指定bean是否存在初始化前的短路操作 ；
4. 创建bean 。

# AbstractBeanDefinition
public void prepareMethodOverrides() throws BeanDefinitionValidationException {
   // Check that lookup methods exist and determine their overloaded status.
   if (hasMethodOverrides()) {
       //获取对应类中方法名的个数
       //如果个数为1时，标记MethodOverride暂未被覆盖，避免参数类型检查的开销
      getMethodOverrides().getOverrides().forEach(this::prepareMethodOverride);
   }
}

protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
   int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
   if (count == 0) {
      throw new BeanDefinitionValidationException(
            "Invalid method override: no method with name '" + mo.getMethodName() +
            "' on class [" + getBeanClassName() + "]");
   }
   else if (count == 1) {
      // Mark override as not overloaded, to avoid the overhead of arg type checking.
      mo.setOverloaded(false);
   }
}

if (bean != null) {
    return bean;
}

# AbstractAutowireCapableBeanFactory
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            Class<?> targetType = determineTargetType(beanName, mbd);
            if (targetType != null) {
                bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                if (bean != null) {
                    bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }
        mbd.beforeInstantiationResolved = (bean != null);
    }
    return bean;
}

# AbstractAutowireCapableBeanFactory
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
   for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
      Object result = bp.postProcessBeforeInstantiation(beanClass, beanName);
      if (result != null) {
         return result;
      }
   }
   return null;
}
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
      throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

• applyBeanPostProcessorsBeforeInstantiation() 实例化前的后处理器的应用 ：bean的实例化前调用，就是将AbstractBeanDefinition转换为BeanWrapper前的处理，给子类一个修改BeanDefinition的机会，也就是说当程序经过这个方法后，bean可能已经不是我们认识的bean了，而是或许成为了一个经过处理的代理bean，可能通过cglib生成的，也可以是其他技术生成的。
• applyBeanPostProcessorsAfterInitialization() 实例化后的后处理器的应用 ：在讲解 '从缓存中获取单例bean' 的时候提到过，Spring中的规则是在bean的初始化后尽可能保证将注册的后处理器的 postProcessAfterInitialization() 方法应用到该bean中，因为如果返回的bean不为空，那么便不会再次经历普通bean的创建过程，所以只能在这里应用后处理器的 postProcessAfterInitialization() 方法。

public class TestA {
    private TestB testB;
}
public class TestB {
    private TestC testC;
}
public class TestC {
    private TestA testA;
}

1. 构造器的循环依赖 ；
2. field属性的循环依赖 （又叫setter循环依赖）。

# AbstractBeanFactory
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

• singletonObjects : 用于保存BeanName和创建bean实例之间的关系，beanName–>bean Instance
• earlySingletonObjects : 也是保存BeanName和bean实例之间的关系，与singletonObjects的不同之处在于，在这个Map里的Bean不是完整的，甚至还不能称之为“Bean”，只是一个Instance。即当一个单例bean被放到这里面后，那么当bean还在创建过程中，就可以通过getBean方法获取到了，其目的是用来检测循环引用。
• singletonFactories : 用于保存BeanName和创建bean的工厂之间的关系，banName–>ObjectFactory

• isSingletonCurrentlyInCreation()：判断当前 singleton bean 是否处于创建中。bean 处于创建中也就是说 bean 在初始化但是没有完成初始化，有一个这样的过程其实和 Spring 解决 bean 循环依赖的理念相辅相成，因为 Spring 解决 singleton bean 的核心就在于提前曝光 bean。
• allowEarlyReference：从字面意思上面理解就是允许提前拿到引用。其实真正的意思是是否允许从 singletonFactories 缓存中通过 getObject() 拿到对象，为什么会有这样一个字段呢？原因就在于 singletonFactories 才是 Spring 解决 singleton bean 的诀窍所在，这个我们后续分析。

1. 首先从一级缓存 singletonObjects 获取 ；
2. 如果没有且当前指定的 beanName 正在创建，就再从二级缓存中 earlySingletonObjects 获取 ；
3. 如果还是没有获取到且允许从 singletonFactories 通过 getObject() 获取，则从三级缓存 singletonFactories 获取 singletonFactory，
4. 如果获取到 singletonFactory ，则通过其 getObject() 获取对象，并将其加入到二级缓存 earlySingletonObjects 中 从三级缓存 singletonFactories 删除，这样就从三级缓存升级到二级缓存了。

# AbstractAutowireCapableBeanFactory
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {
    //.....
    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");
        }
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
    //.....
}

• 单例
• 运行提前暴露 bean
• 当前 bean 正在创建中

# DefaultSingletonBeanRegistry
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(singletonFactory, "Singleton factory must not be null");
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) {
            this.singletonFactories.put(beanName, singletonFactory);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
    }
}

# DefaultSingletonBeanRegistry
protected void addSingleton(String beanName, Object singletonObject) {
    synchronized (this.singletonObjects) {
        this.singletonObjects.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
        this.earlySingletonObjects.remove(beanName);
        this.registeredSingletons.add(beanName);
    }
}

# AbstractBeanFactory
protected <T> T doGetBean(
      String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
      throws BeansException {
    //.....
    if (mbd.isSingleton()) {
        sharedInstance = getSingleton(beanName, () -> {
            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);
    }
    //.....
}

