spring-getBean方法流程

流程圖

（1）呼叫AbstractBeanFactory的doGetBean方法
（2）轉化beanName，如果包含&，刪除之
（3）根據beanName從快取中獲取Bean
（4）Bean 是否 不為null並且args為null
（5）呼叫getObjectForBeanInstance方法
（6） 判斷BeanInstance是否為FactoryBean型別並且不以name為&開頭
（7）判斷BeanInstance不是FactoryBean型別的或者是FactoryBean型別的並且以&開頭
（8）從FactoryBean生成的例項快取中獲取obean
（9）判斷bean是否為null
（10）根據FactoryBean的getObject方法返回Bean
（11）檢查BeanFactory是否包含此Bean定義,如果不包含，繼續自己的父BeanFactory查詢
（12）獲取bean的依賴項
（13）呼叫getObjectForBeanInstance方法
（14）丟擲異常
（15）返回Bean

程式碼註釋

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

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

        // 從快取中獲取BeanInstance，此BeanInstance在我看來，這才是真正建立Bean例項的工廠，根據不同的情況這個Bean例項做處理，或直接返回，如果是FactoryBean並且並且那麼沒有以&開頭則呼叫其方法getObject（）返回例項
 

        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 + "'");
                }
            }
            //真正的bean 返回處理
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // 假設迴圈依賴的時候，如果我們已經開始在當前執行緒中建立此bean例項，但是還沒有建立完成，則失敗；例如此時是A依賴於B，但是B不依賴於A，B也沒有任何屬性依賴於A，則不存在迴圈依賴，那麼無論B初始化，未初始化都不會有以下情況。但是如果B依賴於A，A在獲取依賴的Bean是啟用建立B的方法，那麼B建立過程中就會出現以下情況。就會出現迴圈依賴錯誤。如果A，B 是單例的並且A的建構函式不包含B，B的建構函式不包含A，spring還是可以通過提前暴露例項地址處理這種依賴，但是其它情況spring也無能為力了。迴圈依賴也是一個大問題。。。
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 判斷工廠中是否含有此Bean的定義
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // 如果沒有，查詢父工廠
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // 執行帶有args引數的getbean方法
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    //如果沒有引數，執行標準的getbean方法 
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

            //如果不僅僅是做型別檢查，那麼需要標記此Bean正在建立之中
            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 獲取依賴的Bean
                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);
                    }
                }

                // 這裡終於開始建立Bean例項了，如果是單例的，那麼會建立一個單例的匿名工廠，如果是原型模式的，則不需要建立單例的工廠的，其他的如request、session作用域的，則根據自身的需要
                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 {
                    //把當前的beanName加入正在建立beanName的集合中，會在第一步判斷
                        beforePrototypeCreation(beanName);
                        prototypeInstance = createBean(beanName, mbd, args);
                    }
                    finally {
                    //把當前的beanName從正在建立beanName的集合中移除
                        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;
            }
        }

        // 檢查型別，如果不滿足可以進行型別轉換，如果轉換器進行不了這兩種型別的轉換，則丟擲異常
        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;
    }