首先我们知道spring的启动入口在web.xml里面的一个监听器里

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

然后我们找到它的启动方法

public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
}

里面有一个configureAndRefreshWebApplicationContext()方法代表着启动和刷新容器，我们看到最后一行wac.refresh();这里面就是启动整个spring容器的代码。
我们找到ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); -> refreshBeanFactory(); ->loadBeanDefinitions(beanFactory);
loadBeanDefinitions就是实现加载我们xml里面的bean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(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);
    }

我们可以看到大致流程就是先拿到解析xml的Reader，最后调用loadBeanDefinitions(beanDefinitionReader);,

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            for (String configLocation : configLocations) {
                reader.loadBeanDefinitions(configLocation);
            }
        }
    }

这里面是把我们在web.xml里面配置的xml路径

<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath*:spring/spring-context.xml</param-value>
  </context-param>

读取出来挨个解析,接着我们可以一直往下跟，AbstractBeanDefinitionReader.loadBeanDefinitions(Resource... resources)->XmlBeanDefinitionReader.loadBeanDefinitions();->doLoadBeanDefinitions();->doLoadDocument(inputSource, resource)解析xml节点->registerBeanDefinitions(doc, resource)注册bean->documentReader.registerBeanDefinitions(doc, createReaderContext(resource))我们注意这里有个createReaderContext(resource)这个方法会加载解析xml标签的类，比如<bean/> <context:component-scan/>等等 ，如下图

image.png

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);
        }
    }

delegate.isDefaultNamespace(ele)这边我们先判断一下是不是http://www.springframework.org/schema/beans这个xsd里面的标签，其中包括beans,import,alias,bean。如果不在这几个里面，再去获取特殊的标签handler

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));
    }

对应的handler就会解析相应的标签里面要生成的bean。
举例ComponentScanBeanDefinitionParser (解析component-scan标签下的bean)

public BeanDefinition parse(Element element, ParserContext parserContext) {
        String basePackage = element.getAttribute(BASE_PACKAGE_ATTRIBUTE);
        basePackage = parserContext.getReaderContext().getEnvironment().resolvePlaceholders(basePackage);//获取包路径集合
        String[] basePackages = StringUtils.tokenizeToStringArray(basePackage,
                ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);

        // Actually scan for bean definitions and register them.
        ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
        Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);//解析标签填的包下的class,
        registerComponents(parserContext.getReaderContext(), beanDefinitions, element);

        return null;
    }

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
        Assert.notEmpty(basePackages, "At least one base package must be specified");
        Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
        for (String basePackage : basePackages) {
            Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
            for (BeanDefinition candidate : candidates) {
                ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
                candidate.setScope(scopeMetadata.getScopeName());
                String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);//获取beanname
                if (candidate instanceof AbstractBeanDefinition) {
                    postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
                }
                if (candidate instanceof AnnotatedBeanDefinition) {
                    AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
                }
                if (checkCandidate(beanName, candidate)) {//检验bean是否已经存在
                    BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                    definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                    beanDefinitions.add(definitionHolder);
                    registerBeanDefinition(definitionHolder, this.registry);//注入bean 添加到beanDefinitionMap里面
                }
            }
        }
        return beanDefinitions;
    }