spring的finishBeanFactoryInitialization方法分析

spring源码版本5.0.5

概述

        该方法会实例化所有剩余的非懒加载单例 bean。除了一些内部的 bean、实现了 BeanFactoryPostProcessor 接口的 bean、实现了 BeanPostProcessor 接口的 bean，其他的非懒加载单例 bean 都会在这个方法中被实例化，并且 BeanPostProcessor 的触发也是在这个方法中。

分析

        跟踪到AbstractApplicationContext.refresh()方法，找到代码finishBeanFactoryInitialization(beanFactory)查看实现。

    protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        // Initialize conversion service for this context.
        // 1.初始化此上下文的转换服务
        if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
                beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
            beanFactory.setConversionService(
                    beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
        }

        // Register a default embedded value resolver if no bean post-processor
        // (such as a PropertyPlaceholderConfigurer bean) registered any before:
        // at this point, primarily for resolution in annotation attribute values.
        // 2.如果beanFactory之前没有注册嵌入值解析器，则注册默认的嵌入值解析器：主要用于注解属性值的解析
        if (!beanFactory.hasEmbeddedValueResolver()) {
            beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
        }

        // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
        // 3.初始化LoadTimeWeaverAware Bean实例对象
        String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
        for (String weaverAwareName : weaverAwareNames) {
            getBean(weaverAwareName);
        }

        // Stop using the temporary ClassLoader for type matching.
        beanFactory.setTempClassLoader(null);

        // Allow for caching all bean definition metadata, not expecting further changes.
        // 4.冻结所有bean定义，注册的bean定义不会被修改或进一步后处理，因为马上要创建 Bean 实例对象了
        beanFactory.freezeConfiguration();

        // Instantiate all remaining (non-lazy-init) singletons.
        // 5.实例化所有剩余（非懒加载）单例对象
        beanFactory.preInstantiateSingletons();
    }

继续跟踪到DefaultListableBeanFactory#preInstantiateSingletons

    public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Pre-instantiating singletons in " + this);
        }

        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

        // Trigger initialization of all non-lazy singleton beans...
        //遍历beanNames，触发所有非懒加载单例bean的初始化
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            //Bean实例：不是抽象类 && 是单例 && 不是懒加载
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
              //判断beanName对应的bean是否为FactoryBean
                if (isFactoryBean(beanName)) {
                  //通过getBean(&beanName)拿到的是FactoryBean本身；通过getBean(beanName)拿到的是FactoryBean创建的Bean实例
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        //判断这个FactoryBean是否希望急切的初始化
                        boolean isEagerInit;
                        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                            ((SmartFactoryBean<?>) factory)::isEagerInit,
                                    getAccessControlContext());
                        }
                        else {
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean<?>) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            getBean(beanName);
                        }
                    }
                }
                else {
                  //如果beanName对应的bean不是FactoryBean，只是普通Bean，通过beanName获取bean实例
                    getBean(beanName);
                }
            }
        }

        ......
    }

要理解FactoryBean和Bean的区别

        一般情况下，Spring 通过反射机制利用 bean 的  class 属性指定实现类来实例化 bean。而 FactoryBean 是一种特殊的 bean，它是个工厂 bean，可以自己创建 bean 实例，如果一个类实现了 FactoryBean 接口，则该类可以自己定义创建实例对象的方法，只需要实现它的 getObject() 方法。

注：很多中间件都利用 FactoryBean 来进行扩展。

引入了几个重要的缓存：

• mergedBeanDefinitions 缓存：beanName -> 合并的 bean 定义。
• beanDefinitionMap 缓存：beanName -> BeanDefinition。
• singletonObjects 缓存：beanName -> 单例 bean 对象。
• earlySingletonObjects 缓存：beanName -> 单例 bean 对象，该缓存存放的是早期单例 bean 对象，可以理解成还未进行属性填充、初始化。
• singletonFactories 缓存：beanName -> ObjectFactory。
• singletonsCurrentlyInCreation 缓存：当前正在创建单例 bean 对象的 beanName 集合。

继承跟踪到AbstractBeanFactory#doGetBean创建bean

    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

        //解析beanName，主要是解析别名、去掉FactoryBean的前缀“&”
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        //尝试从缓存中获取beanName对应的实例，通过缓存解决循环依赖问题
        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 {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            // scope为prototype非单例的循环依赖校验：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);
            }

