SpringIOC-11-getBean

Spring源码分析-getBean

前言

接着 Spring IoC：finishBeanFactoryInitialization详解，我们正式开始学习获取 bean 实例方法，该方法是 Spring 最核心的方法。

单击 preInstantiateSingletons 方法里的 getBean(beanName) 代码，进入该方法。

流程图

getBean

实例化Bean的核心方法

/**
* 创建实例的主要入口
*/
@Override
public Object getBean(String name) throws BeansException {
    // 获取name对应的bean实例，如果不存在，则创建一个
    return doGetBean(name, null, null, false);
}

doGetBean

/**
* 实例化bean的主要方法
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
                          @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    //根据name拿到bean的名称
    // 1.解析beanName，主要是解析别名、去掉FactoryBean的前缀“&”
    final String beanName = transformedBeanName(name);
    Object bean;

    // 2.尝试从缓存中获取beanName对应的实例
    // Eagerly check singleton cache for manually registered singletons.
    Object sharedInstance = getSingleton(beanName);
    // 3.如果beanName的实例存在于缓存中
    if (sharedInstance != null && args == null) {
        if (logger.isTraceEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                             "' that is not fully initialized yet - a consequence of a circular reference");
            } else {
                logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        //该方法是FactoryBean接口的调用入口
        // 3.1 返回beanName对应的实例对象（主要用于FactoryBean的特殊处理，普通Bean会直接返回sharedInstance本身）
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    } else {
        //如果singletonObjects缓存里面没有，则走下来
        // Fail if we're already creating this bean instance:
        // We're assumably within a circular reference.
        //如果是scope 是Prototype的，校验是否有出现循环依赖，如果有则直接报错
        // 4.scope为prototype的循环依赖校验：如果beanName已经正在创建Bean实例中，而此时我们又要再一次创建beanName的实例，则代表出现了循环依赖，需要抛出异常
        // 例子：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，就会产生当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
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // 5.获取parentBeanFactory
        //检查是否存在父类容器的定义 一般是没有的
        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        // 5.1 如果parentBeanFactory存在，并且beanName在当前BeanFactory不存在Bean定义，则尝试从parentBeanFactory中获取bean实例
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            // 5.2 将别名解析成真正的beanName
            String nameToLookup = originalBeanName(name);
            // 5.3 尝试在parentBeanFactory中获取bean对象实例
            if (parentBeanFactory instanceof AbstractBeanFactory) {
                return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                    nameToLookup, requiredType, args, typeCheckOnly);
            } else if (args != null) {
                // Delegation to parent with explicit args.
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            } else if (requiredType != null) {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            } else {
                return (T) parentBeanFactory.getBean(nameToLookup);
            }
        }

        if (!typeCheckOnly) {
            // 6.如果不是仅仅做类型检测，而是创建bean实例，这里要将beanName放到alreadyCreated缓存
            markBeanAsCreated(beanName);
        }

        try {
            //父子BeanDefinition合并
            // 7.根据beanName重新获取MergedBeanDefinition（步骤6将MergedBeanDefinition删除了，这边获取一个新的）
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            // 7.1 检查MergedBeanDefinition是否是抽象的
            checkMergedBeanDefinition(mbd, beanName, args);

            // 8.拿到当前bean依赖的bean名称集合，在实例化自己之前，需要先实例化自己依赖的bean
            // Guarantee initialization of beans that the current bean depends on.
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                // 8.1 遍历当前bean依赖的bean名称集合
                for (String dep : dependsOn) {
                    // 8.2 检查dep是否依赖于beanName，即检查是否存在循环依赖
                    if (isDependent(beanName, dep)) {
                        // 8.3 如果是循环依赖则抛异常
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    // 8.4 将dep和beanName的依赖关系注册到缓存中
                    registerDependentBean(dep, beanName);
                    try {
                        // 8.5 获取dep对应的bean实例，如果dep还没有创建bean实例，则创建dep的bean实例
                        getBean(dep);
                    } catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                    }
                }
            }