# DefaultSingletonBeanRegistry
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    synchronized (this.singletonObjects) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            //....
            try {
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            //.....
            if (newSingleton) {
                addSingleton(beanName, singletonObject);
            }
        }
        return singletonObject;
    }
}

1. 首先 A 完成初始化第一步并将自己提前曝光出来（通过 ObjectFactory 将自己提前曝光）；
2. 在A初始化的时候，发现自己依赖对象 B，此时就会去尝试 get(B) ，这个时候发现 B 还没有被创建出来，然后 B 就走创建流程 ；
3. 在 B 初始化的时候，同样发现自己依赖 C，此时就会去尝试 get(C)，C 也没有被创建出来，这个时候 C 又开始初始化进程 ；
4. 但是在C初始化的过程中，发现自己依赖 A，于是尝试 get(A)，这个时候由于 A 已经添加至缓存中（一般都是添加至三级缓存 singletonFactories ），通过 ObjectFactory 提前曝光，所以可以通过 ObjectFactory.getObject() 拿到 A 对象 ；
5. C 拿到 A 对象后顺利完成初始化，然后将自己添加到一级缓存中，回到 B ，B 也可以拿到 C 对象，完成初始化，A 可以顺利拿到 B 完成初始化。到这里整个链路就已经完成了初始化过程了。

# AbstractAutowireCapableBeanFactory
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
        throws BeanCreationException {

    // BeanWrapper是对Bean的包装，其接口中所定义的功能很简单包括设置获取被包装的对象，获取被包装bean的属性描述器
    BeanWrapper instanceWrapper = null;
    // 单例模型，则从未完成的 FactoryBean 缓存中删除
    if (mbd.isSingleton()) {anceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }

    // 使用合适的实例化策略来创建新的实例：工厂方法、构造函数自动注入、简单初始化
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }

    // 包装的实例对象
    final Object bean = instanceWrapper.getWrappedInstance();
    // 包装的实例对象的类型
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // 检测是否有后置处理
    // 如果有后置处理，则允许后置处理修改 BeanDefinition
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                // applyMergedBeanDefinitionPostProcessors
                // 后置处理修改 BeanDefinition
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    // 解决单例模式的循环依赖
    // 单例模式 & 允许循环依赖&当前单例 bean 是否正在被创建
    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");
        }
        // 提前将创建的 bean 实例加入到ObjectFactory 中
        // 这里是为了后期避免循环依赖
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    /*
     * 开始初始化 bean 实例对象
     */
    Object exposedObject = bean;
    try {
        // 对 bean 进行填充，将各个属性值注入，其中，可能存在依赖于其他 bean 的属性
        // 则会递归初始依赖 bean
        populateBean(beanName, mbd, instanceWrapper);
        // 调用初始化方法，比如 init-method 
        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);
        }
    }

    /**
     * 循环依赖处理
     */
    if (earlySingletonExposure) {
        // 获取 earlySingletonReference
        Object earlySingletonReference = getSingleton(beanName, false);
        // 只有在存在循环依赖的情况下，earlySingletonReference 才不会为空
        if (earlySingletonReference != null) {
            // 如果 exposedObject 没有在初始化方法中被改变，也就是没有被增强
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            // 处理依赖
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                for (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.");
                }
            }
        }
    }
    try {
        // 注册 DisposeableBean
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

1. 如果是单例则需要首先清除缓存；
2. createBeanInstance() 实例化 bean，将BeanDefinition转换为BeanWrapper ；
3. MergedBeanDefinitionPostProcessor的应用，bean合并后的处理，Autowire正是通过此方法实现诸如类型的解析；
4. 解决单例模式的循环依赖，提前将bean 的 ObjectFactory 提前曝光（即加入到 singletonFactories 缓存中），这里是为了后期避免循环依赖
5. populateBean() 属性填充 ；
6. initializeBean() 初始化 bean ；
7. 循环依赖检查，之前提到过，在Spring中解决循环依赖只对单例有效，而对于prototype的bean，Spring没有好的解决办法，唯一做的就是抛出异常，在这个步骤里面会检测已经加载的bean是否已经出现了依赖循环，并判断是否需要抛出异常。
8. 注册DisposeableBean，如果配置了destroy-method，这里需要注册以便于在销毁时候调用 ；
9. 完成创建并返回。

# AbstractAutowireCapableBeanFactory
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    // 解析 bean，将 bean 类名解析为 class 引用
    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());
    }

    // 如果存在 Supplier 回调，则使用给定的回调方法初始化策略
    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }

    // 如果工厂方法不为空，则使用工厂方法初始化策略,这里推荐看Factory-Method实例化Bean
    if (mbd.getFactoryMethodName() != null)  {
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }

    boolean resolved = false;
    boolean autowireNecessary = false;
    if (args == null) {
        // constructorArgumentLock 构造函数的常用锁
        synchronized (mbd.constructorArgumentLock) {
            // 如果已缓存的解析的构造函数或者工厂方法不为空，则可以利用构造函数解析
            // 因为需要根据参数确认到底使用哪个构造函数，该过程比较消耗性能，所以采用缓存机制
            if (mbd.resolvedConstructorOrFactoryMethod != null) {
                resolved = true;
                autowireNecessary = mbd.constructorArgumentsResolved;
            }
        }
    }
    // 已经解析好了，直接注入即可
    if (resolved) {
        // 自动注入，调用构造函数自动注入
        if (autowireNecessary) {
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            // 使用默认构造函数构造
            return instantiateBean(beanName, mbd);
        }
    }

