spring-ioc容器初始化过程

这里先从ClassPathXmlApplicationContext的构造来开始分析
看一下 ClassPathXmlApplicationContext 的UML类图

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent) throws BeansException {
    super(parent);
    this.setConfigLocations(configLocations);
    if (refresh) {
        this.refresh();
    }
}

看super(parent)做了些什么,parent卡是传进来的是null

public AbstractApplicationContext(@Nullable ApplicationContext parent) {
    this();
    this.setParent(parent);
}

看this()

public AbstractApplicationContext() {
    this.logger = LogFactory.getLog(this.getClass());
    this.id = ObjectUtils.identityToString(this);
    this.displayName = ObjectUtils.identityToString(this);
    this.beanFactoryPostProcessors = new ArrayList();
    this.active = new AtomicBoolean();
    this.closed = new AtomicBoolean();
    this.startupShutdownMonitor = new Object();
    this.applicationListeners = new LinkedHashSet();
    this.resourcePatternResolver = this.getResourcePatternResolver();
}

这里给当前类(AbstractApplicationContext)的私有变量初始化值。看看setParent()做了些什么

public void setParent(@Nullable ApplicationContext parent) {
    this.parent = parent;
    // 当parent不为空的时候，会对IOC环境进行配置。
    if (parent != null) {
        Environment parentEnvironment = parent.getEnvironment();
        if (parentEnvironment instanceof ConfigurableEnvironment) {
            this.getEnvironment().merge((ConfigurableEnvironment)parentEnvironment);
        }
    }

}

这里super(parent)已经执行完了，继续往下看this.setConfigLocations(configLocations);

public void setConfigLocations(@Nullable String... locations) {
    if (locations != null) {
        Assert.noNullElements(locations, "Config locations must not be null");
        this.configLocations = new String[locations.length];

        for(int i = 0; i < locations.length; ++i) {
            this.configLocations[i] = this.resolvePath(locations[i]).trim();
        }
    } else {
        this.configLocations = null;
    }
}

这里会解析传进来的配置文件路径参数，用的是ant规则匹配。解析完后将路径信息放入configLocations数组里面。往下看refresh默认是true，看refresh方法

public void refresh() throws BeansException, IllegalStateException {
    // var1 = new Object
    Object var1 = this.startupShutdownMonitor;
    synchronized(this.startupShutdownMonitor) {
        // 调用容器准备刷新的方法，此方法中会获取容器的当前时间，给容器设置同步标识
        this.prepareRefresh();
        ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
        this.prepareBeanFactory(beanFactory);

        try {
           
			// beanFactory的后置处理器
            this.postProcessBeanFactory(beanFactory);

            //调用BeanFactoryPostProcessor，激活各种BeanFactory处理器
            this.invokeBeanFactoryPostProcessors(beanFactory);

            //注册拦截Bean创建的Bean处理器，这里只是注册，真正调用是在getBean的时候。
            this.registerBeanPostProcessors(beanFactory);
            
            //为上下文初始化Message源，国际化处理
            this.initMessageSource();
            
            //初始化应用消息广播器，并放入applicationEventMulticaster bean中
            this.initApplicationEventMulticaster();
 
            //留给子类来初始化其他的Bea
            this.onRefresh();
            
            //在所有注册的bean中查找Listener bean，注册到消息广播器中
            this.registerListeners();
            
            //初始化剩下的单实例，非惰性的
            this.finishBeanFactoryInitialization(beanFactory);

            //完成刷新过程，通知生命周期处理器lifecycleProcessor刷新过程，同时发出ContextRefreshEvent通知别人
            this.finishRefresh();
        } catch (BeansException var9) {
            if (this.logger.isWarnEnabled()) {
                this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
            }

            this.destroyBeans();
            this.cancelRefresh(var9);
            throw var9;
        } finally {
            this.resetCommonCaches();
        }

    }
}

这里重点看一下 obtainFreshBeanFactory 方法，它返回一个beanFactory对象，那么我们这里可以猜测应该读取了配置文件，配置了bean等等。追踪一下源码

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
    this.refreshBeanFactory();
    ConfigurableListableBeanFactory beanFactory = this.getBeanFactory();
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Bean factory for " + this.getDisplayName() + ": " + beanFactory);
    }

    return beanFactory;
}
``` 

这里首先调用 refreshBeanFactory 方法刷新beanFactory。源码如下:

```java
protected final void refreshBeanFactory() throws BeansException {
    if (this.hasBeanFactory()) {
        this.destroyBeans();
        this.closeBeanFactory();
    }

    try {
        // 创建一个新的beanFacroty
        DefaultListableBeanFactory beanFactory = this.createBeanFactory();
        // 配置beanFactory的属性
        beanFactory.setSerializationId(this.getId());
        this.customizeBeanFactory(beanFactory);
        this.loadBeanDefinitions(beanFactory);
        Object var2 = this.beanFactoryMonitor;
        synchronized(this.beanFactoryMonitor) {
            this.beanFactory = beanFactory;
        }
    } catch (IOException var5) {
        throw new ApplicationContextException("I/O error parsing bean definition source for " + this.getDisplayName(), var5);
    }
}

可以看出，首先判断是否已经存在beanFactory，有就关闭，然后再创建一个beanFactory.看一下创建 createBeanFactory 的具体实现

protected DefaultListableBeanFactory createBeanFactory() {
    return new DefaultListableBeanFactory(this.getInternalParentBeanFactory());
}

由于前面的parent参数为null,所以这个 getInternalParentBeanFactory 方法返回值为null。这里就实例化了一个 DefaultListableBeanFactory 工厂类。回到前面继续往下看， customizeBeanFactory 方法内部是判断变量赋值的，这里条件都不满足，继续往下看，loadBeanDefinitions 方法是负责给 beanFactory 填充 bean 定义的，看看源码

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
    // 创建 XML 读取器
    XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
    // 配置 beanDefinitionReader
    beanDefinitionReader.setEnvironment(this.getEnvironment());
    beanDefinitionReader.setResourceLoader(this);
    beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
    // 对读取器 设置xml校验。
    this.initBeanDefinitionReader(beanDefinitionReader);
    // 读取配置文件，加载 bean 的定义
    this.loadBeanDefinitions(beanDefinitionReader);
}

看一下 loadBeanDefinitions 方法。

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
    // Resource 是将配置文件封装的一个对象，首先尝试能否直接获取到 Resource 数组。默认是不行的
    Resource[] configResources = this.getConfigResources();
    if (configResources != null) {
        reader.loadBeanDefinitions(configResources);
    }
    // 这里就直接获取配置文件的 localtion，后面还是先获取到Resource[] 数组再去读取xml文件
    String[] configLocations = this.getConfigLocations();
    if (configLocations != null) {
        reader.loadBeanDefinitions(configLocations);
    }

}

看看 loadBeanDefinitions(configLocations) 的源码,实际调用的是下面这个重载类

public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
    // 获取 ResourceLoader 加载器
    ResourceLoader resourceLoader = this.getResourceLoader();
    if (resourceLoader == null) {
        throw new BeanDefinitionStoreException("Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
    } else {
        int loadCount;
        if (!(resourceLoader instanceof ResourcePatternResolver)) {
            Resource resource = resourceLoader.getResource(location);
            loadCount = this.loadBeanDefinitions((Resource)resource);
            if (actualResources != null) {
                actualResources.add(resource);
            }

            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
            }

            return loadCount;
        } else {
            try {
                Resource[] resources = ((ResourcePatternResolver)resourceLoader).getResources(location);
                // 这里对配置文件进行读取，返回读取的配置文件数量
                loadCount = this.loadBeanDefinitions(resources);
                if (actualResources != null) {
                    Resource[] var6 = resources;
                    int var7 = resources.length;

                    for(int var8 = 0; var8 < var7; ++var8) {
                        Resource resource = var6[var8];
                        actualResources.add(resource);
                    }
                }

                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                }

                return loadCount;
            } catch (IOException var10) {
                throw new BeanDefinitionStoreException("Could not resolve bean definition resource pattern [" + location + "]", var10);
            }
        }
    }
}

细看读取过程

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
    Assert.notNull(encodedResource, "EncodedResource must not be null");
    if (this.logger.isInfoEnabled()) {
        this.logger.info("Loading XML bean definitions from " + encodedResource.getResource());
    }

    Set<EncodedResource> currentResources = (Set)this.resourcesCurrentlyBeingLoaded.get();
    if (currentResources == null) {
        currentResources = new HashSet(4);
        this.resourcesCurrentlyBeingLoaded.set(currentResources);
    }

    if (!((Set)currentResources).add(encodedResource)) {
        throw new BeanDefinitionStoreException("Detected cyclic loading of " + encodedResource + " - check your import definitions!");
    } else {
        int var5;
        try {
            // 获取文件流
            InputStream inputStream = encodedResource.getResource().getInputStream();

            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }

                // 这里加载配置文件
                var5 = this.doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            } finally {
                inputStream.close();
            }
        } catch (IOException var15) {
            throw new BeanDefinitionStoreException("IOException parsing XML document from " + encodedResource.getResource(), var15);
        } finally {
            ((Set)currentResources).remove(encodedResource);
            if (((Set)currentResources).isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }

        }

        return var5;
    }
}

来看一下怎么加载的，这个 doLoadBeanDefinitions 方法

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException {
    try {
        // 理论上一个 inputSource 参数就行了，为什么还要 resource 呢？这里是为了设置xml验证模式引入的。这个方法就将InputSource对象转变成了 Document 对象。
        Document doc = this.doLoadDocument(inputSource, resource);
        // 这个方法将document对象变成了Bean放入内存中
        return this.registerBeanDefinitions(doc, resource);
    } catch (BeanDefinitionStoreException var4) {
        throw var4;
    } catch (SAXParseException var5) {
        throw new XmlBeanDefinitionStoreException(resource.getDescription(), "Line " + var5.getLineNumber() + " in XML document from " + resource + " is invalid", var5);
    } catch (SAXException var6) {
        throw new XmlBeanDefinitionStoreException(resource.getDescription(), "XML document from " + resource + " is invalid", var6);
    } catch (ParserConfigurationException var7) {
        throw new BeanDefinitionStoreException(resource.getDescription(), "Parser configuration exception parsing XML from " + resource, var7);
    } catch (IOException var8) {
        throw new BeanDefinitionStoreException(resource.getDescription(), "IOException parsing XML document from " + resource, var8);
    } catch (Throwable var9) {
        throw new BeanDefinitionStoreException(resource.getDescription(), "Unexpected exception parsing XML document from " + resource, var9);
    }
}

看一下这个方法 registerBeanDefinitions

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
    // 获取一个专为 BeanDefinitionDocument 的 reader。
    BeanDefinitionDocumentReader documentReader = this.createBeanDefinitionDocumentReader();
    int countBefore = this.getRegistry().getBeanDefinitionCount();
    // 这里就是开始读取xml定义的文件了
    documentReader.registerBeanDefinitions(doc, this.createReaderContext(resource));
    return this.getRegistry().getBeanDefinitionCount() - countBefore;
}

看一下读取方法 registerBeanDefinitions 的细节

public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
    this.readerContext = readerContext;
    this.logger.debug("Loading bean definitions");
    Element root = doc.getDocumentElement();
    this.doRegisterBeanDefinitions(root);
}

看具体加载根节点的方法 doRegisterBeanDefinitions

protected void doRegisterBeanDefinitions(Element root) {
    BeanDefinitionParserDelegate parent = this.delegate;
    this.delegate = this.createDelegate(this.getReaderContext(), root, parent);
    if (this.delegate.isDefaultNamespace(root)) {
        String profileSpec = root.getAttribute("profile");
        if (StringUtils.hasText(profileSpec)) {
            String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, ",; ");
            if (!this.getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                if (this.logger.isInfoEnabled()) {
                    this.logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec + "] not matching: " + this.getReaderContext().getResource());
                }

                return;
            }
        }
    }

    //xml解析的预处理，可以自己定义一些节点属性等，此方法Spring默认实现为空
    this.preProcessXml(root);
    //把Document对象解析为BeanDefinition对象
    this.parseBeanDefinitions(root, this.delegate);
    //xml解析的后处理，可以在解析完xml之后，实现自己的逻辑。Spring默认实现为空。
    this.postProcessXml(root);
    this.delegate = parent;
}

看一下parseBeanDefinitions方法源码

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
    // 判断是不是Spring的默认空间命名（dubbo的就不是）
    if (delegate.isDefaultNamespace(root)) {
        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)) {
                    this.parseDefaultElement(ele, delegate);
                } else {
                    delegate.parseCustomElement(ele);
                }
            }
        }
    } else {
        // dubbo配置文件读取就是从这里开始的
        delegate.parseCustomElement(root);
    }
}

到这里为止就差不多了，SpringIOC里面配置的bean就是这么来的。

总结

• IOC容器初始化入口是在构造方法中调用refresh开始的。
• 通过ResourceLoader来完成资源文件位置的定位，DefaultResourceLoader是默认的实现，同时上下文本身就给除了ResourceLoader的实现。
• 创建的IOC容器是DefaultListableBeanFactory。
• IOC对Bean的管理和依赖注入功能的实现是通过对其持有的BeanDefinition进行相关操作来完成的。
• 通过BeanDefinitionReader来完成定义信息的解析和Bean信息的注册。
• XmlBeanDefinitionReader是BeanDefinitionReader的实现了，通过它来解析xml配置中的bean定义。
• 实际的处理过程是委托给BeanDefinitionParserDelegate来完成的。得到Bean的定义信息，这些信息在Spring中使用BeanDefinition对象来表示。
• BeanDefinition的注册是由BeanDefinitionRegistry实现的registerBeanDefiition方法进行的。内部使用ConcurrentHashMap来保存BeanDefiition。