一、问题

我们在spring的xml配置文件里经常定义各种各样的配置（tx、bean、mvc、bean等等）。以及集成第三方框架时,也会看到一些spring之外的配置,例如dubbo的配置、security的配置、redis的配置等等。spring作为一个庞大的容器,到底是怎么工作的呢？这篇文章将简单的介绍一下其实现机制,具体细节朋友们可以自己查看相关代码。

二、代码分析

稍微了解一点spring的朋友都知道spring的入口方法就是refresh(),所以我们直接进入refresh()方法【不清楚的可以查看一下ClassPathXmlApplicationContext,一步一步跟踪进去】。

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);//就是这里
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

我们首先会告诉spring的配置文件所在路径,即setLocations(),spring创建完BeanFactory,接下来要做的就是解析配置,完成其他各种属性的封装(BeanDefinition、各种代理、映射关系等等)。这里的loadBeanDefinitions就是解析配置的核心方法。

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 作为BeanFactory的一个核心实例,且实现了BeanDefinitionRegistry接口,在解析之前当然是要给配置文件解析器(XmlBeanDefinitionReader)设置一个存放BeanDefinition的容器(即将注册bean的功能委托给BeanFactory)。然后配置文件解析器就开始它的解析工作了。

我们直接定位到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将配置文件包装成了一个Document对象,接下来就是利用Dom来解析childNode相关属性。

接下来,我们沿着代码定位到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);
        }
    }

核心部分来了,spring迭代根节点<beans/>,一个一个的解析其子节点.如果是<beans/>节点,则走parseDefaultElement方法,我们这里关注的是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));
    }

根据元素的namespaceUri（也就是xml文件头那一串schema）找到对应的NamespaceHandler.【这里就是扩展自定义配置的入口】

我们接下来看一下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);
            }
        }
    }

对应的NamespaceHandler就是根据handlerMappings找到匹配关系的,所以我们只需要看DefaultNamespaceHandlerResolver.getHandlerMappings()即可。

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默认的路径就是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将classpath下的META-INF/spring.handlers中的配置加载到Map中.这个spring.handler文件就是以NamespaceUrl作为key,对应的Handler作为value的键值对,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

···
当我们解析到<context/>配置的时候,我们会根据context的schema,也就是[http://www.springframework.org/schema/context]作为key找到对应的ContextNamespacehandler进行解析的，一切都清楚了。

三、总结

spring这点做得很强大,利用这种类似于Serviceloader的功能来平等对待第三方，方便别人扩展集成。我们也要停下来想一想,当我们自己开发这种核心组件的时候一定要做到"平等对待第三方"以及核心足够小,依赖少。这也是一种修行！