    // 确定解析的构造函数
    // 主要是检查已经注册的 SmartInstantiationAwareBeanPostProcessor
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
        // 构造函数自动注入
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    //使用默认构造函数注入
    return instantiateBean(beanName, mbd);
}

1. 如果存在 Supplier 回调，则调用 obtainFromSupplier() 进行初始化 ;
2. 如果存在工厂方法，则使用工厂方法进行实例化 ;
3. 解析构造函数并进行构造函数的实例化 ：因为一个bean对应的类中可能会有多个构造函数，每个构造函数的参数不同，Spring会根据参数及类型去判断最终会使用哪个构造函数进行实例化。但是，判断的过程是个比较消耗性能的步骤，所以采用缓存机制，如果已经解析过则不需要重复解析，而是直接从 RootBeanDefinition 的属性 resolvedConstructorOrFactoryMethod 缓存的值去取，否则需要再次解析，并将解析的结果添加至 RootBeanDefinition 的属性 resolvedConstructorOrFactoryMethod 中 。
   1. 首先判断缓存，如果缓存中存在，即已经解析过了，则直接使用已经解析了的，根据 constructorArgumentsResolved 参数来判断是使用构造函数自动注入还是默认构造函数注入 ;
   2. 如果缓存中没有，则需要先确定到底使用哪个构造函数来完成解析工作，因为一个类有多个构造函数，每个构造函数都有不同的构造参数，所以需要根据参数来锁定构造函数并完成初始化，如果存在参数则使用相应的带有参数的构造函数，否则使用默认构造函数。

# ConstructorResolver
public BeanWrapper autowireConstructor(final String beanName,final RootBeanDefinition mbd,Constructor [] chosenCtors,final Object [] explicitArgs)
// explicitArgs通过getBean方法传入，如果getBean方法调用的时候指定方法参数那么直接使用
// 如果getBean方法时候没有指定则尝试从配置文件中解析
// 尝试从缓存中获取
// 配置的构造函数参数
// 如果缓存中存在
// 解析参数类型，缓存中的值可能是原始值也可能是最终值
// 如果没有缓存
// 提取配置文件中的配置的构造函数参数
// 用于承载解析后的构造函数参数的值
// 能解析到的参数个数
// 排序给定的构造函数，public构造函数优先参数数量降序，非public构造函数参数数量降序
// 如果已找到选用的构造函数或者需要参数个数小于当前构造函数参数给则终止，以为已经按照参数个数降序排列
// 有参数则根据构造对应参数类型的参数
// 注释上获取参数名称
// 获取参数名探索器
// 获取指定构造函数的参数名称
// 根据名称和数据类型创建参数持有者
// 构造函数没有参数的情况
// 探测是否有不确定性的构造函数存在，例如不同构造函数的参数为父子关系
// 将解析的构造函数加入缓存
// 将构造的实例加入BeanWrapper
}

• 构造函数参数确定。
  • 1.1 根据explicitArgs参数判断，如果传入的explicitArgs参数不为空，那便可以直接确认参数，因为explicitArgs参数是在调用Bean的时候用户指定的，在BeanFactory类中存在这样的方法：Object getBean(String name,Object … args) throws BeanException ；在获取bean的时候，用户不但可以指定bean的名称还可以指定bean所对应类的构造函数或者工厂方法的方法参数，主要用于静态工厂方法的调用，而这里需要给定完全匹配的参数的，所以，便可以判断，如果传入参数explicitArgs不为空，则可以确定构造函数参数就是它。
  • 1.2 缓存中获取，除此之外，确定参数的办法如果之前已经分析过，也就是说构造函数已经记录在缓存中，那么便可以直接拿来使用。而且，这里要提到的是，在缓存中缓存的可能是参数的最终类型也可能是参数的初始类型。
  • 1.3 配置文件获取，如果不能根据传入参数explicitArgs确认构造函数的参数也无法在缓存中得到相关信息，那么只能进行新一轮的分析了。分析从配置文件的构造韩式信息开始，经过之前的分析，Spring配置文件中的信息经过转换会通过BeanDefinition实例承载，也就是参数mbd中包含，那么可以通过调用mbd.getConstructorArgumentValues()来获取配置的构造函数信息。有了配置中的信息便可以获取对应的参数值信息了，获取参数值的信息包括指定值，如：直接指定构造函数中某个值为原始类型String类型，或者是一个对其他bean的引用，而这一处理委托给了resolveConstructorArgments方法，并返回能解析到的参数的个数。
• 构造函数的确定：经过第一步后已经确定了构造函数的参数，接下来的任务就是根据构造函数参数在所有的构造函数中锁定对应的构造函数，而且匹配的方法就是根据参数个数匹配，所以在匹配之前需要对构造函数按照public构造函数优先参数数量降序，非public构造函数参数数量降序。这样可以在遍历的情况下迅速判断排在后面的构造函数参数个数是否符合条件，由于在配置文件中并不是唯一限制使用参数位置索引的方式去创建，同样还支持指定参数名称进行设定参数值的情况，那么这种情况就需要首先确定构造函数中的参数名称，获取参数名称可以有两种方式，一种是通过注解的方式直接获取，另一种方式就是使用Spring提供的工具类ParameterNameDiscoverer来获取，构造函数、参数名称、参数类型、参数值、确定后就可以锁定构造函数以及转换对应的参数类型了。
• 根据确定的构造函数转换对应的参数类型，主要是使用Spring中提供的类型转换器或者用户自定义的类型转换器进行转换。
• 构造函数不确定性的验证，当然，有时候及时构造函数、参数名称、参数类型、参数值都确定后也不一定会直接锁定构造函数，不同构造函数的参数为父子关系，所以Spring在最后又做了一次验证。
• 根据实例化策略以及得到的构造函数及构造函数参数实例化Bean。