            //着重看，大部分是单例的情况
            // 9.针对不同的scope进行bean的创建
            // Create bean instance.
            if (mbd.isSingleton()) {
                //获取单例的实例，单例的核心入口
                // 9.1 scope为singleton的bean创建（新建了一个ObjectFactory，并且重写了getObject方法）
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                        // 9.1.1 创建Bean实例，调用实例化的核心方法
                        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;
                    }
                });
                //该方法是FactoryBean接口的调用入口
                // 9.1.2 返回beanName对应的实例对象
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            } else if (mbd.isPrototype()) {
                // 9.2 scope为prototype的bean创建
                // It's a prototype -> create a new instance.
                Object prototypeInstance = null;
                try {
                    // 9.2.1 创建实例前的操作（将beanName保存到prototypesCurrentlyInCreation缓存中）
                    //将正在实例化的bean加入prototypesCurrentlyInCreation用来阻断循环依赖
                    beforePrototypeCreation(beanName);
                    // 9.2.2 创建Bean实例
                    prototypeInstance = createBean(beanName, mbd, args);
                } finally {
                    // 9.2.3 创建实例后的操作（将创建完的beanName从prototypesCurrentlyInCreation缓存中移除）
                    afterPrototypeCreation(beanName);
                }
                // 9.2.4 返回beanName对应的实例对象
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            } else {
                // 9.3 其他scope的bean创建，可能是request之类的
                // 9.3.1 根据scopeName，从缓存拿到scope实例
                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 {
                    // 9.3.2 其他scope的bean创建（新建了一个ObjectFactory，并且重写了getObject方法）
                    Object scopedInstance = scope.get(beanName, () -> {
                        // 9.3.3 创建实例前的操作（将beanName保存到prototypesCurrentlyInCreation缓存中）
                        beforePrototypeCreation(beanName);
                        try {
                            // 9.3.4 创建bean实例
                            return createBean(beanName, mbd, args);
                        } finally {
                            // 9.3.5 创建实例后的操作（将创建完的beanName从prototypesCurrentlyInCreation缓存中移除）
                            afterPrototypeCreation(beanName);
                        }
                    });
                    // 9.3.6 返回beanName对应的实例对象
                    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) {
            // 如果创建bean实例过程中出现异常，则将beanName从alreadyCreated缓存中移除
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }
    // 10.检查所需类型是否与实际的bean对象的类型匹配
    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && !requiredType.isInstance(bean)) {
        try {
            // 10.1 类型不对，则尝试转换bean类型
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        } catch (TypeMismatchException ex) {
            if (logger.isTraceEnabled()) {
                logger.trace("Failed to convert bean '" + name + "' to required type '" +
                             ClassUtils.getQualifiedName(requiredType) + "'", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    // 11.返回创建出来的bean实例对象
    return (T) bean;
}

getObjectForBeanInstance

获取给定bean实例的对象，或者实例本身或FactoryBean创建的对象

/**
* 获取给定bean实例的对象，或者实例本身或FactoryBean创建的对象
*/
protected Object getObjectForBeanInstance(
    Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

    // Don't let calling code try to dereference the factory if the bean isn't a factory.
    //1. 如果name是以 “&” 号开头的进入方法并返回实例
    if (BeanFactoryUtils.isFactoryDereference(name)) {
        //1.1 如果实现了NullBean直接返回实例
        if (beanInstance instanceof NullBean) {
            return beanInstance;
        }
        //1.2 如果beanInstance不是FactoryBean，则直接报错
        if (!(beanInstance instanceof FactoryBean)) {
            //抛出异常报错
            throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
        }
        //1.3 如果mbd不为空则设置mbd的isFactoryBean为true
        if (mbd != null) {
            mbd.isFactoryBean = true;
        }
        //2. 返回实例
        return beanInstance;
    }


    // 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.
    // 2. 如果beanInstance不是FactoryBean（也就是普通bean），则直接返回beanInstance
    if (!(beanInstance instanceof FactoryBean)) {
        return beanInstance;
    }

