一次性講清楚spring中bean的生命週期之二：FactoryBean的前世今生

前言

在《spring中FactoryBean是什麼bean》一文中，帶着小夥伴學習了spring中的FactoryBean，瞭解了到了FactoryBean其實是一種生產Bean的bean，也就是FactroyBean的前世是Bean，今生還是Bean，小夥伴要疑惑了都是Bean，但是此Bean非彼Bean。今天帶着小夥伴從源碼的角度來分析下FactoryBean，重點是getObjectForBeanInstance方法的分析。

前世

在前面說到FactoryBean的前世是一個Bean，是指是一個FactoryBean的實例。先來看下getObjectForBeanInstance方法，

/**
     * Get the object for the given bean instance, either the bean
     * instance itself or its created object in case of a FactoryBean.
     */
    protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

        // Don't let calling code try to dereference the factory if the bean isn't a factory.
        //1、判斷name是否以&開頭
        if (BeanFactoryUtils.isFactoryDereference(name)) {
            if (beanInstance instanceof NullBean) {
                return beanInstance;
            }
            if (!(beanInstance instanceof FactoryBean)) {
                throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
            }
        }

        // 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.
         //beanInstance不是FactoryBean的實例或name以&開頭
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }

        Object object = null;
        //mbd即beanDefinition爲空，從緩存中取
        if (mbd == null) {
            object = getCachedObjectForFactoryBean(beanName);
        }
        //緩存中沒有，則調用FactoryBean的getObject方法，返回其對象
        if (object == null) {
            // 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;
    }

看該方法上的註釋，

Get the object for the given bean instance, either the bean instance itself or its created object in case of a FactoryBean.

用我蹩腳的英語翻譯過來大概是這個意思，

返回給定的bean Instance的一個對象，該對象可能是bean instance或者是由bean instance（是一個FactoryBean）創建的一個對象。

意思很明白了，該方法有可能返回的是一個FactoryBean的實例，也可能是由FactroyBean生產的實例，關鍵看方法參數中的前兩個，

Object beanInstance  spring容器中的一個bean

String name   可能含有&前綴的名稱

String beanName  bean的規範名稱

RootBeanDefintion mbd  BeanDefinition

看下面的表格更容易理解該方法在各種情況下的返回值，

 

beanInstance	name	返回值
FactoryBean的實例	帶有&	beanInstance
不是FactoryBean的實例	帶有&	beanInstance
FactoryBean的實例	不帶&	beanInstance生產的對象
不是FactoryBean的實例	不帶&	beanInstance

通過上面得表格再結合代碼就很容易理解，只要是返回beanInstance對象，那麼就是FactroyBean的前世，下面看FactoryBean的今生。

今生

這裏有兩個方法需要分析，分別是getCachedObjectForFactoryBean和getObjectFromFactoryBean。第一個方法是從緩存中獲取，也就是說使用FactoryBean生產的bean會單獨放在緩存中，非singletonObjects中，這點務必要注意。

getCachedObjectForFactoryBean

先看下該方法的定義，

@Nullable
    protected Object getCachedObjectForFactoryBean(String beanName) {
        return this.factoryBeanObjectCache.get(beanName);
    }

可以看到很簡單就是通過名稱從factoryBeanObjectCache中取對象。factoryBeanObjectCache肯定是個map了

/** Cache of singleton objects created by FactoryBeans: FactoryBean name to object. */
    private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<>(16);

getObjectFromFactoryBean

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
        if (factory.isSingleton() && containsSingleton(beanName)) {
            synchronized (getSingletonMutex()) {
                Object object = this.factoryBeanObjectCache.get(beanName);
                if (object == null) {
                    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 (shouldPostProcess) {
                            if (isSingletonCurrentlyInCreation(beanName)) {
                                // Temporarily return non-post-processed object, not storing it yet..
                                return object;
                            }
                            beforeSingletonCreation(beanName);
                            try {
                                object = postProcessObjectFromFactoryBean(object, beanName);
                            }
                            catch (Throwable ex) {
                                throw new BeanCreationException(beanName,
                                        "Post-processing of FactoryBean's singleton object failed", ex);
                            }
                            finally {
                                afterSingletonCreation(beanName);
                            }
                        }
                        //把生產的實例對象放到factoryBeanObjectCache緩存中
                        if (containsSingleton(beanName)) {
                            this.factoryBeanObjectCache.put(beanName, object);
                        }
                    }
                }
                return object;
            }
        }
        else {
            //調用getObject方法
            Object object = doGetObjectFromFactoryBean(factory, beanName);
            if (shouldPostProcess) {
                try {
                    object = postProcessObjectFromFactoryBean(object, beanName);
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
                }
            }
            return object;
        }
    }

該方法就比較複雜了，主要有doGetObjectFromFactoryBean、beforeSingletonCreation、postProcessObjectFromFactoryBean、afterSingletonCreation方法，重要的一個是doGetFromFactroyBean，也就是真正幹活生產bean的方法。其定義如下，其餘方法可自行查看

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((PrivilegedExceptionAction<Object>) factory::getObject, acc);
                }
                catch (PrivilegedActionException pae) {
                    throw pae.getException();
                }
            }
            else {
                //調用FactoryBean中的getObject方法，返回其實例對象
                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) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(
                        beanName, "FactoryBean which is currently in creation returned null from getObject");
            }
            object = new NullBean();
        }
        return object;
    }

在上文中的註釋部分已經看到最終調用了getObject方法，也就是返回的是FactoryBean中getObject方法的返回值。

總結

主要分析了FactoryBean的底層源碼，判斷是返回FactoryBean的實例還是返回其生產的實例，主要看bean的類型是否爲FactoryBean和名稱中是否帶&。

推薦：《spring中FactoryBean是什麼bean》

《一次性講清楚spring中bean的生命週期之一：getSingleton方法 》