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spring IOC容器实现探讨


引用自:http://www.blogjava.net/baoyaer/articles/113251.html
sspring IOC容器实现探讨现,一开始我被复杂的接口和类所掩埋,看不清整体的思路和设计,踟蹰于代码丛林中,摸不清前进的方向。一开始我就决定只研读以xml文件做配置文件的XmlFactoryBean的具体实现为主要目标,渐渐地有了点感觉,用UML把spring中的bean工厂体系展现出来之后就更清晰了,让你不得不感叹设计的精巧和复杂。本文只是我个人对spring IOC实现的理解,如有错误,请不吝赐教,谢谢。
首先,需要理解的是spring容器中bean的生命周期,《spring in action》中的那张图是最好的解释,结合这张图和源码来解读spring中IOC的实现将非常容易理解。这张图完整展示了spring容器中一个bean从创建到销毁的整个生命周期。

1. 容器寻找Bean的定义信息并且将其实例化。
2.受用依赖注入,Spring按照Bean定义信息配置Bean的所有属性。
3.如果Bean实现了BeanNameAware接口,工厂调用Bean的setBeanName()方法传递Bean的ID。
4.如果Bean实现了BeanFactoryAware接口,工厂调用setBeanFactory()方法传入工厂自身。
5.如果BeanPostProcessor和Bean关联,那么它们的postProcessBeforeInitialzation()方法将被调用。
6.如果Bean指定了init-method方法,它将被调用。
7.最后,如果有BeanPsotProcessor和Bean关联,那么它们的postProcessAfterInitialization()方法将被调用。
到这个时候,Bean已经可以被应用系统使用了,并且将被保留在Bean Factory中知道它不再需要。有两种方法可以把它从Bean Factory中删除掉。
1.如果Bean实现了DisposableBean接口,destory()方法被调用。
2.如果指定了订制的销毁方法,就调用这个方法。

下面我们将会看到,这些bean创建销毁的每个阶段是如何在源码中实现的。
再看看spring中bean工厂的完整体系,比较复杂,不过我们只需要关注其中的几个核心工厂。

(看不清楚,请下载图片来看,比较清晰)
这些工厂类没有在同一个包内,分布在org.springframework.beans以及它的子包内,把它们放在一起就看的比较清楚了。我们需要关注的是这么两个类:org.springframework.beans.factory.support.AbstractBeanFactory
org.springframework.beans.factory.support.DefaultListableBeanFactory
以及接口:
org.springframework.beans.factory.support.BeanDefinitionRegistry

AbstractBeanFactory作为BeanFactory接口的抽象实现类,是其他工厂类的父类,提供了bean的singlton缓存、singleton/prototype的决定、bean的别名以及bean和它的子类bean的定义合并(bean definition merging for child bean definitions)和销毁。这里有3个重载的getBean方法(实现BeanFactory定义的getBean方法),我们关注下最主要的这个方法:

public Object getBean(String name, Class requiredType, Object[] args) throws BeansException { 

 String beanName = transformedBeanName(name); 

 Object bean = null; 


 // Eagerly check singleton cache for manually registered singletons. 

 Object sharedInstance = null; 

 //从单例缓存中获取 

 synchronized (this.singletonCache) { 

 sharedInstance = this.singletonCache.get(beanName); 

 } 

 if (sharedInstance != null) { 

 if (isSingletonCurrentlyInCreation(beanName)) { 

 if (logger.isDebugEnabled()) { 

 logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + 

 "' that is not fully initialized yet - a consequence of a circular reference"); 

 } 

 } 

 else { 

 if (logger.isDebugEnabled()) { 

 logger.debug("Returning cached instance of singleton bean '" + beanName + "'"); 

 } 

 } 

 bean = getObjectForSharedInstance(name, sharedInstance); 

 } 


 else { 

 // Fail if we're already creating this singleton instance: 

 // We're assumably within a circular reference. 

 if (isSingletonCurrentlyInCreation(beanName)) { 

 throw new BeanCurrentlyInCreationException(beanName); 

 } 


 // Check if bean definition exists in this factory. 