    // 3.走到这边，代表beanInstance是FactoryBean，但name不带有“&”前缀，表示想要获取的是FactoryBean创建的对象实例
    Object object = null;
    // 4.如果mbd不为空则设置mbd的isFactoryBean为true
    if (mbd != null) {
        mbd.isFactoryBean = true;
    } else {
        //5. 如果mbd为空，则尝试从factoryBeanObjectCache缓存中获取该FactoryBean创建的对象实例
        object = getCachedObjectForFactoryBean(beanName);
    }
    //如果缓存中没有拿不到
    if (object == null) {
        // 6. 只有beanInstance是FactoryBean才能走到这边，因此直接强转
        // Return bean instance from factory.
        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
        // Caches object obtained from FactoryBean if it is a singleton.
        //7. 检查beanDefinitionMap中是否有beanName
        if (mbd == null && containsBeanDefinition(beanName)) {
            // 7.1 mbd为空，但是该bean的BeanDefinition在缓存中存在，则获取该bean的MergedBeanDefinition
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        // 8. mbd是否是合成的（这个字段比较复杂，mbd正常情况都不是合成的，也就是false，有兴趣的可以自己查阅资料看看）
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        //重点看
        // 9.从FactoryBean获取对象实例
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    // 10.返回对象实例
    return object;
}

isFactoryDereference

来到BeanFactoryUtils类

是否是FactoryBean，检查name不为空并且name是以 “&” 符号开头的

/**
* 检查是否是FactoryBean
*/
public static boolean isFactoryDereference(@Nullable String name) {
    //如果name不为空并且name是以 “&” 符号开头的
    return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
}

getObjectFromFactoryBean

从FactoryBean中获取对象

/**
* Obtain an object to expose from the given FactoryBean.
* 从FactoryBean中获取对象
*/
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
    // 1.如果是单例，并且已经存在于单例对象缓存中
    //工厂bean是单例的并且singletonObjects的缓存中存在beanName
    if (factory.isSingleton() && containsSingleton(beanName)) {
        synchronized (getSingletonMutex()) {
            // 2.从FactoryBean创建的单例对象的缓存中获取该bean实例
            Object object = this.factoryBeanObjectCache.get(beanName);
            //如果为null
            if (object == null) {
                // 3.调用FactoryBean的getObject方法获取对象实例
                object = doGetObjectFromFactoryBean(factory, beanName);
                // Only post-process and store if not put there already during getObject() call above
                // (e.g. because of circular reference processing triggered by custom getBean calls)
                //4. 再次检查factoryBeanObjectCache是否存在
                Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
                // 5.如果该beanName已经在缓存中存在，则将object替换成缓存中的
                if (alreadyThere != null) {
                    object = alreadyThere;
                } else {
                    if (shouldPostProcess) {
                        //6. 检查beanName是否正在创建如果正在创建直接返回正在创建的实例
                        if (isSingletonCurrentlyInCreation(beanName)) {
                            // Temporarily return non-post-processed object, not storing it yet..
                            return object;
                        }
                        //7. 将正在创建的Bean加入到inCreationCheckExclusions中
                        //在这个集合里面的bean都是正在实例化的bean
                        //如果重复添加就会报错，防止构造方法循环依赖
                        beforeSingletonCreation(beanName);
                        try {
                            //提前暴露bean
                            //8. 对bean实例进行后置处理，执行所有已注册的BeanPostProcessor的postProcessAfterInitialization方法
                            object = postProcessObjectFromFactoryBean(object, beanName);
                        } catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                                            "Post-processing of FactoryBean's singleton object failed", ex);
                        } finally {
                            //9. Bean创建完成从inCreationCheckExclusions中删除beanName
                            afterSingletonCreation(beanName);
                        }
                    }
                    //singletonObjects的缓存中存在beanName
                    if (containsSingleton(beanName)) {
                        //10. 把实例缓存到factoryBeanObjectCache map中，这个是单独缓存FactoryBean类型实例的map
                        this.factoryBeanObjectCache.put(beanName, object);
                    }
                }
            }
            //11. 返回object对象实例
            return object;
        }
    } else {
        //缓存中不存在bean
        //12.调用FactoryBean的getObject方法获取对象实例
        Object object = doGetObjectFromFactoryBean(factory, beanName);
        if (shouldPostProcess) {
            try {
                //提前暴漏bean
                //13. 对bean实例进行后置处理，执行所有已注册的BeanPostProcessor的postProcessAfterInitialization方法
                object = postProcessObjectFromFactoryBean(object, beanName);
            } catch (Throwable ex) {
                throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
            }
        }
        //14. 返回bean的实例
        return object;
    }
}