# AbstractAutowireCapableBeanFactory
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
   try {
      Object beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
      BeanWrapper bw = new BeanWrapperImpl(beanInstance);
      initBeanWrapper(bw);
      return bw;
   }
   catch (Throwable ex) {
      throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), ex);
   }
}

# SimpleInstantiationStrategy
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // 没有覆盖
    // 直接使用反射实例化即可
    if (!bd.hasMethodOverrides()) {
        // 重新检测获取下构造函数
        // 该构造函数是经过前面 N 多复杂过程确认的构造函数
        Constructor<?> constructorToUse;
        synchronized (bd.constructorArgumentLock) {
            // 获取已经解析的构造函数
            constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
            // 如果为 null，从 class 中解析获取，并设置
            if (constructorToUse == null) {
                final Class<?> clazz = bd.getBeanClass();
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged(
                                (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                    }
                    else {
                        //利用反射获取构造器
                        constructorToUse =  clazz.getDeclaredConstructor();
                    }
                    bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                }
                catch (Throwable ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        // 通过BeanUtils直接使用构造器对象实例化bean
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        // 生成CGLIB创建的子类对象
        return instantiateWithMethodInjection(bd, beanName, owner);
    }
}

• 如果该 bean 没有配置 lookup-method、replaced-method 标签或者 @Lookup 注解，则直接通过反射的方式实例化 bean 即可 ；
• 但是如果存在需要覆盖的方法或者动态替换的方法则需要使用 CGLIB 进行动态代理，因为可以在创建代理的同时将动态方法织入类中。

# BeanUtils
public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
    Assert.notNull(ctor, "Constructor must not be null");
    try {
        ReflectionUtils.makeAccessible(ctor);
        return (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
                KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
    }
    // 省略一些 catch 
}

# SimpleInstantiationStrategy
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");
}

# CglibSubclassingInstantiationStrategy
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    return instantiateWithMethodInjection(bd, beanName, owner, null);
}

protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
        @Nullable Constructor<?> ctor, @Nullable Object... args) {

    // 通过CGLIB生成一个子类对象
    return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}

public Object instantiate(@Nullable Constructor<?> ctor, @Nullable Object... args) {
    // 通过 Cglib 创建一个代理类
    Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
    Object instance;
    // 没有构造器，通过 BeanUtils 使用默认构造器创建一个bean实例
    if (ctor == null) {
        instance = BeanUtils.instantiateClass(subclass);
    }
    else {
        try {
            // 获取代理类对应的构造器对象，并实例化 bean
            Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
            instance = enhancedSubclassConstructor.newInstance(args);
        }
        catch (Exception ex) {
            throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
                    "Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
        }
    }

    // 为了避免memory leaks异常，直接在bean实例上设置回调对象
    Factory factory = (Factory) instance;
    factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
            new CglibSubclassingInstantiationStrategy.LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
            new CglibSubclassingInstantiationStrategy.ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
    return instance;
}

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
       throws BeanCreationException {
    //......
    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");
        }
        //为避免后期循环依赖，可以在bean初始化完成前将创建实例的ObjectFactory加入工厂  
        //依赖处理：在Spring中会有循环依赖的情况，例如，当A中含有B的属性，而B中又含有A的属性时就会  
        //构成一个循环依赖，此时如果A和B都是单例，那么在Spring中的处理方式就是当创建B的时候，涉及  
        //自动注入A的步骤时，并不是直接去再次创建A，而是通过放入缓存中的ObjectFactory来创建实例，  
        //这样就解决了循环依赖的问题。
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
    //......
}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(singletonFactory, "Singleton factory must not be null");
    synchronized (this.singletonObjects) {
       if (!this.singletonObjects.containsKey(beanName)) {
          this.singletonFactories.put(beanName, singletonFactory);
          this.earlySingletonObjects.remove(beanName);
          this.registeredSingletons.add(beanName);
       }
    }
}

• earlySingletonExposure：从字面的意思就是提早曝光的单例。需要确认有哪些条件影响这个值；
• mbd.isSingleton()：是否是单例；
• this.allowCirularReference：是否允许循环依赖，在配置文件中并没有找到如何配置，但是在AbstractRefreshableApplicationContext中提供了设置函数，可以通过硬编码的方法进行设置或者通过自定义名称空间进行设置；
• isSingletonCurrentlyInCreation(beanName)：该bean是否在创建中。在Spring中，会有个专门的属性默认为DefaultSingletonBeanRegistry的singletonsCurrentlyInCreation来记录bean的加载状态，在bean开始创建前会将beanName记录在属性中，在bean创建结束后会将beanName移除。那么我们跟随代码一路走下来可以对这个属性的记录并没有多少印象，这个状态是在哪里记录的呢？不同scope的记录位置不一样，我们以singleton为例，在singleton下记录属性的函数是在DefaultSingletonBeanRegistry类的 public Object getSingleton(String beanName,ObjectFactory singletonFactory) 函数的beforeSingletonCreation(beanName)和afterSingletonCreation(beanName)中，在这两段函数中分别this.singletonsCurrentlyInCreation.add(beanName)与this.singletonsCurrentlyInCreation.remove(beanName)来进行状态的记录与移除。