 //检测bean是否定义在父工厂 

 if (getParentBeanFactory() != null && !containsBeanDefinition(beanName)) { 

 // Not found -> check parent. 

 if (getParentBeanFactory() instanceof AbstractBeanFactory) { 

 // Delegation to parent with args only possible for AbstractBeanFactory. 

 return ((AbstractBeanFactory) getParentBeanFactory()).getBean(name, requiredType, args); 

 } 

 else if (args == null) { 

 // No args -> delegate to standard getBean method. 

 return getParentBeanFactory().getBean(name, requiredType); 

 } 

 else { 

 throw new NoSuchBeanDefinitionException(beanName, 

 "Cannot delegate to parent BeanFactory because it does not supported passed-in arguments"); 

 } 

 } 


 //获取BeanDefinition 

 RootBeanDefinition mergedBeanDefinition = getMergedBeanDefinition(beanName, false); 

 checkMergedBeanDefinition(mergedBeanDefinition, beanName, requiredType, args); 


 // Create bean instance. 

 //创建bean,如果为为singleton 

 if (mergedBeanDefinition.isSingleton()) { 

 synchronized (this.singletonCache) { 

 // Re-check singleton cache within synchronized block. 

 sharedInstance = this.singletonCache.get(beanName); 

 if (sharedInstance == null) { 

 if (logger.isDebugEnabled()) { 

 logger.debug("Creating shared instance of singleton bean '" + beanName + "'"); 

 } 

 this.currentlyInCreation.add(beanName); 

 try { 

 sharedInstance = createBean(beanName, mergedBeanDefinition, args); 

 //加进单例缓存 

 addSingleton(beanName, sharedInstance); 

 } 

 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. 

 destroyDisposableBean(beanName); 

 throw ex; 

 } 

 finally { 

 this.currentlyInCreation.remove(beanName); 

 } 

 } 

 } 

 bean = getObjectForSharedInstance(name, sharedInstance); 

 } 

 //如果是prototype 

 else { 

 // It's a prototype -> create a new instance. 

 bean = createBean(beanName, mergedBeanDefinition, args); 

 } 

 } 


 // Check if required type matches the type of the actual bean instance. 

 if (requiredType != null && !requiredType.isAssignableFrom(bean.getClass())) { 

 throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); 

 } 

 return bean; 

 }


当你调用getBean获取一个bean的时候,spring首先查找单例缓存中是否已经有这个bean,有的话直接返回(首先会判断是否已经创建),如果没有,spring就开始创建这个bean:首先获取bean的定义(BeanDefinition),检测bean是否定义在父工厂中,有的话调用父工厂的getBean方法;没有就检测bean是singleton还是prototype,如果是singleton就是创建bean并加入缓存以便下次直接调用,如果是prototype,就在每次调用时重新创建一个bean实例。注意createBean(beanName, mergedBeanDefinition, args); 这个方法,这是创建bean的核心方法,并且是一个abstract方法,将被子类实现。
看看是如何获取bean的定义的,

protected RootBeanDefinition getMergedBeanDefinition(String name, boolean includingAncestors) 

 throws BeansException { 


 String beanName = transformedBeanName(name); 


 // Efficiently check whether bean definition exists in this factory. 

 if (includingAncestors && !containsBeanDefinition(beanName) && 

 getParentBeanFactory() instanceof AbstractBeanFactory) { 

 return ((AbstractBeanFactory) getParentBeanFactory()).getMergedBeanDefinition(beanName, true); 

 } 


 // Resolve merged bean definition locally. 

 return getMergedBeanDefinition(beanName, getBeanDefinition(beanName)); 

 }



调用重载的getMergedBeanDefinition合并父工厂和子工厂中的bean定义,注意getBeanDefinition(beanName),这是一个抽象方法,延迟到子类实现以便提供获取bean定义的具体方法,这个方法和 createBean 一样都是template method模式的应用。看看它们的定义:

protected abstract BeanDefinition getBeanDefinition(String beanName) throws BeansException; 


protected abstract Object createBean( 

 String beanName, RootBeanDefinition mergedBeanDefinition, Object[] args) throws BeanCreationException;