            // Check if bean definition exists in this factory.
            BeanFactory parentBeanFactory = getParentBeanFactory();
            //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.
                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);
                }
            }

            if (!typeCheckOnly) {
              //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 {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                //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，如使用@DependsOn
                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);
                        }
                    }
                }

                // Create bean instance.
                if (mbd.isSingleton()) {
                   //scope为singleton的bean创建（新建了一个ObjectFactory，并且重写了getObject方法，在里面创建bean
                    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;
                        }
                    });
                    //返回beanName对应的实例对象
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }

                //其它非单例的情况，暂不分析
                ......
            }
            catch (BeansException ex) {
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
        }

        ......
        return (T) bean;
    }

查看DefaultSingletonBeanRegistry#getSingleton

    public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "Bean name must not be null");
        synchronized (this.singletonObjects) {
            //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);
            if (singletonObject == null) {
                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 + "'");
                }
                //创建单例前的操作，把bean放入正准备创建的一个Set中singletonsCurrentlyInCreation，如果重复会报异常
                //如果存在构造器循环依赖的时候（A(B b),B(C c),C(A a)），会在这点报出异常
                beforeSingletonCreation(beanName);
                boolean newSingleton = false;
                boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = new LinkedHashSet<>();
                }
                try {
                    //执行singletonFactory的getObject方法获取bean实例，就是执行传入方法createBean
                    singletonObject = singletonFactory.getObject();
                    newSingleton = true;
                }
                
                ......
                
                finally {
                    if (recordSuppressedExceptions) {
                        this.suppressedExceptions = null;
                    }
                    //创建单例后的操作，在singletonsCurrentlyInCreation中移除
                    afterSingletonCreation(beanName);
                }
                if (newSingleton) {
                    addSingleton(beanName, singletonObject);
                }
            }
            return singletonObject;
        }
    }

继承跟踪AbstractAutowireCapableBeanFactory#createBean

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

        ......
        
        try {
            //验证及准备覆盖的方法（对override属性进行标记及验证）
            mbdToUse.prepareMethodOverrides();
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                    beanName, "Validation of method overrides failed", ex);
        }

        try {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            //实例化前的处理，如果有实现InstantiationAwareBeanPostProcessor的BeanPostProcessor可以直接返回真正的bean实例
            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 {
            //创建Bean实例（一般真正创建Bean的方法）
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            if (logger.isDebugEnabled()) {
                logger.debug("Finished creating instance of bean '" + beanName + "'");
            }
            return beanInstance;
        }
        ......
    }

实例化前的处理，给 InstantiationAwareBeanPostProcessor 一个机会返回代理对象来替代真正的 bean 实例，从而跳过 Spring 默认的实例化过程，达到“短路”效果。会执行 InstantiationAwareBeanPostProcessor 的 postProcessBeforeInstantiation 方法，该方法可以返回 bean 实例的代理，从而跳过 Spring 默认的实例化过程。

 查看AbstractAutowireCapableBeanFactory#doCreateBean，这个方法非常重要

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

        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
          //根据beanName、mbd、args，使用对应的策略创建Bean实例，并返回包装类BeanWrappe
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        // todo ！！！！！！！！这里获取出来的对象是原生对象！！！！！
        final Object bean = instanceWrapper.getWrappedInstance();
        Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            mbd.resolvedTargetType = beanType;
        }

        ......

        // Eagerly cache singletons to be able to resolve circular references
        // even when triggered by lifecycle interfaces like BeanFactoryAware.
        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");
            }
            //注意这点，曝光beanName的ObjectFactory，用于解决循环引用。 在开始通过doGetBean取值的时候调用了getSingleton就调用到这个工厂来了
            addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            //todo  对bean进行属性填充；其中，可能存在依赖于其他bean的属性，则会递归初始化依赖的bean实例
            //设置属性，非常重要，比如使用@Autowired注入的值等
            populateBean(beanName, mbd, instanceWrapper);
            
            // todo 经过AOP处理,原生对象转换成了代理对象,跟进去
            //执行后置处理器，aop就是在这里完成的处理
            exposedObject = initializeBean(beanName, exposedObject, mbd);
        }
        
        ......
        
        return exposedObject;
    }

实例化对象createBeanInstance，就是选出一个策略来实例化一个对象, 那有什么策略呢? 这就看程序员是怎么配置的了, 程序员可以配置工厂方法,指定构造方法,或者是程序员没有做出任何干涉,让Spring按自己的方式去实例化

参考：

https://www.cnblogs.com/ZhuChangwu/p/11755973.html

https://blog.csdn.net/v123411739/category_8589693.html