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {  
    Object exposedObject = bean;  
    if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {  
        for (BeanPostProcessor bp : getBeanPostProcessors()) {  
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {  
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;  
                exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);  
                if (exposedObject == null) {  
                    return exposedObject;  
                }  
            }  
        }  
    }  
    return exposedObject;  
}

# AbstractAutowireCapableBeanFactory
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;  
                //返回值为是否继续填充bean  
                if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {  
                    continueWithPropertyPopulation = false;  
                    break;  
                }  
            }  
        }  
    }  
    //如果后处理器发出停止填充命令则终止后续的执行  
    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.  
        //根据名称自动注入  
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {  
            autowireByName(beanName, mbd, bw, newPvs);  
        }  

        // Add property values based on autowire by type if applicable.  
        //根据类型自动注入  
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {  
            autowireByType(beanName, mbd, bw, newPvs);  
        }  

        pvs = newPvs;  
    }  
    //后处理器已经初始化  
    boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();  
    //需要依赖检查  
    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;  
                    //对所有需要依赖检查的属性进行后处理  
                    pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);  
                    if (pvs == null) {  
                        return;  
                    }  
                }  
            }  
        }  
        if (needsDepCheck) {  
            //依赖检查，对应depends-on属性，3.0已经弃用此属性  
            checkDependencies(beanName, mbd, filteredPds, pvs);  
        }  
    }  
    //将属性应用到bean中  
    //将所有ProtertyValues中的属性填充至BeanWrapper中。  
    applyPropertyValues(beanName, mbd, bw, pvs);  
}

# AbstractAutowireCapableBeanFactory
protected void autowireByName(
        String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

    // 寻找bw中需要依赖注入的属性（Bean 对象中非简单属性）
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
    for (String propertyName : propertyNames) {
        // 如果容器中包含指定名称的 bean，则将该 bean 注入到 bean中
        if (containsBean(propertyName)) {
            // 递归初始化相关 bean
            Object bean = getBean(propertyName);
            // 为指定名称的属性赋予属性值  
            pvs.add(propertyName, bean);
            // 属性依赖注入
            registerDependentBean(propertyName, beanName);
            if (logger.isDebugEnabled()) {
                logger.debug("Added autowiring by name from bean name '" + beanName +
                        "' via property '" + propertyName + "' to bean named '" + propertyName + "'");
            }
        }
        else {
            if (logger.isTraceEnabled()) {
                logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
                        "' by name: no matching bean found");
            }
        }
    }
}

# AbstractAutowireCapableBeanFactory
protected String[] unsatisfiedNonSimpleProperties(AbstractBeanDefinition mbd, BeanWrapper bw) {
    Set<String> result = new TreeSet<>();
    PropertyValues pvs = mbd.getPropertyValues();
    PropertyDescriptor[] pds = bw.getPropertyDescriptors();
    for (PropertyDescriptor pd : pds) {
        if (pd.getWriteMethod() != null && !isExcludedFromDependencyCheck(pd) && !pvs.contains(pd.getName()) &&
                !BeanUtils.isSimpleProperty(pd.getPropertyType())) {
            result.add(pd.getName());
        }
    }
    return StringUtils.toStringArray(result);
}

# AbstractAutowireCapableBeanFactory
protected void autowireByType(  
        String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {  

    TypeConverter converter = getCustomTypeConverter();  
    if (converter == null) {  
        converter = bw;  
    }  

    Set<String> autowiredBeanNames = new LinkedHashSet<String>(4);  
    //寻找bw中需要依赖注入的属性  
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);  
    for (String propertyName : propertyNames) {  
        try {  
            PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);  
            // Don't try autowiring by type for type Object: never makes sense,  
            // even if it technically is a unsatisfied, non-simple property.  
            if (!Object.class.equals(pd.getPropertyType())) {  
                //探测指定属性的set方法  
                MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);  
                // Do not allow eager init for type matching in case of a prioritized post-processor.  
                boolean eager = !PriorityOrdered.class.isAssignableFrom(bw.getWrappedClass());  
                DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);  
                //解析指定beanName的属性所匹配的值，并把解析到的属性名称存储在autowiredBeanNames中，  
                Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);  
                if (autowiredArgument != null) {  
                    pvs.add(propertyName, autowiredArgument);  
                }  
                for (String autowiredBeanName : autowiredBeanNames) {  
                    //注册依赖  
                    registerDependentBean(autowiredBeanName, beanName);  
                    if (logger.isDebugEnabled()) {  
                        logger.debug("Autowiring by type from bean name '" + beanName + "' via property '" +  
                                propertyName + "' to bean named '" + autowiredBeanName + "'");  
                    }  
                }  
                autowiredBeanNames.clear();  
            }  
        }  
        catch (BeansException ex) {  
            throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, propertyName, ex);  
        }  
    }  
}

@Autowired
private List<Test> tests;

