Spring bean加載2--FactoryBean情況處理

在Spring bean加載過程中,每次bean實例在返回前都會調用getObjectForBeanInstance來處理FactoryBean的情況.
這邊的FactoryBean,Spring設計用於新建復雜bean的,聯想下GOF設計模式的創建型,一樣的為了解決復雜的bean實例化過程.
其實這邊的FactoryBean就是一個factory method[gof定義的意圖:定義一個用於創建對象的接口,讓子類決定實例化哪個類.Factory Method使一個類的實例化延遲到其子類].

這邊的處理主要涉及3個方法,我們來看看3個方法各自的職責:

1. getObjectForBeanInstance: 參數校驗之類的准備工作
2. getObjectFromFactoryBean: 單例時,確保實例是全局唯一的
3. doGetObjectFromFactoryBean: 實實在在的實例化

參數校驗之類的准備工作

AbstractBeanFactory的getObjectForBeanInstance處理.

1. 如果name制定要獲取FactoryBean本身實例,而beanInstance卻又不是FactoryBean,直接拋異常
2. 如果不需要調用getObject實例化,直接返回實例
3. 嘗試從緩存中獲取實例[FactoryBeanRegistrySupport負責]
4. 准備beanDefinition,委托getObjectFromFactoryBean處理

單例時保障實例全局唯一

FactoryBeanRegistrySupport的getObjectFromFactoryBean處理
FactoryBeanRegistrySupport負責FactoryBean相關的操作,並緩存FactoryBean的getObject實例化的bean.

1. 判斷factory是單例,同時已經new好了(後面需要調用getObject獲取目標對象,這邊需要factory已經實例化)
2. 單例時,先嘗試去緩存找;如果找不到或者不是單例,委托doGetObjectFromFactoryBean實例化一個
3. 由於這樣新建就沒有機會調用BeanPostProcessor了,所以這邊直接調用其postProcessAfterInitialization[職責職責職責,AbstractAutowireCapableBeanFactory干的]
4. 緩沖得到的實例

實例化,干活的

FactoryBeanRegistrySupport的doGetObjectFromFactoryBean處理
這邊做的事很少,就是調用factoryBean的getObject,然後如果實例化得到的對象為空拋異常.

源碼摘要

AbstractBeanFactory

/** * Get the object for the given bean instance, either the bean * instance itself or its created object in case of a FactoryBean. * 返回bean 實例的對象,可能是實例本身,也可能是FactoryBean新建的對象 * @param beanInstance the shared bean instance * @param name name that may include factory dereference prefix * @param beanName the canonical bean name * @param mbd the merged bean definition * @return the object to expose for the bean */
protected Object getObjectForBeanInstance(
        Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {

    // 如果想要獲取FactoryBean本身,那麼beanInstance必須是FactoryBean的實例
    // Don't let calling code try to dereference the factory if the bean isn't a factory.
    if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
        throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
    }

    // 如果instance不是FactoryBean實例,或者想要獲取的就是FactoryBean實例,那麼直接返回就好
    // Now we have the bean instance, which may be a normal bean or a FactoryBean.
    // If it's a FactoryBean, we use it to create a bean instance, unless the
    // caller actually wants a reference to the factory.
    if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
        return beanInstance;
    }

    Object object = null;
    if (mbd == null) {
        // 獲取緩存的實例
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        // 緩存中沒有對象,那麼從頭准備bean defition實例化一個
        // Return bean instance from factory.
        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
        // Caches object obtained from FactoryBean if it is a singleton.
        if (mbd == null && containsBeanDefinition(beanName)) {
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

