spring bean的加载

一般调用方式是

MyJavaBean bean = (MyJavaBean) bf.getBean("myJavaBean");

首先获取BeanFactory（DefaultListableBeanFactory类），然后调用AbstractBeanFactory类中的getBean方法

@Override
    public Object getBean(String name) throws BeansException {
        assertBeanFactoryActive();
        return getBeanFactory().getBean(name);
    }

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

真正的调用在doGetBean中

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

//获取beanName 主要为处理别名和&beanName这两种形式

        final String beanName = transformedBeanName(name);
        Object bean;

/**     
分别从singletonObjects(beanName->beanName instance)，earlySingletonObjects(beanName->beanName instance)主要用于检测循环引用，可以在bean还在创建的过程中提前获取bean，singletonFactories(beanName->objectFactory)三个缓存中获取bean
*/  
        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 + "'");
                }
            }
/**
如果通过缓存获取到了bean的实例，还需要对该实例进行验证
因为不管是从缓存中获取到的bean还是我们通过不同的scope策略加载的bean，都是bean的原始形式，不一定是我们最终需要的bean
例如我们定义一个工厂bean，那么此时获取的是工厂bean本身，但实际上我们需要的是bean中定义的factory－bean方法返回的bean
*/
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {

//如果没有从缓存中获取bean实例，首先检查是否存在循环引用，只有在单例情况下才尝试解决循环引用

            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

//若已经加载的BeanDefinition中不包含该beanName，则从（父容器）parentBeanFactory中检测，若存在，则调用parentBeanFactory加载该bean

            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);
                }
            }

//如果不仅仅做类型检测则是要创建bean，在这里做记录,并清除mergedBeanDefinitions中的缓存

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
/**         
(1)验证beanName对应的BeanDefinition是否存在 若不存在直接抛异常
(2)若该bean是子bean则要合并父类的相关属性
(3)合并后要验证mergedBeanDefinition，该bean不能为抽象bean
*/
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

//若存在依赖则递归实例化依赖的bean，该依赖由depends-on属性引入

                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dependsOnBean : dependsOn) {
                        if (isDependent(beanName, dependsOnBean)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
                        }
                        registerDependentBean(dependsOnBean, beanName);
                        getBean(dependsOnBean);
                    }
                }

//根据不同的scope创建bean的实例

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

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

                else {
                    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;
            }
        }

//检查获取的bean实例是否符合bean的实际类型

        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            try {
                return getTypeConverter().convertIfNecessary(bean, requiredType);
            }
            catch (TypeMismatchException ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Failed to convert bean '" + name + "' to required type [" +
                            ClassUtils.getQualifiedName(requiredType) + "]", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        return (T) bean;
    }