spring源码：入口

概述

我相信很多人都像我一样迷惑，spring到底从哪里开始看，大多数视频，资料等，讲述从 AbstractWebApplicationContext.refresh 方法开始看，或给出一个这样的例子，

ApplicationContext context = new ClassPathXmlApplicationContext("services.xml", "daos.xml");

开始debug，但仔细想一想，我们的项目是放在 tomcat 中运行的，tomcat 是如何加载 spring 的？又是怎么调用 refresh 方法的？是什么标志 spring 已经被启动？这些问题我们接下来深入了解。

寻找入口

在 web.xml 中，我们要配置一个监听器：ContextLoaderListener。关于 tomcat 是如何加载 web.xml，并启动监听器的，大家自己百度下，没有研究过 tomcat 源码就不做叙述了，到时候抽时间会给大家补上。

这段注释说明了此处关联了 spring，意思是说：辅助监听启动和关闭 spring 的根（WebApplicationContext），简单地依托 ContextLoader 和 ContextCleanupListener。自从 spring3.1起，支持注入底层应用上下文，凭借ContextLoaderListener(WebApplicationContext) 构造函数。在Servlet3.0+环境中允许编程式配置。

提到的构造函数：

/*** Create a new {@code ContextLoaderListener} with the given application context. This* constructor is useful in Servlet 3.0+ environments where instance-based* registration of listeners is possible through the {@link javax.servlet.ServletContext#addListener}* API.* <p>The context may or may not yet be {@linkplain* org.springframework.context.ConfigurableApplicationContext#refresh() refreshed}. If it* (a) is an implementation of {@link ConfigurableWebApplicationContext} and* (b) has <strong>not</strong> already been refreshed (the recommended approach),* then the following will occur:* <ul>* <li>If the given context has not already been assigned an {@linkplain* org.springframework.context.ConfigurableApplicationContext#setId id}, one will be assigned to it</li>* <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to* the application context</li>* <li>{@link #customizeContext} will be called</li>* <li>Any {@link org.springframework.context.ApplicationContextInitializer ApplicationContextInitializer org.springframework.context.ApplicationContextInitializer ApplicationContextInitializers}* specified through the "contextInitializerClasses" init-param will be applied.</li>* <li>{@link org.springframework.context.ConfigurableApplicationContext#refresh refresh()} will be called</li>* </ul>* If the context has already been refreshed or does not implement* {@code ConfigurableWebApplicationContext}, none of the above will occur under the* assumption that the user has performed these actions (or not) per his or her* specific needs.* <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.* <p>In any case, the given application context will be registered into the* ServletContext under the attribute name {@link* WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} and the Spring* application context will be closed when the {@link #contextDestroyed} lifecycle* method is invoked on this listener.* @param context the application context to manage* @see #contextInitialized(ServletContextEvent)* @see #contextDestroyed(ServletContextEvent)*/public ContextLoaderListener(WebApplicationContext context) {super(context);}

英文不是很好，你们用有道词典试着翻译下，我们在浏览的过程中看到有许多 servlet 相关的东西，如果不熟悉看下 servlet3.1规范。我们学习源码以官方文档为主！！！

进入父类（ContextLoader）构造方法：

public ContextLoader(WebApplicationContext context) {this.context = context;
}

到这里大家可能会疑惑，在哪里启动？当 tomcat 加载监听器时，会进行初始化，找到初始化函数 ContextLoaderListener.contextinitialized 函数：

/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {initWebApplicationContext(event.getServletContext());
}

进入父类的 ContextLoader.initWebApplicationContext 函数：

/*** Initialize Spring's web application context for the given servlet context,* using the application context provided at construction time, or creating a new one* according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and* "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.* @param servletContext current servlet context* @return the new WebApplicationContext* @see #ContextLoader(WebApplicationContext)* @see #CONTEXT_CLASS_PARAM* @see #CONFIG_LOCATION_PARAM*/public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");}servletContext.log("Initializing Spring root WebApplicationContext");Log logger = LogFactory.getLog(ContextLoader.class);if (logger.isInfoEnabled()) {logger.info("Root WebApplicationContext: initialization started");}long startTime = System.currentTimeMillis();try {// Store context in local instance variable, to guarantee that// it is available on ServletContext shutdown.if (this.context == null) {this.context = createWebApplicationContext(servletContext);}if (this.context instanceof ConfigurableWebApplicationContext) {ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;if (!cwac.isActive()) {// The context has not yet been refreshed -> provide services such as// setting the parent context, setting the application context id, etcif (cwac.getParent() == null) {// The context instance was injected without an explicit parent ->// determine parent for root web application context, if any.ApplicationContext parent = loadParentContext(servletContext);cwac.setParent(parent);}configureAndRefreshWebApplicationContext(cwac, servletContext);}}servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);ClassLoader ccl = Thread.currentThread().getContextClassLoader();if (ccl == ContextLoader.class.getClassLoader()) {currentContext = this.context;}else if (ccl != null) {currentContextPerThread.put(ccl, this.context);}if (logger.isInfoEnabled()) {long elapsedTime = System.currentTimeMillis() - startTime;logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");}return this.context;}catch (RuntimeException | Error ex) {logger.error("Context initialization failed", ex);servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);throw ex;}}

其中有两个主要方法，createWebApplicationContext 和 configureAndRefreshWebApplicationContext，这里不细述这些逻辑，主要是找到 spring 的入口，等学 springmvc 时我们再回来细说。

/*** Instantiate the root WebApplicationContext for this loader, either the* default context class or a custom context class if specified.* <p>This implementation expects custom contexts to implement the* {@link ConfigurableWebApplicationContext} interface.* Can be overridden in subclasses.* <p>In addition, {@link #customizeContext} gets called prior to refreshing the* context, allowing subclasses to perform custom modifications to the context.* @param sc current servlet context* @return the root WebApplicationContext* @see ConfigurableWebApplicationContext*/
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {Class<?> contextClass = determineContextClass(sc);if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {throw new ApplicationContextException("Custom context class [" + contextClass.getName() +"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");}return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

BeanUtils.instantiateClass 方法主要用到了反射，返回一个 contextClass 类型的类并强转成 ConfigurableWebApplicationContext 类型。

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {if (ObjectUtils.identityToString(wac).equals(wac.getId())) {// The application context id is still set to its original default value// -> assign a more useful id based on available informationString idParam = sc.getInitParameter(CONTEXT_ID_PARAM);if (idParam != null) {wac.setId(idParam);}else {// Generate default id...wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +ObjectUtils.getDisplayString(sc.getContextPath()));}}wac.setServletContext(sc);String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);if (configLocationParam != null) {wac.setConfigLocation(configLocationParam);}// The wac environment's #initPropertySources will be called in any case when the context// is refreshed; do it eagerly here to ensure servlet property sources are in place for// use in any post-processing or initialization that occurs below prior to #refreshConfigurableEnvironment env = wac.getEnvironment();if (env instanceof ConfigurableWebEnvironment) {((ConfigurableWebEnvironment) env).initPropertySources(sc, null);}customizeContext(sc, wac);wac.refresh();
}

wac.refresh 方法会跳入 AbstractApplicationContext.refrsh 中，我们算是找到了spring的入口。

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spring源码：入口

概述

寻找入口

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