Spring bean加载2--FactoryBean情况处理

文章由LinuxBoy分享于2019-04-01 11:04:57热评（421）

Spring bean加载2--FactoryBean情况处理

在Spring bean加载过程中,每次bean实例在返回前都会调用getObjectForBeanInstance来处理FactoryBean的情况.
这边的FactoryBean,Spring设计用于新建复杂bean的,联想下GOF设计模式的创建型,一样的为了解决复杂的bean实例化过程.
其实这边的FactoryBean就是一个factory method[gof定义的意图:定义一个用于创建对象的接口,让子类决定实例化哪个类.Factory Method使一个类的实例化延迟到其子类].

这边的处理主要涉及3个方法,我们来看看3个方法各自的职责:

getObjectForBeanInstance: 参数校验之类的准备工作
getObjectFromFactoryBean: 单例时,确保实例是全局唯一的
doGetObjectFromFactoryBean: 实实在在的实例化

参数校验之类的准备工作

AbstractBeanFactory的getObjectForBeanInstance处理.

如果name制定要获取FactoryBean本身实例,而beanInstance却又不是FactoryBean,直接抛异常
如果不需要调用getObject实例化,直接返回实例
尝试从缓存中获取实例[FactoryBeanRegistrySupport负责]
准备beanDefinition,委托getObjectFromFactoryBean处理

单例时保障实例全局唯一

FactoryBeanRegistrySupport的getObjectFromFactoryBean处理
FactoryBeanRegistrySupport负责FactoryBean相关的操作,并缓存FactoryBean的getObject实例化的bean.

判断factory是单例,同时已经new好了(后面需要调用getObject获取目标对象,这边需要factory已经实例化)
单例时,先尝试去缓存找;如果找不到或者不是单例,委托doGetObjectFromFactoryBean实例化一个
由于这样新建就没有机会调用BeanPostProcessor了,所以这边直接调用其postProcessAfterInitialization[职责职责职责,AbstractAutowireCapableBeanFactory干的]
缓冲得到的实例

实例化,干活的

FactoryBeanRegistrySupport的doGetObjectFromFactoryBean处理
这边做的事很少,就是调用factoryBean的getObject,然后如果实例化得到的对象为空抛异常.

源码摘要

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 实例的对象,可能是实例本身,也可能是FactoryBean新建的对象 * @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) {

    // 如果想要获取FactoryBean本身,那么beanInstance必须是FactoryBean的实例
    // 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实例,或者想要获取的就是FactoryBean实例,那么直接返回就好
    // 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) {
        // 获取缓存的实例
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        // 缓存中没有对象,那么从头准备bean defition实例化一个
        // 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. * 获取缓存的,通过FactoryBean暴露出来的对象 * @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);
    // 类似<重构>中Introduce NUll Object,只是这边的NULL_OBJECT是Object类型的,没有解决Martin说的"不需要询问对象类型,就可以直接调用行为的方法"
    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是单例的同时,还得已经被实例化(感觉这个检验怪怪的,没想明白)
    if (factory.isSingleton() && containsSingleton(beanName)) {
        synchronized (getSingletonMutex()) {
            Object object = this.factoryBeanObjectCache.get(beanName);
            if (object == null) {
                // 缓存中找不到,就自己干吧,实例化
                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,没有作用在实际想要的实例上,这边补一个
                            // 也就是说,BeanPostProcessor的postProcessBeforeInitialization不会作用在FactoryBean上
                            object = postProcessObjectFromFactoryBean(object, beanName);
                        }
                        catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                    "Post-processing of FactoryBean's singleton object failed", ex);
                        }
                    }
                    // 缓存
                    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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