Spring IOC解析（二）

1. 初始化

大致单步跟了下Spring IOC的初始化过程，整个脉络很庞大，初始化的过程主要就是读取XML资源文件，并解析，最终注册到BeanFactory中：

在完成初始化的过程后，Bean们就在BeanFactory中蓄势以待地等调用了。下面通过一个具体的例子，来详细地学习一下初始化过程，例如当加载下面一个bean：

<bean id="XiaoWang" class="com.springstudy.talentshow.SuperInstrumentalist">
    <property name="instruments">
        <list>
            <ref bean="piano"/>
            <ref bean="saxophone"/>
        </list>
    </property>
</bean>

加载时需要读取、解析、注册bean，这个过程具体的调用栈如下所示：

下面对每一步的关键的代码进行详细分析：

1.1 准备

保存配置位置，并刷新

在调用ClassPathXmlApplicationContext后，先会将配置位置信息保存到configLocations，供后面解析使用，之后，会调用AbstractApplicationContext的refresh方法进行刷新：

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, 
        ApplicationContext parent) throws BeansException {
    super(parent);
    // 保存位置信息，比如`com/springstudy/talentshow/talent-show.xml`
    setConfigLocations(configLocations);
    if (refresh) {
        // 刷新
        refresh();
    }
}
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();
        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);
        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);
            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);
            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);
            // Initialize message source for this context.
            initMessageSource();
            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();
            // Initialize other special beans in specific context subclasses.
            onRefresh();
            // Check for listener beans and register them.
            registerListeners();
            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);
            // Last step: publish corresponding event.
            finishRefresh();
        }
        catch (BeansException ex) {
            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();
            // Reset 'active' flag.
            cancelRefresh(ex);
            // Propagate exception to caller.
            throw ex;
        }
    }
}

创建载入BeanFactory

protected final void refreshBeanFactory() throws BeansException {
    // ... ...
    DefaultListableBeanFactory beanFactory = createBeanFactory();
    // ... ...
    loadBeanDefinitions(beanFactory);
    // ... ...
}

创建XMLBeanDefinitionReader

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
     throws BeansException, IOException {
    // Create a new XmlBeanDefinitionReader for the given BeanFactory.
    XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
    // ... ...
    // Allow a subclass to provide custom initialization of the reader,
    // then proceed with actually loading the bean definitions.
    initBeanDefinitionReader(beanDefinitionReader);
    loadBeanDefinitions(beanDefinitionReader);
}

1.2 读取

创建处理每一个resource

public int loadBeanDefinitions(String location, Set<Resource> actualResources)
     throws BeanDefinitionStoreException {
    // ... ...
    // 通过Location来读取Resource
    Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
    int loadCount = loadBeanDefinitions(resources);
    // ... ...
}
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
    Assert.notNull(resources, "Resource array must not be null");
    int counter = 0;
    for (Resource resource : resources) {
        // 载入每一个resource
        counter += loadBeanDefinitions(resource);
    }
    return counter;
}

处理XML每个元素

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
    // ... ...
    NodeList nl = root.getChildNodes();
    for (int i = 0; i < nl.getLength(); i++) {
        Node node = nl.item(i);
        if (node instanceof Element) {
            Element ele = (Element) node;
            if (delegate.isDefaultNamespace(ele)) {
                // 处理每个xml中的元素，可能是import、alias、bean
                parseDefaultElement(ele, delegate);
            }
            else {
                delegate.parseCustomElement(ele);
            }
        }
    }
    // ... ...
}

解析和注册bean

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
    // 解析
    BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
    if (bdHolder != null) {
        bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
        try {
            // 注册
            // Register the final decorated instance.
            BeanDefinitionReaderUtils.registerBeanDefinition(
                bdHolder, getReaderContext().getRegistry());
        }
        catch (BeanDefinitionStoreException ex) {
            getReaderContext().error("Failed to register bean definition with name '" +
                    bdHolder.getBeanName() + "'", ele, ex);
        }
        // Send registration event.
        getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
    }
}

本步骤中，通过parseBeanDefinitionElement将XML的元素解析为BeanDefinition，然后存在BeanDefinitionHolder中，然后再利用BeanDefinitionHolder将BeanDefinition注册，实质就是把BeanDefinition的实例put进BeanFactory中，和后面将详细的介绍解析和注册过程。

1.3 解析

处理每个Bean的元素

public AbstractBeanDefinition parseBeanDefinitionElement(
        Element ele, String beanName, BeanDefinition containingBean) {
    // ... ...
    // 创建beandefinition
    AbstractBeanDefinition bd = createBeanDefinition(className, parent);
    parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
    bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
    parseMetaElements(ele, bd);
    parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
    parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
    // 处理“Constructor”
    parseConstructorArgElements(ele, bd);
    // 处理“Preperty”
    parsePropertyElements(ele, bd);
    parseQualifierElements(ele, bd);
    // ... ...
}

处理属性的值

public Object parsePropertyValue(Element ele, BeanDefinition bd, String propertyName) {
    String elementName = (propertyName != null) ?
                    "<property> element for property '" + propertyName + "'" :
                    "<constructor-arg> element";
    // ... ...
    if (hasRefAttribute) {
    // 处理引用
        String refName = ele.getAttribute(REF_ATTRIBUTE);
        if (!StringUtils.hasText(refName)) {
            error(elementName + " contains empty 'ref' attribute", ele);
        }
        RuntimeBeanReference ref = new RuntimeBeanReference(refName);
        ref.setSource(extractSource(ele));
        return ref;
    }
    else if (hasValueAttribute) {
    // 处理值
        TypedStringValue valueHolder = new TypedStringValue(ele.getAttribute(VALUE_ATTRIBUTE));
        valueHolder.setSource(extractSource(ele));
        return valueHolder;
    }
    else if (subElement != null) {
    // 处理子类型（比如list、map等）
        return parsePropertySubElement(subElement, bd);
    }
    // ... ...
}

1.4 注册

public static void registerBeanDefinition(
        BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
        throws BeanDefinitionStoreException {
    // Register bean definition under primary name.
    String beanName = definitionHolder.getBeanName();
    registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
    // Register aliases for bean name, if any.
    String[] aliases = definitionHolder.getAliases();
    if (aliases != null) {
        for (String alias : aliases) {
            registry.registerAlias(beanName, alias);
        }
    }
}
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
        throws BeanDefinitionStoreException {
    // ......
    // 将beanDefinition注册
    this.beanDefinitionMap.put(beanName, beanDefinition);
    // ......
}

注册过程中，最核心的一句就是：this.beanDefinitionMap.put(beanName, beanDefinition)，也就是说注册的实质就是以beanName为key，以beanDefinition为value，将其put到HashMap中。

2. 注入依赖

当完成初始化IOC容器后，如果bean没有设置lazy-init(延迟加载)属性，那么bean的实例就会在初始化IOC完成之后，及时地进行初始化。初始化时会先建立实例，然后根据配置利用反射对实例进行进一步操作，具体流程如下所示：

创建bean的实例

创建bean的实例过程函数调用栈如下所示：

注入bean的属性

注入bean的属性过程函数调用栈如下所示：

在创建bean和注入bean的属性时，都是在doCreateBean函数中进行的，我们重点看下：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, 
        final Object[] args) {
    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 创建bean的实例
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    // ... ...
    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        // 初始化bean的实例，如注入属性
        populateBean(beanName, mbd, instanceWrapper);
        if (exposedObject != null) {
            exposedObject = initializeBean(beanName, exposedObject, mbd);
        }
    }
    // ... ...
}

理解了以上两个过程，这是spring最重要的过程了。