doGetObjectFromFactoryBean

调用getObject获取bean实例

/**
 * 调用getObject获取bean实例
 */
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
    throws BeanCreationException {

    Object object;
    // 1.调用FactoryBean的getObject方法获取bean对象实例
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                // 1.1 带有权限验证的
                object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
            } catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        } else {
            // 1.2 不带权限
            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.
    // 2.getObject返回的是空值，并且该FactoryBean正在初始化中，则直接抛异常，不接受一个尚未完全初始化的FactoryBean的getObject返回的空值
    if (object == null) {
        //2.1 如果该Bean 正在实例化说明出现了循环依赖直接报错
        if (isSingletonCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(
                beanName, "FactoryBean which is currently in creation returned null from getObject");
        }
        //2.2 如果第一次实例化为空 转换为NullBean
        object = new NullBean();
    }
    //3.返回创建好的bean对象实例
    return object;
}

很简单的方法，就是直接调用 FactoryBean 的 getObject 方法来获取到对象实例。

细心的同学可以发现，该方法是以 do 开头， do 开头的方法是最终进行实际操作的方法，例如本方法就是 FactoryBean 最终实际进行创建 bean 对象实例的方法。

postProcessObjectFromFactoryBean

是一个扩展点，使用BeanPostProcessor对bean实例化之前进行一些调用

@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
    return applyBeanPostProcessorsAfterInitialization(object, beanName);
}

/**
 * 是一个扩展点
 * 执行BeanPostProcessor的postProcessAfterInitialization接口
 */
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
    throws BeansException {

    Object result = existingBean;
    // 1.遍历所有注册的BeanPostProcessor实现类，调用postProcessAfterInitialization方法
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
        // 2.在bean初始化后，调用postProcessAfterInitialization方法
        Object current = processor.postProcessAfterInitialization(result, beanName);
        if (current == null) {
            // 3.如果返回null，则不会调用后续的BeanPostProcessors
            return result;
        }
        result = current;
    }
    return result;
}

这边走的是 AbstractAutowireCapableBeanFactory 里的方法。通过前面的介绍，我们知道创建的 BeanFactory 为 DefaultListableBeanFactory，而 DefaultListableBeanFactory 继承了 AbstractAutowireCapableBeanFactory，因此这边会走 AbstractAutowireCapableBeanFactory 的重写方法。

markBeanAsCreated

/**
 * 清除alreadyCreated中的缓存

 */
protected void markBeanAsCreated(String beanName) {
    if (!this.alreadyCreated.contains(beanName)) {
        synchronized (this.mergedBeanDefinitions) {
            // 1.如果alreadyCreated缓存中不包含beanName
            if (!this.alreadyCreated.contains(beanName)) {
                // Let the bean definition get re-merged now that we're actually creating
                // the bean... just in case some of its metadata changed in the meantime.
                // 2.将beanName的MergedBeanDefinition从mergedBeanDefinitions缓存中移除，
                // 在之后重新获取MergedBeanDefinition，避免BeanDefinition在创建过程中发生变化
                clearMergedBeanDefinition(beanName);
                // 3.将beanName添加到alreadyCreated缓存中，代表该beanName的bean实例已经创建（或即将创建）
                this.alreadyCreated.add(beanName);
            }
        }
    }
}

