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。