基本了解了AbstractBeanFactory 后,我们来看看它的子类的AbstractAutowireCapableBeanFactory ,这个类实现了createBean() ,在这个方法中我们将看到与上面bean的生命周期图对应的bean的创建过程,英文注释已经非常清楚:

protected Object createBean(String beanName, RootBeanDefinition mergedBeanDefinition, Object[] args) 

 throws BeanCreationException { 


 if (logger.isDebugEnabled()) { 

 logger.debug("Creating instance of bean '" + beanName + 

 "' with merged definition [" + mergedBeanDefinition + "]"); 

 } 


 Object bean = null; 


 // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. 

 if (mergedBeanDefinition.hasBeanClass()) { 

 bean = applyBeanPostProcessorsBeforeInstantiation(mergedBeanDefinition.getBeanClass(), beanName); 

 if (bean != null) { 

 return bean; 

 } 

 } 


 // Guarantee initialization of beans that the current one depends on. 

 if (mergedBeanDefinition.getDependsOn() != null) { 

 for (int i = 0; i < mergedBeanDefinition.getDependsOn().length; i++) { 

 getBean(mergedBeanDefinition.getDependsOn()[i]); 

 } 

 } 


 BeanWrapper instanceWrapper = null; 

 Object originalBean = null; 

 String errorMessage = null; 


 try { 

 // Instantiate the bean. 

 errorMessage = "Instantiation of bean failed"; 


 if (mergedBeanDefinition.getFactoryMethodName() != null) { 

 instanceWrapper = instantiateUsingFactoryMethod(beanName, mergedBeanDefinition, args); 

 } 

 else if (mergedBeanDefinition.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR || 

 mergedBeanDefinition.hasConstructorArgumentValues() ) { 

 instanceWrapper = autowireConstructor(beanName, mergedBeanDefinition); 

 } 

 else { 

 // No special handling: simply use no-arg constructor. 

 instanceWrapper = instantiateBean(beanName, mergedBeanDefinition); 

 } 

 bean = instanceWrapper.getWrappedInstance(); 


 // Eagerly cache singletons to be able to resolve circular references 

 // even when triggered by lifecycle interfaces like BeanFactoryAware. 

 if (isAllowCircularReferences() && isSingletonCurrentlyInCreation(beanName)) { 

 if (logger.isDebugEnabled()) { 

 logger.debug("Eagerly caching bean with name '" + beanName + 

 "' to allow for resolving potential circular references"); 

 } 

 addSingleton(beanName, bean); 

 } 


 // Initialize the bean instance. 

 errorMessage = "Initialization of bean failed"; 

 populateBean(beanName, mergedBeanDefinition, instanceWrapper); 


 if (bean instanceof BeanNameAware) { 

 if (logger.isDebugEnabled()) { 

 logger.debug("Invoking setBeanName on BeanNameAware bean '" + beanName + "'"); 

 } 

 ((BeanNameAware) bean).setBeanName(beanName); 

 } 


 if (bean instanceof BeanFactoryAware) { 

 if (logger.isDebugEnabled()) { 

 logger.debug("Invoking setBeanFactory on BeanFactoryAware bean '" + beanName + "'"); 

 } 

 ((BeanFactoryAware) bean).setBeanFactory(this); 

 } 


 originalBean = bean; 

 bean = applyBeanPostProcessorsBeforeInitialization(bean, beanName); 

 invokeInitMethods(beanName, bean, mergedBeanDefinition); 

 bean = applyBeanPostProcessorsAfterInitialization(bean, beanName); 

 } 

 catch (BeanCreationException ex) { 

 throw ex; 

 } 

 catch (Throwable ex) { 

 throw new BeanCreationException( 

 mergedBeanDefinition.getResourceDescription(), beanName, errorMessage, ex); 

 } 


 // Register bean as disposable, and also as dependent on specified "dependsOn" beans. 

 registerDisposableBeanIfNecessary(beanName, originalBean, mergedBeanDefinition); 


 return bean; 

 }


通过instanceof操作符来判断bean是否实现了用于生命周期回调的接口,然后调用相应的回调方法,可以看到spring充分实践了针对接口编程的原则,虽然很夸张的一个方法一个接口的地步,不过这些接口也是作为mark interface用于标记回调。结合上面的生命周期图看这段代码将很好理解。有了bean的创建方法,那么如何获取bean在配置文件中的定义信息呢?也就是在哪里实现了 getBeanDefinition方法呢?答案就在AbstractAutowireCapableBeanFactory 的子类DefaultListableBeanFactory中。