/**
 * 移除指定bean的合并bean定义，在下次访问时重新创建它
 * @param beanName the bean name to clear the merged definition for
 */
protected void clearMergedBeanDefinition(String beanName) {
    //从mergedBeanDefinitions中获取BeanDefinition
    RootBeanDefinition bd = this.mergedBeanDefinitions.get(beanName);
    if (bd != null) {
        //设置 stale属性为true
        bd.stale = true;
    }
}

这边会将 beanName 对应的 MergedBeanDefinition 移除，然后在之后的代码重新获取，主要是为了使用最新的 MergedBeanDefinition 来进行创建操作。

isDependent

循环依赖检查

/**
 * 循环依赖检查
 */
private boolean isDependent(String beanName, String dependentBeanName, @Nullable Set<String> alreadySeen) {
    // 已经检查过的直接跳过
    if (alreadySeen != null && alreadySeen.contains(beanName)) {
        return false;
    }
    // 1.将别名解析为真正的名称
    String canonicalName = canonicalName(beanName);
    // 2.拿到依赖canonicalName的beanName集合
    Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
    // 3.如果dependentBeans为空，则两者必然还未确定依赖关系，返回false
    if (dependentBeans == null) {
        return false;
    }
    // 4.如果dependentBeans包含dependentBeanName，则表示两者已确定依赖关系，返回true
    if (dependentBeans.contains(dependentBeanName)) {
        return true;
    }
    // 5.循环检查，即检查依赖canonicalName的所有beanName是否存在被dependentBeanName依赖的（即隔层依赖）
    for (String transitiveDependency : dependentBeans) {
        if (alreadySeen == null) {
            alreadySeen = new HashSet<>();
        }
        //加入到set
        alreadySeen.add(beanName);
        if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
            return true;
        }
    }
    return false;
}

这边引入了一个缓存 dependentBeanMap：beanName -> 所有依赖 beanName 对应的 bean 的 beanName 集合。内容比较简单，就是检查依赖 beanName 的集合中是否包含 dependentBeanName，隔层依赖也算。例如：A 依赖了 B，B 依赖了 C，则 A 也算依赖了 C。

registerDependentBean

/**
 * Register a dependent bean for the given bean,
 * to be destroyed before the given bean is destroyed.
 * 注册依赖的bean和bean name的映射关系
 */
public void registerDependentBean(String beanName, String dependentBeanName) {
    // 1.解析别名
    String canonicalName = canonicalName(beanName);

    synchronized (this.dependentBeanMap) {
        //2. 拿到依赖canonicalName的beanName集合
        //3.将dependentBeanName添加到依赖canonicalName的beanName集合中
        Set<String> dependentBeans =
            this.dependentBeanMap.computeIfAbsent(canonicalName, k -> new LinkedHashSet<>(8));
        // 5.如果dependentBeans包含dependentBeanName，则表示依赖关系已经存在，直接返回
        if (!dependentBeans.add(dependentBeanName)) {
            return;
        }
    }
    //4.如果依赖关系还没有注册，则将两者的关系注册到dependentBeanMap和dependenciesForBeanMap缓存
    synchronized (this.dependenciesForBeanMap) {
        Set<String> dependenciesForBean =
            this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName, k -> new LinkedHashSet<>(8));
        dependenciesForBean.add(canonicalName);
    }
}

这边又引入了一个跟 dependentBeanMap 类似的缓存，dependenciesForBeanMap：beanName -> beanName 对应的 bean 依赖的所有 bean 的 beanName 集合。

这两个缓存很容易搞混，举个简单例子：例如 B 依赖了 A，则 dependentBeanMap 缓存中应该存放一对映射：其中 key 为 A，value 为含有 B 的 Set；而 dependenciesForBeanMap 缓存中也应该存放一对映射：其中 key 为：B，value 为含有 A 的 Set。

getSingleton

针对不同的scope进行bean的创建

入口代码

//获取单例的实例，单例的核心入口
// 9.1 scope为singleton的bean创建（新建了一个ObjectFactory，并且重写了getObject方法）
sharedInstance = getSingleton(beanName, () -> {
    try {
        // 9.1.1 创建Bean实例，调用实例化的核心方法
        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;
    }
});

