Spring中的ContextLoaderListener详细解析
作者:遇见更好的自己、
前言
我们已经了解到,在web容器即Tomact容器启动web应用即servlet应用时,会触发ServletContextEvent时间,这个事件会被ServletContextListener监听,监听到了以后会通过ContextLoaderListener监听器(ServletContextListener的一个实现类)去初始化Spring容器其实就是去初始化一些Bean。即默认WEB-INF/applicationContext.xml文件的配置信息。
关于有如下两点需要理解:
1.web项目自身:接收web容器启动web应用的通知,开始自身配置的解析加载,创建bean实例,通过一个WebApplicationContext来维护spring项目的主容器相关的bean,以及其他一些组件。
2.web容器:web容器使用ServletContext来维护每一个web应用,ContextLoaderListener将spring容器,即WebApplicationContext,作为ServletContext的一个attribute,key为WebApplicationContext.class.getName() + “.ROOT”,保存在ServletContext中,从而web容器和spring项目可以通过ServletContext来交互。
ContextLoaderListener只是作为一个中间层来建立spring容器和web容器的关联关系,而实际完成以上两个角度的工作是通过ContextLoader来进行的,即在ContextLoader中定义以上逻辑,
ContextLoaderListener的实现
ContextLoaderListener的源码如下:
public class ContextLoaderListener extends ContextLoader implements ServletContextListener { /** * Create a new {@code ContextLoaderListener} that will create a web application * context based on the "contextClass" and "contextConfigLocation" servlet * context-params. See {@link ContextLoader} superclass documentation for details on * default values for each. * <p>This constructor is typically used when declaring {@code ContextLoaderListener} * as a {@code <listener>} within {@code web.xml}, where a no-arg constructor is * required. * <p>The created 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. * @see ContextLoader * @see #ContextLoaderListener(WebApplicationContext) * @see #contextInitialized(ServletContextEvent) * @see #contextDestroyed(ServletContextEvent) */ public ContextLoaderListener() { } /** * 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); } /** * Initialize the root web application context. */ @Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); } /** * Close the root web application context. */ @Override public void contextDestroyed(ServletContextEvent event) { closeWebApplicationContext(event.getServletContext()); ContextCleanupListener.cleanupAttributes(event.getServletContext()); }
可以看到ContextLoaderListener继承了ServletContextListener并实现了ContextLoader类。
在web容器启动的时候,ContextLoaderListener就会监听到。
并将监听事件传入到方法:contextInitialized(ServletContextEvent event)中,然后,调用了父类的方法去初始化initWebApplicationContext:
Spring容器初始化
接下来我们的重点是Spring容器是如何初始化的:
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) { // 创建一个WebApplicationContext // 具体类型如果是指定了contextClass则使用该指定的; // 默认使用XmlWebApplicationContext 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, etc if (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); } // 核心方法,完成配置加载,BeanDefinition定义和bean对象创建 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; } }
在看上述代码前,我们需要了解ContextLoader的一些重要含义
ContextLoader主要负责加载spring主容器,即root ApplicationContext,在设计层面主要定义了contextId,contextConfigLocation,contextClass,contextInitializerClasses。这些参数都可以在配置中指定,如web.xml的context-param标签,或者是基于Java编程方式配置的WebApplicationInitializer中定义,作为分别为:contextId:当前容器的id,主要给底层所使用的BeanFactory,在进行序列化时使用。
- contextConfigLocation:配置文件的位置,默认为WEB-INF/applicationContext.xml,可以通过在web.xml使用context-param标签来指定其他位置,其他名字或者用逗号分隔指定多个。在配置文件中通过beans作为主标签来定义bean。这样底层的BeanFactory会解析beans标签以及里面的bean,从而来创建BeanDefinitions集合,即bean的元数据内存数据库。
- contextClass:当前所使用的WebApplicationContext的类型,如果是在WEB-INF/applicationContext.xml中指定beans,则使用XmlWebApplicationContext,如果是通过注解,如@Configuration,@Component等,则是AnnotationConfigWebApplicationContext,通过扫描basePackages指定的包来创建bean。
- contextInitializerClasses:ApplicationContextInitializer的实现类,即在调用ApplicationContext的refresh加载beanDefinition和创建bean之前,对WebApplicationContext进行一些初始化
configureAndRefreshWebApplicationContext源码如下:
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 information String 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 #refresh ConfigurableEnvironment env = wac.getEnvironment(); if (env instanceof ConfigurableWebEnvironment) { ((ConfigurableWebEnvironment) env).initPropertySources(sc, null); } customizeContext(sc, wac); // ApplicationContext的核心方法:在refresh方法中完成ApplicationContext的启动 // 即spring容器的各个核心组件的创建,如beanDefinitions,enviromnet等 wac.refresh(); }
总结
通过上面的分析可知,ContextLoader完成spring主容器的创建,工作主要是负责从ServletContext中,具体为ServletContext从web.xml文件或者WebApplicationInitializer的实现类中,获取WebApplicationContext的相关配置信息,如使用contextClass指定使用哪种WebApplicationContext实现,contextConfigLocation指定spring容器的配置文件在哪里,以及获取WebApplicationContextInitializers来在进行spring容器创建之前,对WebApplicationContext进行加工处理。
而spring容器的创建,则是使用spring-context包的ApplicationContext,spring-beans包的BeanFactory,来完成从配置中获取beans定义并创建BeanDefinition,获取一些资源属性值,以及完成单例bean的创建等。具体在之后关于ApplicationContext和BeanFactory体系结构的文章中分析。
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