# DefaultListableBeanFactory
public Object resolveDependency(DependencyDescriptor descriptor, String beanName, Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
    descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());  
    if (descriptor.getDependencyType().equals(ObjectFactory.class)) {  
        //ObjectFactory类注入的特殊处理  
        return new DependencyObjectFactory(descriptor, beanName);  
    }  
    else if (descriptor.getDependencyType().equals(javaxInjectProviderClass)) {  
        //javaxInjectProviderClass类注入的特殊处理  
        return new DependencyProviderFactory().createDependencyProvider(descriptor, beanName);  
    }  
    else {  
        //通用处理逻辑  
        return doResolveDependency(descriptor, descriptor.getDependencyType(), beanName, autowiredBeanNames, typeConverter);  
    }  
}  

protected Object doResolveDependency(DependencyDescriptor descriptor, Class<?> type, String beanName,  
        Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {  
    /* 
     * 用于支持Spring中新增的注解@Value 
     */  
    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()));  
    }  
    //如果解析器没有成功解析，则需要考虑各种情况  
    //属性是数组类型  
    if (type.isArray()) {  
        Class<?> componentType = type.getComponentType();  
        //根据属性类型找到beanFactory中所有类型的匹配bean，  
        //返回值的构成为：key=匹配的beanName,value=beanName对应的实例化后的bean（通过getBean(beanName)返回）  
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType, descriptor);  
        if (matchingBeans.isEmpty()) {  
            //如果autowire的require属性为true而找到的匹配项却为空则只能抛出异常  
            if (descriptor.isRequired()) {  
                raiseNoSuchBeanDefinitionException(componentType, "array of " + componentType.getName(), descriptor);  
            }  
            return null;  
        }  
        if (autowiredBeanNames != null) {  
            autowiredBeanNames.addAll(matchingBeans.keySet());  
        }  
        TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());  
        //通过转换器将bean的值转换为对应的type类型  
        return converter.convertIfNecessary(matchingBeans.values(), type);  
    }  
    //属性是Collection类型  
    else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {  
        Class<?> elementType = descriptor.getCollectionType();  
        if (elementType == null) {  
            if (descriptor.isRequired()) {  
                throw new FatalBeanException("No element type declared for collection [" + type.getName() + "]");  
            }  
            return null;  
        }  
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType, descriptor);  
        if (matchingBeans.isEmpty()) {  
            if (descriptor.isRequired()) {  
                raiseNoSuchBeanDefinitionException(elementType, "collection of " + elementType.getName(), descriptor);  
            }  
            return null;  
        }  
        if (autowiredBeanNames != null) {  
            autowiredBeanNames.addAll(matchingBeans.keySet());  
        }  
        TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());  
        return converter.convertIfNecessary(matchingBeans.values(), type);  
    }  
    //属性是Map类型  
    else if (Map.class.isAssignableFrom(type) && type.isInterface()) {  
        Class<?> keyType = descriptor.getMapKeyType();  
        if (keyType == null || !String.class.isAssignableFrom(keyType)) {  
            if (descriptor.isRequired()) {  
                throw new FatalBeanException("Key type [" + keyType + "] of map [" + type.getName() +  
                        "] must be assignable to [java.lang.String]");  
            }  
            return null;  
        }  
        Class<?> valueType = descriptor.getMapValueType();  
        if (valueType == null) {  
            if (descriptor.isRequired()) {  
                throw new FatalBeanException("No value type declared for map [" + type.getName() + "]");  
            }  
            return null;  
        }  
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType, descriptor);  
        if (matchingBeans.isEmpty()) {  
            if (descriptor.isRequired()) {  
                raiseNoSuchBeanDefinitionException(valueType, "map with value type " + valueType.getName(), descriptor);  
            }  
            return null;  
        }  
        if (autowiredBeanNames != null) {  
            autowiredBeanNames.addAll(matchingBeans.keySet());  
        }  
        return matchingBeans;  
    }  
    else {  
        Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);  
        if (matchingBeans.isEmpty()) {  
            if (descriptor.isRequired()) {  
                raiseNoSuchBeanDefinitionException(type, "", descriptor);  
            }  
            return null;  
        }  
        if (matchingBeans.size() > 1) {  
            String primaryBeanName = determinePrimaryCandidate(matchingBeans, descriptor);  
            if (primaryBeanName == null) {  
                throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());  
            }  
            if (autowiredBeanNames != null) {  
                autowiredBeanNames.add(primaryBeanName);  
            }  
            return matchingBeans.get(primaryBeanName);  
        }  
        // We have exactly one match.  
        Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();  
        if (autowiredBeanNames != null) {  
            autowiredBeanNames.add(entry.getKey());  
        }  
        //已经确定只有一个匹配项  
        return entry.getValue();  
    }  
}

protected Map<String, Object> findAutowireCandidates(
        @Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
    //在BeanFactory找到所有Type类型的beanName
    String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
            this, requiredType, true, descriptor.isEager());
    Map<String, Object> result = new LinkedHashMap<>(candidateNames.length);

    //遍历所有的beanName,通过getBean获取
    for (String candidate : candidateNames) {
        if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
            //
            addCandidateEntry(result, candidate, descriptor, requiredType);
        }
    }
    return result;
}

private void addCandidateEntry(Map<String, Object> candidates, String candidateName,
        DependencyDescriptor descriptor, Class<?> requiredType) {

    Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);
    if (!(beanInstance instanceof NullBean)) {
        candidates.put(candidateName, beanInstance);
    }
}

public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
        throws BeansException {
    //通过类型找到beanName,然后再找到其实例
    return beanFactory.getBean(beanName);
}

# AbstractAutowireCapableBeanFactory
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {  
    if (pvs == null || pvs.isEmpty()) {  
        return;  
    }  

    MutablePropertyValues mpvs = null;  
    List<PropertyValue> original;  

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

    if (pvs instanceof MutablePropertyValues) {  
        mpvs = (MutablePropertyValues) pvs;  
        //如果mpvs中的值已经被转换为对应的类型那么可以直接设置到beanwapper中  
        if (mpvs.isConverted()) {  
            // Shortcut: use the pre-converted values as-is.  
            try {  
                bw.setPropertyValues(mpvs);  
                return;  
            }  
            catch (BeansException ex) {  
                throw new BeanCreationException(  
                        mbd.getResourceDescription(), beanName, "Error setting property values", ex);  
            }  
        }  
        original = mpvs.getPropertyValueList();  
    }  
    else {  
        //如果pvs并不是使用MutablePropertyValues封装的类型，那么直接使用原始的属性获取方法  
        original = Arrays.asList(pvs.getPropertyValues());  
    }  

    TypeConverter converter = getCustomTypeConverter();  
    if (converter == null) {  
        converter = bw;  
    }  
    //获取对应的解析器  
    BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);  

    // Create a deep copy, resolving any references for values.  
    List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());  
    boolean resolveNecessary = false;  
    //遍历属性，将属性转换为对应类的对应属性的类型  
    for (PropertyValue pv : original) {  
        if (pv.isConverted()) {  
            deepCopy.add(pv);  
        }  
        else {  
            String propertyName = pv.getName();  
            Object originalValue = pv.getValue();  
            Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);  
            Object convertedValue = resolvedValue;  
            boolean convertible = bw.isWritableProperty(propertyName) &&  
                    !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);  
            if (convertible) {  
                convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);  
            }  
            // Possibly store converted value in merged bean definition,  
            // in order to avoid re-conversion for every created bean instance.  
            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 {  
                resolveNecessary = true;  
                deepCopy.add(new PropertyValue(pv, convertedValue));  
            }  
        }  
    }  
    if (mpvs != null && !resolveNecessary) {  
        mpvs.setConverted();  
    }  

    // Set our (possibly massaged) deep copy.  
    try {  
        bw.setPropertyValues(new MutablePropertyValues(deepCopy));  
    }  
    catch (BeansException ex) {  
        throw new BeanCreationException(  
                mbd.getResourceDescription(), beanName, "Error setting property values", ex);  
    }  
}

public class MyTestBean {
    private String name ;

    public MyTestBean(String name) {
        this.name = name;
    }

    public MyTestBean() {
    }
    //...get set方法
}

<bean id="myTestBean"  class="chenhao.spring01.MyTestBean">
        <property name="name" value="chenhao"></property>
</bean>

@Override
public void setValue(final @Nullable Object value) throws Exception {
    //获取writeMethod，也就是我们MyTestBean的setName方法
    final Method writeMethod = (this.pd instanceof GenericTypeAwarePropertyDescriptor ?
            ((GenericTypeAwarePropertyDescriptor) this.pd).getWriteMethodForActualAccess() :
            this.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 (PrivilegedActionException ex) {
            throw ex.getException();
        }
    }
    else {
        ReflectionUtils.makeAccessible(writeMethod);
        //通过反射调用方法进行赋值
        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>) () -> {
            // 激活 Aware 方法
            invokeAwareMethods(beanName, bean);
            return null;
        }, getAccessControlContext());
    }
    else {
        // 对特殊的 bean 处理：Aware、BeanClassLoaderAware、BeanFactoryAware
        invokeAwareMethods(beanName, bean);
    }

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

    try {
        // 激活用户自定义的 init 方法
        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()) {
        // 后处理器
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }
    return wrappedBean;
}

1. 激活 Aware 方法；
2. 后置处理器的应用；
3. 激活自定义的 init 方法。

public class HelloBean {
    public void say()
    {
        System.out.println("Hello");
    }
}

//定义实现BeanFactoryAware的bean
public class MyBeanAware implements BeanFactoryAware {
    private BeanFactory beanFactory;
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.beanFactory = beanFactory;
    }
    public void testAware()
    {
        //通过hello这个bean id从beanFactory获取实例  
        HelloBean hello = (HelloBean)beanFactory.getBean("hello");
        hello.say();
    }
}

//applicationContext.xml 配置文件
<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="myBeanAware" class="com.chenhao.spring.MyBeanAware">
    </bean>
    <bean id="hello" class="com.chenhao.spring.HelloBean">
    </bean>
</beans>

//测试
public class Test {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
        MyBeanAware test = (MyBeanAware)ctx.getBean("myBeanAware");
        test.testAware();
    }
}

//输出
hello

# AbstractAutowireCapableBeanFactory
private void invokeAwareMethods(final String beanName, final Object bean) {  
    if (bean instanceof Aware) {  
        if (bean instanceof BeanNameAware) {  
            ((BeanNameAware) bean).setBeanName(beanName);  
        }  
        if (bean instanceof BeanClassLoaderAware) {  
            ((BeanClassLoaderAware) bean).setBeanClassLoader(getBeanClassLoader());  
        }  
        if (bean instanceof BeanFactoryAware) {  
            ((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);  
        }  
    }
}