代码解析

/**
 * Return the (raw) singleton object registered under the given name,
 * creating and registering a new one if none registered yet.
 * 返回一个实例化的单例对象，如果没有实例化就进行实例化
 */
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    // 1.加锁，避免重复创建单例对象
    synchronized (this.singletonObjects) {
        // 2.首先检查beanName对应的bean实例是否在缓存中存在，如果已经存在，则直接返回
        Object singletonObject = this.singletonObjects.get(beanName);
        //第一次调用必定为空
        // 3.beanName对应的bean实例不存在于缓存中，则进行Bean的创建
        if (singletonObject == null) {
            // 4.当bean工厂的单例处于destruction状态时，不允许进行单例bean创建，抛出异常
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                                                          "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                                                          "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            // 5.创建单例前的操作
            //把beanName添加到singletonsCurrentlyInCreation Set容器中，在这个集合里面的bean都是正在实例化的bean
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            // suppressedExceptions用于记录异常相关信息
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                // 6.执行singletonFactory的getObject方法获取bean实例
                //如果这里有返回值，就代表这个bean已经结束创建了，已经完全创建成功
                //调到lamda表达式的createBean(beanName, mbd, args);的方法
                singletonObject = singletonFactory.getObject();
                // 标记为新的单例对象
                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;
                }
                // 7.创建单例后的操作
                //bean创建完成后singletonsCurrentlyInCreation要删除该bean
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                // 8.如果是新的单例对象，将beanName和对应的bean实例添加到缓存中（singletonObjects、registeredSingletons）
                //创建对象成功时，把对象缓存到singletonObjects缓存中,bean创建完成时放入一级缓存
                addSingleton(beanName, singletonObject);
            }
        }
        // 9.返回创建出来的单例对象
        return singletonObject;
    }
}

beforeSingletonCreation&afterSingletonCreation

添加正在实例化的bean，解决循环依赖

/**
 * Callback before singleton creation.
 * <p>The default implementation register the singleton as currently in creation.
 * 添加正在实例化的bean
 * 如果再次添加一个存在的就会直接报错
 * 标识正在创建的bean
 */
protected void beforeSingletonCreation(String beanName) {
    // 把beanName添加到singletonsCurrentlyInCreation Set容器中，在这个集合里面的bean都是正在实例化的bean
    // 先校验beanName是否为要在创建检查排除掉的（inCreationCheckExclusions缓存），如果不是，
    // 则将beanName加入到正在创建bean的缓存中（Set），如果beanName已经存在于该缓存，会返回false抛出异常（这种情况出现在构造器的循环依赖）
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
    }
}


/**
 * Callback after singleton creation.
 * <p>The default implementation marks the singleton as not in creation anymore.
 * 完成实例化从inCreationCheckExclusions中删除实例化完成的beanName
 */
protected void afterSingletonCreation(String beanName) {
    // 先校验beanName是否为要在创建检查排除掉的（inCreationCheckExclusions缓存），如果不是，
    // 则将beanName从正在创建bean的缓存中（Set）移除，如果beanName不存在于该缓存，会返回false抛出异常
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
        throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
    }
}

inCreationCheckExclusions 是要在创建检查排除掉的 beanName 集合，正常为空，可以不管。这边主要是引入了 singletonsCurrentlyInCreation 缓存：当前正在创建的 bean 的 beanName 集合。在 beforeSingletonCreation 方法中，通过添加 beanName 到该缓存，可以预防出现构造器循环依赖的情况。

