getBean方法會呼叫到AbstractBeanFactory的doGetBean()方法中去。下面看看AbstractBeanFactory 的類圖結構

　一、AbstractBeanFactory doGetBean()方法

　　這個程式碼塊比較主要點在getSingleton()方法上

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

    ...
    // 1.嘗試獲取單例物件
    Object sharedInstance = getSingleton(beanName);
    ...

    // 2.建立物件並獲取單例物件
    if (mbd.isSingleton()) {
        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;
            }
        });
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
    }

    ...

    return (T) bean;
}

　　下面是第1步中getSingleton()的具體程式碼

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    //嘗試從快取C中獲取單例物件
    Object singletonObject = this.singletonObjects.get(beanName);
    //為獲取到並且物件在建立中
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            //從早期快取A中獲取物件
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    //獲取物件
                    singletonObject = singletonFactory.getObject();
                    //放入早期快取A
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    //清除該物件的單例工廠快取B
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

　　下面是第2步中getSingleton()的具體程式碼

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    synchronized (this.singletonObjects) {
        //從快取C中獲取
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            //標記當前物件正在建立
            beforeSingletonCreation(beanName);
            ...
            //呼叫實現類的createBean方法
            singletonObject = singletonFactory.getObject();
            newSingleton = true;
            
            ...
            //如果是單例則做對應的快取操作
            if (newSingleton) {
                addSingleton(beanName, singletonObject);
            }
            //清除當前正在建立的快取
            afterSingletonCreation(beanName);
        }
        return singletonObject;
    }
}

protected void addSingleton(String beanName, Object singletonObject) {
    synchronized (this.singletonObjects) {
        //快取C
        this.singletonObjects.put(beanName, singletonObject);
        //清除快取B
        this.singletonFactories.remove(beanName);
        //清除快取A
        this.earlySingletonObjects.remove(beanName);
        this.registeredSingletons.add(beanName);
    }
}

二、AbstractBeanFactory createBean()方法，建立SpringBean物件的實現

　　createBean方法是一個抽象方法，由實現類實現，下面是實現類的結構

　　這個實現類createBean方法實現如下，首先獲取Class物件，再處理InstantiationAwareBeanPostProcessor介面的實現，然後建立Bean

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
        throws BeanCreationException {
    
    RootBeanDefinition mbdToUse = mbd;
    //獲得Bean的Class物件
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    //執行 InstantiationAwareBeanPostProcessor 介面的實現
    Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
    if (bean != null) {
        return bean;
    }
    //建立Bean
    Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    if (logger.isTraceEnabled()) {
        logger.trace("Finished creating instance of bean '" + beanName + "'");
    }
    return beanInstance;
    
}

　　　doCreateBean方法包含了，通過反射建立物件，填充物件成員變數等操作