# AbstractAutowireCapableBeanFactory
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)  
        throws BeansException {  

    Object result = existingBean;  
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {  
        result = beanProcessor.postProcessBeforeInitialization(result, beanName);  
        if (result == null) {  
            return result;  
        }  
    }  
    return result;  
}

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)  
        throws BeansException {  

    Object result = existingBean;  
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {  
        result = beanProcessor.postProcessAfterInitialization(result, beanName);  
        if (result == null) {  
            return result;  
        }  
    }  
    return result;  
}

# AbstractAutowireCapableBeanFactory
protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
        throws Throwable {

    // 是否实现 InitializingBean
    // 如果实现了 InitializingBean 接口，则只掉调用bean的 afterPropertiesSet()
    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isDebugEnabled()) {
            logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                    ((InitializingBean) bean).afterPropertiesSet();
                    return null;
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            // 直接调用 afterPropertiesSet()
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }

    if (mbd != null && bean.getClass() != NullBean.class) {
        // 判断是否指定了 init-method()，
        // 如果指定了 init-method()，则再调用制定的init-method
        String initMethodName = mbd.getInitMethodName();
        if (StringUtils.hasLength(initMethodName) &&
                !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                !mbd.isExternallyManagedInitMethod(initMethodName)) {
            // 利用反射机制执行初始化方法
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}

public class InitializingBeanTest {

    private String name;
    //... get/set方法
    public void setOtherName(){
        this.name = "wangjiafu";
        System.out.println("InitializingBeanTest setOtherName " + this.name);
    }
}

//applicationContext.xml 配置文件
<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="initializingBeanTest" class="com.wangjiafu.spring.InitializingBeanTest"
            init-method="setOtherName">
        <property name="name" value="wangjiafu123"/>
    </bean>
</beans>

//测试
public class Test {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
        InitializingBeanTest test = (InitializingBeanTest)ctx.getBean("initializingBeanTest");
    }
}

//执行结果
InitializingBeanTest setOtherName wangjiafu

protected void invokeCustomInitMethod(String beanName, final Object bean, RootBeanDefinition mbd)
        throws Throwable {

    String initMethodName = mbd.getInitMethodName();
    Assert.state(initMethodName != null, "No init method set");
    Method initMethod = (mbd.isNonPublicAccessAllowed() ?
            BeanUtils.findMethod(bean.getClass(), initMethodName) :
            ClassUtils.getMethodIfAvailable(bean.getClass(), initMethodName));

    if (initMethod == null) {
        if (mbd.isEnforceInitMethod()) {
            throw new BeanDefinitionValidationException("Could not find an init method named '" +
                    initMethodName + "' on bean with name '" + beanName + "'");
        }
        else {
            if (logger.isTraceEnabled()) {
                logger.trace("No default init method named '" + initMethodName +
                        "' found on bean with name '" + beanName + "'");
            }
            // Ignore non-existent default lifecycle methods.
            return;
        }
    }

    if (logger.isTraceEnabled()) {
        logger.trace("Invoking init method  '" + initMethodName + "' on bean with name '" + beanName + "'");
    }
    Method methodToInvoke = ClassUtils.getInterfaceMethodIfPossible(initMethod);

    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            ReflectionUtils.makeAccessible(methodToInvoke);
            return null;
        });
        try {
            AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
                    methodToInvoke.invoke(bean), getAccessControlContext());
        }
        catch (PrivilegedActionException pae) {
            InvocationTargetException ex = (InvocationTargetException) pae.getException();
            throw ex.getTargetException();
        }
    }
    else {
        try {
            ReflectionUtils.makeAccessible(initMethod);
            initMethod.invoke(bean);
        }
        catch (InvocationTargetException ex) {
            throw ex.getTargetException();
        }
    }
}

# AbstractBeanFactory
protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
   if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {
      if (mbd.isSingleton()) {
          /*
           * 单例模式下注册需要销毁的bean，此方法中会处理实现DisposableBean的bean，
           * 并对所有的bean使用DestructionAwareBeanPostProcessor处理
           * DisposableBean DestructionAwareBeanPostProcessors。
           */
         // Register a DisposableBean implementation that performs all destruction
         // work for the given bean: DestructionAwareBeanPostProcessors,
         // DisposableBean interface, custom destroy method.
         registerDisposableBean(beanName, new DisposableBeanAdapter(
               bean, beanName, mbd, getBeanPostProcessorCache().destructionAware));
      }
      else {
         // 自定义scope的处理
         Scope scope = this.scopes.get(mbd.getScope());
         if (scope == null) {
            throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
         }
         scope.registerDestructionCallback(beanName, new DisposableBeanAdapter(
               bean, beanName, mbd, getBeanPostProcessorCache().destructionAware));
      }
   }
}

# DefaultSingletonBeanRegistry
/** Disposable bean instances: bean name to disposable instance. */
private final Map<String, Object> disposableBeans = new LinkedHashMap<>();
public void registerDisposableBean(String beanName, DisposableBean bean) {
   synchronized (this.disposableBeans) {
      this.disposableBeans.put(beanName, bean);
   }
}