为什么无法解决构造器循环依赖？

我们之前在 Spring IoC：finishBeanFactoryInitialization详解中的代码块getSingleton 提过，getSingleton 方法是解决循环引用的核心代码。解决逻辑的第一句话：“我们先用构造函数创建一个 “不完整” 的 bean 实例”，从这句话可以看出，构造器循环依赖是无法解决的，因为当构造器出现循环依赖，我们连 “不完整” 的 bean 实例都构建不出来。Spring 能解决的循环依赖有：通过 setter 注入的循环依赖、通过属性注入的循环依赖。

addSingleton

将实例化完成的单例对象添加到单例对象缓存

/**
 * Add the given singleton object to the singleton cache of this factory.
 * <p>To be called for eager registration of singletons.
 * 将实例化完成的单例对象添加到单例对象缓存
 */
protected void addSingleton(String beanName, Object singletonObject) {
    synchronized (this.singletonObjects) {
        // 1.添加到单例对象缓存，添加进一级缓存
        this.singletonObjects.put(beanName, singletonObject);
        //2.将单例工厂缓存移除（已经不需要），删除三级缓存
        this.singletonFactories.remove(beanName);
        //3.将早期单例对象缓存移除（已经不需要），删除二级缓存
        this.earlySingletonObjects.remove(beanName);
        // 4.添加到已经注册的单例对象缓存
        this.registeredSingletons.add(beanName);
    }
}

beforePrototypeCreation&afterPrototypeCreation

使用prototypesCurrentlyInCreation 来阻断prototype的循环依赖

/**
 * Callback before prototype creation.
 * <p>The default implementation register the prototype as currently in creation.
 * 将正在实例化的bean加入prototypesCurrentlyInCreation
 * 用来阻断循环依赖
 */
@SuppressWarnings("unchecked")
protected void beforePrototypeCreation(String beanName) {
    // 1.拿到当前线程中正在创建的prototype的bean的beanName集合
    Object curVal = this.prototypesCurrentlyInCreation.get();
    // 2.如果为空，则将ThreadLocal设置成当前的beanName
    if (curVal == null) {
        this.prototypesCurrentlyInCreation.set(beanName);
        // 3.如果不为空，并且是String类型，则代表目前只有一个beanName，将之前和当前的一起封装成Set<String>，设置到ThreadLocal中
    } else if (curVal instanceof String) {
        Set<String> beanNameSet = new HashSet<>(2);
        beanNameSet.add((String) curVal);
        beanNameSet.add(beanName);
        this.prototypesCurrentlyInCreation.set(beanNameSet);

        // 4.如果不为空，并且不是String，则必然是Set<String>类型，将当前的beanName加到Set中去
    } else {
        Set<String> beanNameSet = (Set<String>) curVal;
        beanNameSet.add(beanName);
    }
}



/**
 * Callback after prototype creation.
 * <p>The default implementation marks the prototype as not in creation anymore.
 * 创建成功后从prototypesCurrentlyInCreation移除正在创建的bean
 */
@SuppressWarnings("unchecked")
protected void afterPrototypeCreation(String beanName) {
    // 1.拿到当前线程中正在创建的prototype的bean的beanName集合
    Object curVal = this.prototypesCurrentlyInCreation.get();
    // 2.如果是String类型，则代表目前只有一个beanName，则直接移除
    if (curVal instanceof String) {
        this.prototypesCurrentlyInCreation.remove();
        // 3.如果是Set类型，则从Set从移除beanName
    } else if (curVal instanceof Set) {
        Set<String> beanNameSet = (Set<String>) curVal;
        beanNameSet.remove(beanName);
        if (beanNameSet.isEmpty()) {
            this.prototypesCurrentlyInCreation.remove();
        }
    }
}

该方法和代码块beforeSingletonCreation&afterSingletonCreation的两个方法类似。主要是在进行 bean 实例的创建前，将 beanName 添加到 prototypesCurrentlyInCreation 缓存；bean 实例创建后，将 beanName 从 prototypesCurrentlyInCreation 缓存中移除。这边 prototypesCurrentlyInCreation 存放的类型为 Object，在只有一个 beanName 的时候，直接存该 beanName，也就是 String 类型；当有多个 beanName 时，转成 Set 来存放。

总结

本文介绍了获取 bean 实例的大部分内容，包括先从缓存中检查、 FactoryBean 的 bean 创建、实例化自己的依赖（depend-on 属性）、创建 bean 实例的前后一些标记等，在下篇文章中，将解析创建 bean 的内容。