Spring Bean的生命周期之我的理解（三）------BeanFactory和FactoryBean

前言

大家可能使用Spring框架已经很久了，但是对其中的一些概念可能会比较模糊，比如BeanFactory和FactoryBean，诈一看，两个没啥区别，仔细研究后发现，其实本质上是两个不同的单元。

概念

BeanFactory

字面意思讲就是Bean的工厂，是Spring重要的IOC容器，维护着整个Bean的生命周期。

FactoryBean

字面意思就是个工厂Bean，属于Spring Bean的一种，是一种特殊的Bean。可以借助工厂Bean得到某一个特殊Bean的实例。

源码

说到获取某一个Bean的实例，不得不说的一个比较重要的类《AbstractBeanFactory》，所有的获取Bean的方法都会由此类的doGetBean方法所接管。

    protected <T> T doGetBean(            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)            throws BeansException {        // 没细看这个方法，感觉没大用        final String beanName = transformedBeanName(name);        Object bean;        // Eagerly check singleton cache for manually registered singletons.        // 从正在创建的单例类型Bean缓存中查看是否已经存在        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 + "'");                }            }            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);        }        else {            // Fail if we're already creating this bean instance:            // We're assumably within a circular reference.            // 从正在创建的原型类型Bean缓存中查看是否已经存在            if (isPrototypeCurrentlyInCreation(beanName)) {                throw new BeanCurrentlyInCreationException(beanName);            }            // Check if bean definition exists in this factory.            // 查看工厂中是否已经存在该Bean的定义            BeanFactory parentBeanFactory = getParentBeanFactory();            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {                // Not found -> check parent.                String nameToLookup = originalBeanName(name);                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) {                // 标记Bean的状态为已经创建                markBeanAsCreated(beanName);            }            try {                // 如果有父Bean定义，则做一次合并操作                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);                // 对合并后的Bean定义再做一次检查                checkMergedBeanDefinition(mbd, beanName, args);                // Guarantee initialization of beans that the current bean depends on.                // 检测Bean的依赖关系，优先初始化依赖的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);                        getBean(dep);                    }                }                // Create bean instance.                // 创建单例类型Bean的实例，也就是如果当前的Bean是单例类型的，执行此逻辑                if (mbd.isSingleton()) {                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {                        @Override                        public Object getObject() throws BeansException {                            try {                                return createBean(beanName, mbd, args);                            }                            catch (BeansException ex) {                                // Explicitly remove instance from singleton cache: It might have been put there                                // eagerly by the creation process, to allow for circular reference resolution.                                // Also remove any beans that received a temporary reference to the bean.                                destroySingleton(beanName);                                throw ex;                            }                        }                    });                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);                }                // 创建原型类型的Bean的实例                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);                }                // 创建作用域类型的Bean的实例                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, new ObjectFactory<Object>() {                            @Override                            public Object getObject() throws BeansException {                                beforePrototypeCreation(beanName);                                try {                                    return createBean(beanName, mbd, args);                                }                                finally {                                    afterPrototypeCreation(beanName);                                }                            }                        });                        bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);                    }                    catch (IllegalStateException ex) {                        throw new BeanCreationException(beanName,                                "Scope '" + scopeName + "' is not active for the current thread; consider " +                                "defining a scoped proxy for this bean if you intend to refer to it from a singleton",                                ex);                    }                }            }            catch (BeansException ex) {                cleanupAfterBeanCreationFailure(beanName);                throw ex;            }        }        // Check if required type matches the type of the actual bean instance.        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {            try {                return getTypeConverter().convertIfNecessary(bean, requiredType);            }            catch (TypeMismatchException ex) {                if (logger.isDebugEnabled()) {                    logger.debug("Failed to convert bean '" + name + "' to required type '" +                            ClassUtils.getQualifiedName(requiredType) + "'", ex);                }                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());            }        }        return (T) bean;    }

误区

我曾经误认为BeanFactory会调用FactoryBean的实现来获得Bean的实例，其实是不正确的，只有特殊的实现了FactoryBean接口的Bean的实例才会从FactoryBean的实现中获取，其他的时候都是通过AbstractAutowireCapableBeanFactory的createBean方法获取。

待续

下一节我将分析AbstractAutowireCapableBeanFactory源码的createBean方法。