Spring源码：IOC原理解析（二）

版权声明：本文为博主原创文章，转载请注明出处，欢迎交流学习！

接着上一章节的内容，我们来分析当new一个FileSystemXmlApplicationContext对象的时候，spring到底做了那些事。FileSystemXmlApplicationContext类的内容主要是定义了若干重载的构造方法，核心构造方法如下：

/**
     * Create a new FileSystemXmlApplicationContext with the given parent,
     * loading the definitions from the given XML files.
     *
     * loading all bean definitions and creating all singletons.
     * Alternatively, call refresh manually after further configuring the context.
     *
     */
    public FileSystemXmlApplicationContext(
            String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

从方法说明可以看出，在这个构造方法里加载所有bean定义并创建bean单例实例。其中的refresh()方法就是IOC容器初始化的入口，refresh()方法位AbstractApplicationContext类中，这是一个抽象类，它实现了ApplicationContext的基础功能，这里使用了模版方法模式，给实现它的子类提供了统一的模板：

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.告诉子类刷新内部bean工厂
            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) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

refresh()方法里列出了IOC容器初始化的步骤，第一个方法是初始化准备，这里只是设置启动日期和活动标识以及执行属性源的初始化。我们重点看第二个方法obtainFreshBeanFactory()，它告诉子类刷新内部bean工厂，返回了一个ConfigurableListableBeanFactory，跟踪这个方法：

/**
     * Tell the subclass to refresh the internal bean factory.
     * @return the fresh BeanFactory instance
     * @see #refreshBeanFactory()
     * @see #getBeanFactory()
     */
    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;

/**
     * Return the internal bean factory of this application context.
     * Can be used to access specific functionality of the underlying factory.
     *
     */
    ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;

obtainFreshBeanFactory()方法的第一行调用了refreshBeanFactory()方法，这是一个抽象方法，由它的子类来实现，方法的第二行调用了getBeanFactory()，这是在其父接口中定义的一个空方法。抽象方法refreshBeanFactory()在其子类子类AbstractRefreshableApplicationContext中实现：

/**
     * This implementation performs an actual refresh of this context's underlying
     * bean factory, shutting down the previous bean factory (if any) and
     * initializing a fresh bean factory for the next phase of the context's lifecycle.
     *
     * 此实现执行该上下文的底层bean工厂的实际刷新，关闭以前的bean工厂（如果有的话），
     * 并为上下文生命周期的下一阶段初始化一个新的bean工厂
     */
    @Override
    protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

这个方法被final关键字修饰，也就是说不可以被重写，IOC容器的初始化就是在这个方法中完成的。第一步先判断有没有现有的工厂，有的话就销毁掉，然后创建一个默认的工厂，也就是DefaultListableBeanFactory ，接下来两行代码是设置bean工厂的一些属性，注意看loadBeanDefinitions(beanFactory)这行，当创建了一个默认的bean工厂后，加载bean定义，这跟我们上一章节使用原始方式初始化bean工厂类似。从这里不难看出，FileSystemXmlApplicationContext的构造方法中其实已经包含了我们上一章节中原始的初始化过程。接下来我们跟踪一下loadBeanDefinitions(beanFactory)的实现，这个方法是由AbstractXmlApplicationContext抽象类实现的：

/**
     * Loads the bean definitions via an XmlBeanDefinitionReader.装载bean定义通过XmlBeanDefinitionReader
     *
     */
    @Override
    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // Configure the bean definition reader with this context's
        // resource loading environment.
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // then proceed with actually loading the bean definitions.
        initBeanDefinitionReader(beanDefinitionReader);
        loadBeanDefinitions(beanDefinitionReader);
    }

方法的第一行首先定义了一个Reader，这个Reader就是用来读取xml配置文件的，最后一行就是真正载入bean定义的实现过程，代码如下：

/**
     * Load the bean definitions with the given XmlBeanDefinitionReader.
     *
     */
    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            reader.loadBeanDefinitions(configLocations);
        }
    }

上面的方法调用了XmlBeanDefinitionReader类的loadBeanDefinitions(EncodedResource encodedResource)方法：

/**
     * Load bean definitions from the specified XML file.
     * rows BeanDefinitionStoreException in case of loading or parsing errors
     */
    public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if (logger.isInfoEnabled()) {
            logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }

        Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
        if (currentResources == null) {
            currentResources = new HashSet<>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            }
            finally {
                inputStream.close();
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }

从方法说明可以看出，这个方法是从指定的xml文件中加载bean定义，try块中的代码才是载入bean定义的过程。spring将资源返回的输入流包装以后传给了doLoadBeanDefinitions()方法，我们进入这个方法，代码如下：

/**
     * Actually load bean definitions from the specified XML file.
     *
     */
    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            Document doc = doLoadDocument(inputSource, resource);
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (SAXParseException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
        }
        catch (SAXException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "XML document from " + resource + " is invalid", ex);
        }
        catch (ParserConfigurationException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Parser configuration exception parsing XML from " + resource, ex);
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "IOException parsing XML document from " + resource, ex);
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Unexpected exception parsing XML document from " + resource, ex);
        }
    }

/**
     * Actually load the specified document using the configured DocumentLoader.
     *
     */
    protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

从try块中的代码可以看出，spring使用documentLoader将资源转换成了Document资源，spring使用的documentLoader为DefaultDocumentLoader，loadDocument方法定义在此类中：

/**
     * Load the {@link Document} at the supplied {@link InputSource} using the standard JAXP-configured
     * XML parser.
     */
    @Override
    public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
            ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

        DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
        if (logger.isDebugEnabled()) {
            logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
        }
        DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
        return builder.parse(inputSource);
    }

从这里不难看出，这就是我们非常熟悉的DOM解析xml了，可以想象spring是根据XSD文件规定的格式解析了xml文件的各节点及属性。我们再来回头看看registerBeanDefinitions(doc, resource)方法，

/**
     * Register the bean definitions contained in the given DOM document.
     * Called by {@code loadBeanDefinitions}.
     *
     */
    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        int countBefore = getRegistry().getBeanDefinitionCount();
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

方法说明很明确的告诉我们，这个方法是注册给定的DOM文档中包含的bean定义。到这里思路就很明确了，spring将包装的输入流解析成DOM文档，然后将DOM中包含的bean定义信息注册到IOC容器持有的Map<String，BeanDefinition>对象中。只要我们的IOC容器持有了bean定义，就能正确的生产bean实例。

通过阅读源码，我们分析了Spring IOC的实现原理。有些实现细节并没有去深究，更重要的是去理解它的核心思想和实现思路。