Spring IOC核心源码学习

曾经终败给现在 2022-07-30 14:56 11阅读 0赞

转自：[Yikun][][\-Spring IOC核心源码学习][Yikun]

**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，并解析，最终注册到Bean Factory中：  
![20160419124844360][]  
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：

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：  
![20160419124847834][]  
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**  
![20160419124853344][]  
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：  
![20160419124857538][]  
创建bean的实例  
创建bean的实例过程函数调用栈如下所示：  
![20160419124900704][]  
注入bean的属性  
注入bean的属性过程函数调用栈如下所示：  
![20160419124904006][]  
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，我们就可以自己实现一个简单的Spring框架了。于是，我根据自己的理解实现了一个简单的IOC框架Simple Spring，有兴趣可以看看。  
  
转自：[http://yikun.github.io/2015/05/29/Spring-IOC%E6%A0%B8%E5%BF%83%E6%BA%90%E7%A0%81%E5%AD%A6%E4%B9%A0/][Yikun]

[Yikun]: http://yikun.github.io/2015/05/29/Spring-IOC%E6%A0%B8%E5%BF%83%E6%BA%90%E7%A0%81%E5%AD%A6%E4%B9%A0/
[20160419124844360]: /images/20220724/aa9a8aaae3794b0a852f03c391d74e7a.png
[20160419124847834]: /images/20220724/cb6de27e8b98417ea5433880b4c0097e.png
[20160419124853344]: /images/20220724/da1360459ffb404e81709b651efbfde6.png
[20160419124857538]: /images/20220724/14538647edb6422a804fcf9963d8f96f.png
[20160419124900704]: /images/20220724/cd150c086a324da28af96f5632889b5d.png
[20160419124904006]: /images/20220724/6e65d60906e4461b9d9c2c26f4182178.png