一、Tomcat引导Spring容器的加载:ContextLoaderListener
- 在我的上一篇文章:Spring源码分析(一):从哪里开始看spring源码,分析过在web容器,如tomcat,启动web应用时,会通过监听器的方式,通知ServletContextListener,web容器开始启动web应用了,ServletContextListener可以自定义初始化逻辑。ContextLoaderListener就是ServletContextListener接口的一个实现类,主要负责加载spring主容器相关的bean,即默认WEB-INF/applicationContext.xml文件的配置信息。
- ContextLoaderListener通过实现ServletContextListener接口,将spring容器融入web容器当中。这个可以分两个角度来理解:
- 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。
- 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的实现如下:
package org.springframework.web.context;
**
* Bootstrap listener to start up and shut down Spring's root {@link WebApplicationContext}.
* Simply delegates to {@link ContextLoader} as well as to {@link ContextCleanupListener}.
*
* <p>As of Spring 3.1, {@code ContextLoaderListener} supports injecting the root web
* application context via the {@link #ContextLoaderListener(WebApplicationContext)}
* constructor, allowing for programmatic configuration in Servlet 3.0+ environments.
* See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
*/
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
*/
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());
}
}
二、加载Spring容器:ContextLoader
- 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。
- initWebApplicationContext方法:创建和初始化spring主容器对应的WebApplicationContext,主要完成两个操作:
- 创建WebApplicationContext对象实例并调用refresh方法完成从contextConfigLocation指定的配置中,加载BeanDefinitions和创建bean实例;核心源码实现如下:
// 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()) {
// 设置parent WebApplicationContext,
// 对root WebApplicationContext来说,通常为null
// 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);
}
}
configureAndRefreshWebApplicationContext方法主要完成对ApplicationContext配置文件地址contextConfigLocation的设值,调用ApplicationContextInitializers,最后调用ApplicationContext的refresh方法完成bean容器的创建。
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
...
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);
}
// 使用ApplicationContextInitializer对ApplicationContext进行初始化
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();
}
- 将创建好的WebApplicationContext实例作为将创建好的WebApplicationContext实例,作为一个attribute保存在ServletContext当中,核心源码实现如下:
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
其中 WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE为:
/**
* Context attribute to bind root WebApplicationContext to on successful startup.
* <p>Note: If the startup of the root context fails, this attribute can contain
* an exception or error as value. Use WebApplicationContextUtils for convenient
* lookup of the root WebApplicationContext.
* @see org.springframework.web.context.support.WebApplicationContextUtils#getWebApplicationContext
* @see org.springframework.web.context.support.WebApplicationContextUtils#getRequiredWebApplicationContext
*/
String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
三、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。