DefaultListableBeanFactory 不仅继承了AbstractAutowireCapableBeanFactory ,还实现了BeanDefinitionRegistry和ConfigurableListableBeanFactory接口。其中ConfigurableListableBeanFactory接口是定义了分析和修改bean定义信息的回调方法,暂时不去管它。看看BeanDefinitionRegistry接口:

public interface BeanDefinitionRegistry { 

 int getBeanDefinitionCount(); 

 String[] getBeanDefinitionNames(); 

 BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException; 

 String[] getAliases(String beanName) throws NoSuchBeanDefinitionException; 

 void registerAlias(String beanName, String alias) throws BeansException; 

 void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) 

 throws BeansException; 

 ...... 

}



一系列用于获取bean定义信息的方法,这个接口我们将在后面的代码中看到,作为一个mark inteface。注意咯,这个接口中非常关键的一个方法就是registerBeanDefinition,这个方法用于向bean工厂注册解析的每个bean定义,我们将在xml文件的解析的环节看到调用这个方法注册bean定义信息。

DefaultListableBeanFactory实现了BeanDefinitionRegistry 接口,可以看到它实现了关键的registerBeanDefinition用于将bean的定义注册到工厂类中,其中维护了一个Map用于存储bean的定义信息:

public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) 

 throws BeanDefinitionStoreException { 


 Assert.hasText(beanName, "Bean name must not be empty"); 

 Assert.notNull(beanDefinition, "Bean definition must not be null"); 


 if (beanDefinition instanceof AbstractBeanDefinition) { 

 try { 

 ((AbstractBeanDefinition) beanDefinition).validate(); 

 } 

 catch (BeanDefinitionValidationException ex) { 

 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName, 

 "Validation of bean definition failed", ex); 

 } 

 } 


 Object oldBeanDefinition = this.beanDefinitionMap.get(beanName); 

 if (oldBeanDefinition != null) { 

 if (!isAllowBeanDefinitionOverriding()) { 

 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName, 

 "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName + 

 "': there's already [" + oldBeanDefinition + "] bound"); 

 } 

 else { 

 if (logger.isInfoEnabled()) { 

 logger.info("Overriding bean definition for bean '" + beanName + 

 "': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]"); 

 } 

 } 

 } 

 else { 

 this.beanDefinitionNames.add(beanName); 

 } 

 this.beanDefinitionMap.put(beanName, beanDefinition); 


 // Remove corresponding bean from singleton cache, if any. 

 // Shouldn't usually be necessary, rather just meant for overriding 

 // a context's default beans (e.g. the default StaticMessageSource 

 // in a StaticApplicationContext). 

 removeSingleton(beanName); 

 } 

 beanDefinitionMap就是我们存储工厂类中注册的bean的信息,用bean name做为key: 

/** Map of bean definition objects, keyed by bean name */ 

 private final Map beanDefinitionMap = new HashMap();



DefaultListableBeanFactory 重写了
protected abstract BeanDefinition getBeanDefinition(String beanName) throws BeansException;
模板方法用于提供bean定义信息给getBean方法用于创建bean实例。getBeanDefinition的从beanDefinitionMap 查找bean定义即可:

public BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException { 

 BeanDefinition bd = (BeanDefinition) this.beanDefinitionMap.get(beanName); 

 if (bd == null) { 

 if (logger.isDebugEnabled()) { 

 logger.debug("No bean named '" + beanName + "' found in " + toString()); 

 } 

 throw new NoSuchBeanDefinitionException(beanName); 

 } 

 return bd; 

 }



看到了这里是不是觉的有点糊涂,我还是建议有兴趣地找来spring1.2的源码来看看就很清晰了,充分利用eclipse的F3跳转结合上面的那张UML图。
OK,有了注册bean定义信息的方法(registerBeanDefinition),有了获取bean定义信息的方法(getBeanDefinition ),有了创建bean的方法(createBean),那么在哪里调用这些方法呢?createBean我们已经知道是在BeanFactory的getBean方法时调用,getBeanDefinition 又是在creatBean时调用用于获取bean的定义信息,那么注册bean定义信息的方法肯定是在xml文件解析阶段被调用。
XmlBeanFactory继承DefaultListableBeanFactory ,没有定义新的方法,只是有两个重载的构造函数:

public class XmlBeanFactory extends DefaultListableBeanFactory { 

 private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this); 

 public XmlBeanFactory(Resource resource) throws BeansException { 

 this(resource, null); 