FactoryBeanRegistrySupport

/** * Obtain an object to expose from the given FactoryBean, if available * in cached form. Quick check for minimal synchronization. * 獲取緩存的,通過FactoryBean暴露出來的對象 * @param beanName the name of the bean * @return the object obtained from the FactoryBean, * or {@code null} if not available */
protected Object getCachedObjectForFactoryBean(String beanName) {
    Object object = this.factoryBeanObjectCache.get(beanName);
    // 類似<重構>中Introduce NUll Object,只是這邊的NULL_OBJECT是Object類型的,沒有解決Martin說的"不需要詢問對象類型,就可以直接調用行為的方法"
    return (object != NULL_OBJECT ? object : null);
}

/** * Obtain an object to expose from the given FactoryBean. * @param factory the FactoryBean instance * @param beanName the name of the bean * @param shouldPostProcess whether the bean is subject to post-processing * @return the object obtained from the FactoryBean * @throws BeanCreationException if FactoryBean object creation failed * @see org.springframework.beans.factory.FactoryBean#getObject() */
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
    // factory是單例的同時,還得已經被實例化(感覺這個檢驗怪怪的,沒想明白)
    if (factory.isSingleton() && containsSingleton(beanName)) {
        synchronized (getSingletonMutex()) {
            Object object = this.factoryBeanObjectCache.get(beanName);
            if (object == null) {
                // 緩存中找不到,就自己干吧,實例化
                object = doGetObjectFromFactoryBean(factory, beanName);
                // Only post-process and store if not put there already during getObject() call above
                // (e.g. because of circular reference processing triggered by custom getBean calls)
                Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
                if (alreadyThere != null) {
                    object = alreadyThere;
                }
                else {
                    if (object != null && shouldPostProcess) {
                        try {
                            // 外面BeanPostProcessor作用在factory上,沒有作用在實際想要的實例上,這邊補一個
                            // 也就是說,BeanPostProcessor的postProcessBeforeInitialization不會作用在FactoryBean上
                            object = postProcessObjectFromFactoryBean(object, beanName);
                        }
                        catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                    "Post-processing of FactoryBean's singleton object failed", ex);
                        }
                    }
                    // 緩存
                    this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
                }
            }
            return (object != NULL_OBJECT ? object : null);
        }
    }
    else {
        Object object = doGetObjectFromFactoryBean(factory, beanName);
        if (object != null && shouldPostProcess) {
            try {
                object = postProcessObjectFromFactoryBean(object, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
            }
        }
        return object;
    }
}

/** * Obtain an object to expose from the given FactoryBean. * @param factory the FactoryBean instance * @param beanName the name of the bean * @return the object obtained from the FactoryBean * @throws BeanCreationException if FactoryBean object creation failed * @see org.springframework.beans.factory.FactoryBean#getObject() */
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
        throws BeanCreationException {

    Object object;
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                    @Override
                    public Object run() throws Exception {
                            return factory.getObject();
                        }
                    }, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            object = factory.getObject();
        }
    }
    catch (FactoryBeanNotInitializedException ex) {
        throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    }

    // Do not accept a null value for a FactoryBean that's not fully
    // initialized yet: Many FactoryBeans just return null then.
    if (object == null && isSingletonCurrentlyInCreation(beanName)) {
        throw new BeanCurrentlyInCreationException(
                beanName, "FactoryBean which is currently in creation returned null from getObject");
    }
    return object;
}
/** * Post-process the given object that has been obtained from the FactoryBean. * The resulting object will get exposed for bean references. * <p>The default implementation simply returns the given object as-is. * Subclasses may override this, for example, to apply post-processors. * @param object the object obtained from the FactoryBean. * @param beanName the name of the bean * @return the object to expose * @throws org.springframework.beans.BeansException if any post-processing failed */
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) throws BeansException {
    return object;
}

AbstractAutowireCapableBeanFactory

/** * Applies the {@code postProcessAfterInitialization} callback of all * registered BeanPostProcessors, giving them a chance to post-process the * object obtained from FactoryBeans (for example, to auto-proxy them). * @see #applyBeanPostProcessorsAfterInitialization */
@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
    return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessAfterInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;
}

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