bean的创建过程--doGetBean

protected <T> T doGetBean(String name, Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {
        //处理别名，提取对应的beanName
        final String beanName = this.transformedBeanName(name);
        /**
         * 检查缓存中或者实例工厂中是否有对应的实例
         * 为什么会首先使用这段代码呢？
         * 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的原则是不等bean创建完就会将创建bean的ObjectFactory提早曝光，也就是将ObjectFactory加入到缓存中，一旦下个bean创建时候需要依赖上个bean则直接使用ObjectFactory
         */
        //直接尝试从缓存获取或者singletonFactories中的ObjectFactory中获取
        Object sharedInstance = this.getSingleton(beanName);
        Object bean;
        if (sharedInstance != null && args == null) {
            if (this.logger.isDebugEnabled()) {
                if (this.isSingletonCurrentlyInCreation(beanName)) {
                    this.logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference");
                } else {
                    this.logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            //返回对应的实例，有时候存在诸如BeanFactory的情况并不是直接返回实例本身而是返回指定方法返回的实例
            bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, (RootBeanDefinition)null);
        } else {
            //只有在单例情况下才会尝试解决循环依赖，如果是原型模式下 则直接抛出如下异常
            if (this.isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            BeanFactory parentBeanFactory = this.getParentBeanFactory();
            //如果beanDefinitionMap中也就是所有已经加载的类中不包括beanname则尝试从parentBeanFactory中检测
            if (parentBeanFactory != null && !this.containsBeanDefinition(beanName)) {
                String nameToLookup = this.originalBeanName(name);
                //递归到BeanFactory中寻找
                if (args != null) {
                    return parentBeanFactory.getBean(nameToLookup, args);
                }

                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }

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

            try {
                final RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
                this.checkMergedBeanDefinition(mbd, beanName, args);
                String[] dependsOn = mbd.getDependsOn();
                String[] var11;
                //若存在依赖则需要递归实例化依赖的bean
                if (dependsOn != null) {
                    var11 = dependsOn;
                    int var12 = dependsOn.length;

                    for(int var13 = 0; var13 < var12; ++var13) {
                        String dependsOnBean = var11[var13];
                        if (this.isDependent(beanName, dependsOnBean)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
                        }
                        //缓存依赖调用
                        this.registerDependentBean(dependsOnBean, beanName);
                        this.getBean(dependsOnBean);
                    }
                }
                //实例化依赖的bean后便可以实例化mbd本身
                //singleton模式的创建
                if (mbd.isSingleton()) {
                    sharedInstance = this.getSingleton(beanName, new ObjectFactory<Object>() {
                        public Object getObject() throws BeansException {
                            try {
                                return AbstractBeanFactory.this.createBean(beanName, mbd, args);
                            } catch (BeansException var2) {
                                AbstractBeanFactory.this.destroySingleton(beanName);
                                throw var2;
                            }
                        }
                    });
                    bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                } else if (mbd.isPrototype()) {
                    //protoType模式的创建
                    var11 = null;

                    Object prototypeInstance;
                    try {
                        this.beforePrototypeCreation(beanName);
                        prototypeInstance = this.createBean(beanName, mbd, args);
                    } finally {
                        this.afterPrototypeCreation(beanName);
                    }

                    bean = this.getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                } else {
                    //指定的scope上实例化bean
                    String scopeName = mbd.getScope();
                    Scope scope = (Scope)this.scopes.get(scopeName);
                    if (scope == null) {
                        throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
                    }

                    try {
                        Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                            public Object getObject() throws BeansException {
                                AbstractBeanFactory.this.beforePrototypeCreation(beanName);

                                Object var1;
                                try {
                                    var1 = AbstractBeanFactory.this.createBean(beanName, mbd, args);
                                } finally {
                                    AbstractBeanFactory.this.afterPrototypeCreation(beanName);
                                }

                                return var1;
                            }
                        });
                        bean = this.getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    } catch (IllegalStateException var21) {
                        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", var21);
                    }
                }
            } catch (BeansException var23) {
                this.cleanupAfterBeanCreationFailure(beanName);
                throw var23;
            }
        }
        //检查需要的类型是否符合bean的实际类型
        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            try {
                return this.getTypeConverter().convertIfNecessary(bean, requiredType);
            } catch (TypeMismatchException var22) {
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Failed to convert bean '" + name + "' to required type [" + ClassUtils.getQualifiedName(requiredType) + "]", var22);
                }

                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        } else {
            return bean;
        }
    }

从上述代码可以看出bean的加载经历了一个复杂的过程，以上仅能粗略的了解整个Spring加载bean的过程。加载的步骤大致如下

1：转换对应的beanName

因为传入的参数不仅是beanName也可能是别名，也可能是FactoryBean，所以需要解析：去除FactoryBean的修饰符，也就是如果name="&aa"，那么会首先去除&而使name="aa"。

取指定alias所表示的最终beanName，例如别名A指向名称B的bean则返回B;

2：尝试从缓存中加载单例

单例在spring容器内只会创建一次，后续获取都是从单例缓存中获取。如果获取不到则从singletonFactories中加载。因为在创建单例bean的时候会存在依赖注入的情况，而在创建的时候为了

避免循环依赖，在Spring中创建bean的原则是不等bean创建完就会将创建bean的ObjectFactory提早曝光，也就是将ObjectFactory加入到缓存中，一旦下个bean创建时候需要依赖上个bean则直接使用ObjectFactory

3：bean的实例化

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。

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

5：加测parentBeanFactory

如果缓存中没有取到则直接转到父类工厂上加载。

6：将存储xml配置文件的GernericBeanDefinition转换为RootBeanDefinition

7：寻找依赖

8：针对不同的scope进行bean的创建

9：类型转换

缓存池获取bean实例

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    //一级缓存：存放的是已经完成实例化，属性填充和初始化步骤的单例bean实例
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && this.isSingletonCurrentlyInCreation(beanName)) {
        synchronized(this.singletonObjects) {
            //二级缓存：存放的是提前暴露的单例bean实例，可能是代理对象，也可能是未经代理的原对象，但都还没有完成初始化的步骤
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                //三级缓存：存放的是ObjectFactory的匿名内部类实例
                ObjectFactory<?> singletonFactory = (ObjectFactory)this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    //回调ObjectFactory#getObject方法获取提前暴露的引用
                    singletonObject = singletonFactory.getObject();
                    //移入二级缓存
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    //移除三级缓存
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject != NULL_OBJECT ? singletonObject : null;
}