一赃份、問(wèn)題

我們?cè)趕pring的xml配置文件里經(jīng)常定義各種各樣的配置（tx饿这、bean抛丽、mvc、bean等等）晌缘。以及集成第三方框架時(shí),也會(huì)看到一些spring之外的配置,例如dubbo的配置齐莲、security的配置、redis的配置等等磷箕。spring作為一個(gè)龐大的容器,到底是怎么工作的呢选酗？這篇文章將簡(jiǎn)單的介紹一下其實(shí)現(xiàn)機(jī)制,具體細(xì)節(jié)朋友們可以自己查看相關(guān)代碼。

二岳枷、代碼分析

稍微了解一點(diǎn)spring的朋友都知道spring的入口方法就是refresh(),所以我們直接進(jìn)入refresh()方法【不清楚的可以查看一下ClassPathXmlApplicationContext,一步一步跟蹤進(jìn)去】芒填。

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();
            }
...由于篇幅，我省略了一部分

而解析spring配置的入口就在obtainFreshBeanFactory().refreshBeanFactory()中:

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);//就是這里
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

我們首先會(huì)告訴spring的配置文件所在路徑,即setLocations(),spring創(chuàng)建完BeanFactory,接下來(lái)要做的就是解析配置,完成其他各種屬性的封裝(BeanDefinition空繁、各種代理殿衰、映射關(guān)系等等)。這里的loadBeanDefinitions就是解析配置的核心方法盛泡。

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);
    }

DefaultListableBeanFactory 作為BeanFactory的一個(gè)核心實(shí)例,且實(shí)現(xiàn)了BeanDefinitionRegistry接口,在解析之前當(dāng)然是要給配置文件解析器(XmlBeanDefinitionReader)設(shè)置一個(gè)存放BeanDefinition的容器(即將注冊(cè)bean的功能委托給BeanFactory)闷祥。然后配置文件解析器就開(kāi)始它的解析工作了。

我們直接定位到XmlBeanDefinitionReader的doLoadBeanDefinitions方法:

try {
            Document doc = doLoadDocument(inputSource, resource);
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }

利用w3c的DOM將配置文件包裝成了一個(gè)Document對(duì)象,接下來(lái)就是利用Dom來(lái)解析childNode相關(guān)屬性傲诵。

接下來(lái),我們沿著代碼定位到DefaultBeaDefinitionDocumentReader中的的parseDefinitions方法:

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        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)) {
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

核心部分來(lái)了,spring迭代根節(jié)點(diǎn)<beans/>,一個(gè)一個(gè)的解析其子節(jié)點(diǎn).如果是<beans/>節(jié)點(diǎn),則走parseDefaultElement方法,我們這里關(guān)注的是parseCustomElement():

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
        String namespaceUri = getNamespaceURI(ele);
        NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
        if (handler == null) {
            error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
            return null;
        }
        return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
    }

根據(jù)元素的namespaceUri（也就是xml文件頭那一串schema）找到對(duì)應(yīng)的NamespaceHandler.【這里就是擴(kuò)展自定義配置的入口】

我們接下來(lái)看一下resolve()方法:

public NamespaceHandler resolve(String namespaceUri) {
        Map<String, Object> handlerMappings = getHandlerMappings();
        Object handlerOrClassName = handlerMappings.get(namespaceUri);
        if (handlerOrClassName == null) {
            return null;
        }
        else if (handlerOrClassName instanceof NamespaceHandler) {
            return (NamespaceHandler) handlerOrClassName;
        }
        else {
            String className = (String) handlerOrClassName;
            try {
                Class<?> handlerClass = ClassUtils.forName(className, this.classLoader);
                if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
                    throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
                            "] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
                }
                NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
                namespaceHandler.init();
                handlerMappings.put(namespaceUri, namespaceHandler);
                return namespaceHandler;
            }
            catch (ClassNotFoundException ex) {
                throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" +
                        namespaceUri + "] not found", ex);
            }
            catch (LinkageError err) {
                throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" +
                        namespaceUri + "]: problem with handler class file or dependent class", err);
            }
        }
    }

對(duì)應(yīng)的NamespaceHandler就是根據(jù)handlerMappings找到匹配關(guān)系的,所以我們只需要看DefaultNamespaceHandlerResolver.getHandlerMappings()即可凯砍。

private Map<String, Object> getHandlerMappings() {
        if (this.handlerMappings == null) {
            synchronized (this) {
                if (this.handlerMappings == null) {
                    try {
                        Properties mappings =
                                PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);//handlerMappingsLocation默認(rèn)的路徑就是META-INF/spring.handlers
                        if (logger.isDebugEnabled()) {
                            logger.debug("Loaded NamespaceHandler mappings: " + mappings);
                        }
                        Map<String, Object> handlerMappings = new ConcurrentHashMap<String, Object>(mappings.size());
                        CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
                        this.handlerMappings = handlerMappings;
                    }
                    catch (IOException ex) {
                        throw new IllegalStateException(
                                "Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
                    }
                }
            }
        }
        return this.handlerMappings;
    }

spring將classpath下的META-INF/spring.handlers中的配置加載到Map中.這個(gè)spring.handler文件就是以NamespaceUrl作為key,對(duì)應(yīng)的Handler作為value的鍵值對(duì),例如:
···
http://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandler
http://www.springframework.org/schema/jee=org.springframework.ejb.config.JeeNamespaceHandler
http://www.springframework.org/schema/lang=org.springframework.scripting.config.LangNamespaceHandler
http://www.springframework.org/schema/task=org.springframework.scheduling.config.TaskNamespaceHandler
http://www.springframework.org/schema/cache=org.springframework.cache.config.CacheNamespaceHandler

···
當(dāng)我們解析到<context/>配置的時(shí)候,我們會(huì)根據(jù)context的schema,也就是[http://www.springframework.org/schema/context]作為key找到對(duì)應(yīng)的ContextNamespacehandler進(jìn)行解析的箱硕，一切都清楚了。

三悟衩、總結(jié)

spring這點(diǎn)做得很強(qiáng)大,利用這種類(lèi)似于Serviceloader的功能來(lái)平等對(duì)待第三方剧罩，方便別人擴(kuò)展集成。我們也要停下來(lái)想一想,當(dāng)我們自己開(kāi)發(fā)這種核心組件的時(shí)候一定要做到"平等對(duì)待第三方"以及核心足夠小,依賴(lài)少座泳。這也是一種修行惠昔！