Spring MVC源码分析（一）：ContextLoaderListener的设计与实现

ContextLoaderListener
在我的Spring源码分析（一）：从哪里开始看spring源码这篇文章，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，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());
    }
}

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：

1.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。

2.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。

3.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。

4.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：

1.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();
    }

2.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";

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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Spring MVC源码分析（一）：ContextLoaderListener的设计与实现

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