BeanDefinition到Bean

当 BeanDefinition 注册完毕以后, Spring Bean 工厂就可以随时根据需要进行实例化了。对于 XmlBeanFactory 来说,实例化默认是延迟进行的,也就是说在 getBean 的时候才会;而对于 ApplicationContext 来说,实例化会在容器启动后通过 AbstractApplicationContext 中 reflash 方法自动进行,主要经过方法链: reflesh() à finishBeanFactoryInitialization (factory) à DefaultListableBeanFactory.preInstantiateSingletons (), 在这里会根据注册的 BeanDefinition 信息依此调用 getBean(beanName) 。而真正实例化的逻辑和 BeanFactory 是“殊途同归”的,所有有关 Bean 实例化都可以从 getBean(beanName) 入手。

  AbstractBeanFactory 有四个 getBean 的重载方法,不管调用哪个方法最终都是会调用另一个 doGetBean 方法: 

java代码:

    public Object getBean(String name) throws BeansException {  
            return getBean(name, null, null);  
        }  

        public Object getBean(String name, Class requiredType) throws BeansException {  
            return getBean(name, requiredType, null);  
        }  

        public Object getBean(String name, Object[] args) throws BeansException {  
            return getBean(name, null, args);  
        }  

        /** 
         * Return an instance, which may be shared or independent, of the specified bean. 
         * @param name the name of the bean to retrieve 
         * @param requiredType the required type of the bean to retrieve 
         * @param args arguments to use if creating a prototype using explicit arguments to a 
         * static factory method. It is invalid to use a non-null args value in any other case. 
         * @return an instance of the bean 
         * @throws BeansException if the bean could not be created 
         */  
        public Object getBean(String name, Class requiredType, Object[] args) throws BeansException {  
            return doGetBean(name, requiredType, args, false);  
        }  

doGetBean方法比较长一点,见下面注释:

Java代码

    protected Object doGetBean(  
    final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)   
    throws BeansException {  
    //bean name处理,去除FactoryBean前缀等  
         final String beanName = transformedBeanName(name);  
         Object bean = null;  

    //先从singleton缓存中查看是否已经实例化过该Bean,根据是否有缓存分为两个分支分别处理  
        Object sharedInstance = getSingleton(beanName);  
        if (sharedInstance != null && args == null) {  
    // 分支一,若缓存中获取到了并且该BeanDefinition信息表明该bean是singleton的,直接将获取到的缓存Bean  
    //(有可能是半成品)交给getObjectForBeanInstance处理  
     /*.........省略logger部分代码............*/  
    //调用getObjectForBeanInstance处理  
         bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);  
        }else {  
    // 分之二:没有缓存,则需要从头实例化该bean  
                // We're assumably within a circular reference.  
          if (isPrototypeCurrentlyInCreation(beanName)) {   
               throw new BeanCurrentlyInCreationException(beanName);}  

    // 检查BeanDefinition是否在当前工厂或父工厂  
                BeanFactory parentBeanFactory = getParentBeanFactory();  
                if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {  
                    // Not found -> check parent.  
                    String nameToLookup = originalBeanName(name);  
                    if (args != null) {  
    // 父工厂getBean  
                        return parentBeanFactory.getBean(nameToLookup, args);  
                    }  
                    else {  
                        // No args -> delegate to standard getBean method.  
                        return parentBeanFactory.getBean(nameToLookup, requiredType);  
                    }  
                }  
    //将bean加入“正在创建”的集合,完成后会remove,对应afterSingletonCreation/afterPrototypeCreation方法  
                if (!typeCheckOnly) {  
                    markBeanAsCreated(beanName);  
                }  

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

    // 解决依赖关系,将依赖的bean提前实例化  
                String[] dependsOn = mbd.getDependsOn();  
                if (dependsOn != null) {  
                    for (int i = 0; i < dependsOn.length; i++) {  
                        String dependsOnBean = dependsOn[i];  
                        getBean(dependsOnBean);  
                        registerDependentBean(dependsOnBean, beanName);  
                    }  
                }  