 } 

 public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException { 

 super(parentBeanFactory); 

 this.reader.loadBeanDefinitions(resource); 

 } 

}



初始化XmlBeanFactory时调用XmlBeanDefinitionReader读取xml配置文件,而XmlBeanDefinitionReader的loadBeanDefinitions方法,载入xml文件并且调用XmlBeanDefinitionParser 解析xml文件同时注册bean定义信息到bean工厂,这几个类的协作以及它们继承体系也非常清晰,在此不再详述(累啊),XmlBeanDefinitionParser是一个接口,具体的实现类是org.springframework.beans.factory.xml.DefaultXmlBeanDefinitionParser,我们关注的就是它是怎么注册bean信息到bean工厂的:

protected int parseBeanDefinitions(Element root) throws BeanDefinitionStoreException { 



 if (IMPORT_ELEMENT.equals(node.getNodeName())) { 

 importBeanDefinitionResource(ele); 

 } 

 else if (ALIAS_ELEMENT.equals(node.getNodeName())) { 

 String name = ele.getAttribute(NAME_ATTRIBUTE); 

 String alias = ele.getAttribute(ALIAS_ATTRIBUTE); 

 this.beanDefinitionReader.getBeanFactory().registerAlias(name, alias); 

 } 

 else if (BEAN_ELEMENT.equals(node.getNodeName())) { 

 beanDefinitionCount++; 

 BeanDefinitionHolder bdHolder = parseBeanDefinitionElement(ele, false); 

 //关键代码,调用工具类将bean定义注册到beanFactory 

 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, this.beanDefinitionReader.getBeanFactory()); 

 } 


}


工具类BeanDefinitionReaderUtils中的注册方法:

public static void registerBeanDefinition( 

 BeanDefinitionHolder bdHolder, BeanDefinitionRegistry beanFactory) throws BeansException { 


 // Register bean definition under primary name.,注册bean 

 beanFactory.registerBeanDefinition(bdHolder.getBeanName(), bdHolder.getBeanDefinition()); 


 // Register aliases for bean name, if any.,注册别名 

 if (bdHolder.getAliases() != null) { 

 for (int i = 0; i < bdHolder.getAliases().length; i++) { 

 beanFactory.registerAlias(bdHolder.getBeanName(), bdHolder.getAliases()[i]); 

 } 

 } 

 }


注意这里的beanFactory是BeanDefinitionRegistry类型 ,DefaultListableBeanFactory实现了这个接口。
更多内容,比如spring是怎么把bean的属性注入的?答案在BeanWrapperImpl中的一系列setPropertyXXX方法。BeanFactory是如何实例化bean的呢?具体展开请看org.springframework.beans.factory.support.InstantiationStrategy 接口,一个典型的策略模式的应用,两个实现类:SimpleInstantiationStrategy和CglibSubclassingInstantiationStrategy。我们知道spring有3种注入方式:setter、构造函数以及Method Inject。这里的CglibSubclassingInstantiationStrategy使用cglib库用于实现method inject。这样一篇小文容不下这么多内容,还是有兴趣的自己找源码读读。

写到这里,我对自己的文字表达能力产生怀疑,我能不能把这些所有的信息都说清楚呢?感觉很不满意,将就吧,也算是自己的学习总结。
读spring源码这段时间,最深的几点体会:
1)单元测试非常重要,充分感受到了单元测试作为项目文档的优势。看不懂一个类,找到这个类的测试类就OK!
2)针对接口编程的原则,spring处处体现了这个原则,繁多的接口带来的是松耦合、高内聚并且易于扩展的架构,但也带来了一定的理解难度。作为通用型框架也许需要这么考虑,在实际项目中的取舍还是要自己把握。
3)设计模式的使用,spring中应用了很多模式,比如template method,比如strategy和factory method、visitor、singleton等,简直是一本模式实践的良好教材。

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