    // 这里又需要根据bean的类型分为三种情况:singleton、prototype、request/session  
                if (mbd.isSingleton()) {  
                               //通过自定义ObjectFactory实例化Bean,此结果可能是半成品(是FactoryBean等)  
                    sharedInstance = getSingleton(beanName, new ObjectFactory() {  
                        public Object getObject() throws BeansException {  
                            try {  
                              //真正实例化装配的逻辑在createBean方法中  
                                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;  
                            }  
                        }  
                    });  
                            //上一步半成品的Bean交给getObjectForBeanInstance方法处理  
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);  
                }  

                else if (mbd.isPrototype()) {  
                    // It's a prototype -> create a new instance.  
                    Object prototypeInstance = null;  
                    try {  
                        beforePrototypeCreation(beanName);  
                         //真正实例化装配的逻辑在createBean方法中  
                        prototypeInstance = createBean(beanName, mbd, args);  
                    }  
                    finally {  
                        afterPrototypeCreation(beanName);  
                    }  
                        //上一步半成品的Bean交给getObjectForBeanInstance方法处理  
                   bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);  
                }  

                else {  
                                //request、session 的bean  
                    String scopeName = mbd.getScope();  
                    final Scope scope = (Scope) this.scopes.get(scopeName);  
                    if (scope == null) {  
            throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");  
                    }  
                    try {  
                        Object scopedInstance = scope.get(beanName, new ObjectFactory() {  
                            public Object getObject() throws BeansException {  
                                beforePrototypeCreation(beanName);  
                                try {  
                             //真正实例化装配的逻辑在createBean方法中  
                                    return createBean(beanName, mbd, args);  
                                }  
                                finally {  
                                    afterPrototypeCreation(beanName);  
                                }  
                            }  
                        });  
                           //上一步半成品的Bean交给getObjectForBeanInstance方法处理  
                    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);  
                    }  
                }  
            }  

            // Check if required type matches the type of the actual bean instance.  
            if (requiredType != null && bean != null &&  
                                  !requiredType.isAssignableFrom(bean.getClass())) {  
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());  
            }  
            return bean;  
        }  

通过注释,可以整理出较清晰的逻辑: 检查Bean缓存,已经有缓存的Bean对象(有可能是半成品)则交给getObjectForBeanInstance方法处理;否则先根据Bean的生命周期类型分别实例化,每种情况大致都分两步,第一步都交给createBean方法生产一个半成品的bean对象,然后同样是将半成品的bean交给getObjectForBeanInstance方法处理。所以关键的逻辑就在这两个方法了,下面以singleton为例看看这两个方法,具体代码:

Java代码

    sharedInstance = getSingleton(beanName, new ObjectFactory() {  
                        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);  

getSingleton方法做的工作主要是实例化bean之前和之后回调beforeSingletonCreation/afterSingletonCreation、实例化bean、以及将bean对象缓存起来,具体实例化bean是通过回调匿名对象ObjectFactory的getObject方法实现的,从代码中明显看到主要是createBean方法。这里要特别注意下,缓存的bean对象是createBean生产的,这个方法生产的bean只是“半成品”,有可能是个factoryBean,真正返回给客户端使用的bean还必须进行下一步getObjectBeanInstance处理。所以缓存的bean对象可以认为是“半成品”,这就和前文代码中的注释相呼应(缓存中若取到了bean,必须进行getObjectBeanInstance处理)。

 对createBean方法实际上在“Spring IOC之BeanFactory”中已经有过分析,所以流程就不再简单重复,直接按步骤:
  1. resolveBeanClass(mbd, beanName);

  2. mbd.prepareMethodOverrides();//Spring IOC之BeanFactory已有解释,不重复

  3. Object bean = resolveBeforeInstantiation(beanName, mbd);

    解释下,这里主要对于一些代理的bean做处理,我们平常经常会配置一些AOP模块,对于需要aop增强的bean实际上都会经过spring代理织入,而这些bean的代理逻辑应该就在这里处理了,具体看看这部分的代码:

Java代码

    /** 
         * Apply before-instantiation post-processors, resolving whether there is a 
         * before-instantiation shortcut for the specified bean. 
         * @param beanName the name of the bean 
         * @param mbd the bean definition for the bean 
         * @return the shortcut-determined bean instance, or null if none 
         */  
        protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {  
            Object bean = null;  
            if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {  
                // Make sure bean class is actually resolved at this point.  
                if (mbd.hasBeanClass() && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {  
                    bean = applyBeanPostProcessorsBeforeInstantiation(mbd.getBeanClass(), beanName);  
                    if (bean != null) {  
                        bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);  
                    }  
                }  
                mbd.beforeInstantiationResolved = Boolean.valueOf(bean != null);  
            }  
            return bean;  
        }  

    /** 
         * Apply InstantiationAwareBeanPostProcessors to the specified bean definition 
         * (by class and name), invoking their postProcessBeforeInstantiation methods. 
         * 

Any returned object will be used as the bean instead of actually instantiating * the target bean. A null return value from the post-processor will * result in the target bean being instantiated. * @param beanClass the class of the bean to be instantiated * @param beanName the name of the bean * @return the bean object to use instead of a default instance of the target bean, or null * @throws BeansException if any post-processing failed * @see InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation */ protected Object applyBeanPostProcessorsBeforeInstantiation(Class beanClass, String beanName) throws BeansException { for (Iterator it = getBeanPostProcessors().iterator(); it.hasNext();) { BeanPostProcessor beanProcessor = (BeanPostProcessor) it.next(); if (beanProcessor instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) beanProcessor; Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName); if (result != null) { return result; } } } return null; } //AbstractAutoProxyCreator public Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException { Object cacheKey = getCacheKey(beanClass, beanName); if (!this.targetSourcedBeans.contains(cacheKey)) { if (this.advisedBeans.contains(cacheKey) || this.nonAdvisedBeans.contains(cacheKey)) { return null; } if (isInfrastructureClass(beanClass, beanName) || shouldSkip(beanClass, beanName)) { this.nonAdvisedBeans.add(cacheKey); return null; } } // Create proxy here if we have a custom TargetSource. // Suppresses unnecessary default instantiation of the target bean: // The TargetSource will handle target instances in a custom fashion. TargetSource targetSource = getCustomTargetSource(beanClass, beanName); if (targetSource != null) { this.targetSourcedBeans.add(beanName); Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource); Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource); this.proxyTypes.put(cacheKey, proxy.getClass()); return proxy; } return null; }

对于AutoProxy部分代码要放到spring aop部分中去写了,这里只要明白如果是代理bean在postProcessBeforeInstantiation阶段会返回一个proxy,这时候bean已经算是实例化好了,再调用applyBeanPostProcessorsAfterInitialization即BeanPostProcessor.postProcessAfterInitialization回调进行属性的设置。最后的结果就可以返回bean对象了,只是一个Proxy Bean Object.

  1. Object beanInstance = doCreateBean(beanName, mbd, args);//如果是代理bean就不会走到这一步

    解释下:对于非代理的bean,基本上实例化的逻辑就在doCreateBean这个方法了,这个方法在“Spring IOC之BeanFactory”中已经有过分析,所以不重复解释了,贴一个bean生命周期的流程图做为createBean方法的结尾:
    BeanDefinition到Bean_第1张图片
    上图不是我所画,来源于spring IOC容器介绍。

    半成品出来之后还需要调用getObjectForBeanInstance进一步处理,这个方法:

    protected Object getObjectForBeanInstance(  
                Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {  

            // 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());  
            }  

            // 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) {  
                object = getCachedObjectForFactoryBean(beanName);  
            }  
            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;  
        }  

开始部分是逻辑检查,是否是FactoryBean,后面是从factoryBean中获取实际的Bean。获取bean也是先检查缓存,没有缓存再getObjectFromFactoryBean获取,详细再doGetObjectFromFactoryBean方法中,最后将获取的bean缓存起来。代码就不贴了。

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