//@WebAppConfiguration是一个类级注释，始于Spring3.2，用于声明集成测试加载的ApplicationContext应该是WebApplicationContext。
//Spring会以value属性（默认为"src/main/webapp"）指定的目录路径来为测试加载WebApplicationContext。要覆盖默认值，请通过value属性指定一个显式资源路径。
//请注意， @WebAppConfiguration必须在单个测试类中或在测试类层次结构中与@ContextConfiguration结合使用。
//从Spring Framework 4.0开始，此注释可用作创建自定义组成的注释的元注释。
@Documented
@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface WebAppConfiguration {
	//指向Web应用程序根目录的资源路径。
	//不包括Spring资源前缀的路径(例如，classpath:, file:等等)将被解释为文件系统资源，并且该路径不应以斜杠结尾。
	//也可以指向classpath中的资源路径，而不是文件系统。eg.@WebAppConfiguration("classpath:testResources")
	//默认以"src/main/webapp"作为文件系统资源。 请注意，这是Web应用程序根目录的标准目录，该目录遵循WAR的标准Maven项目布局。
	String value() default "src/main/webapp";
}

//该接口为Web应用程序提供配置。在应用程序运行时，它是只读的，但是如果实现支持，则可以重新加载。
//此接口给通用的ApplicationContext接口添加了getServletContext()方法，并定义了一个众所周知的应用程序属性名称，在引导过程中必须将根上下文绑定到该名称。
//像通用应用程序上下文一样，Web应用程序上下文是分层的。 每个应用程序只有一个root context，而应用程序中的每个servlet（包括MVC框架中的调度程序servlet）都有自己的子上下文。
//除了标准的应用程序上下文生命周期功能外，WebApplicationContext的实现类还需要检测ServletContextAware Bean并相应地调用setServletContext方法。
public interface WebApplicationContext extends ApplicationContext {
	String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
	String SCOPE_REQUEST = "request";
	//......
	String SERVLET_CONTEXT_BEAN_NAME = "servletContext";
	//......
	//返回此应用程序的标准Servlet API ServletContext。除PortletContext外，它还可用于Portlet应用程序。
	ServletContext getServletContext();
}

/**
 * 是BeanPostProcessor接口的实现类，将ServletContext传递给实现了ServletContextAware接口的Bean。
 * web应用程序上下文（Web application contexts）将自动将这个类注册到其底层bean工厂。 应用程序不直接使用它。
 * 
 * @see org.springframework.web.context.ServletContextAware
 * @see org.springframework.web.context.support.XmlWebApplicationContext#postProcessBeanFactory
 */
public class ServletContextAwareProcessor implements BeanPostProcessor {
	private ServletContext servletContext;
	private ServletConfig servletConfig;
	
}

public interface ServletContextAware extends Aware {
	/**
	 * Set the {@link ServletContext} that this object runs in.
	 * <p>Invoked after population of normal bean properties but before an init
	 * callback like InitializingBean's {@code afterPropertiesSet} or a
	 * custom init-method. Invoked after ApplicationContextAware's
	 * {@code setApplicationContext}.
	 * @param servletContext ServletContext object to be used by this object
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 * @see org.springframework.context.ApplicationContextAware#setApplicationContext
	 */
	void setServletContext(ServletContext servletContext);
}

public abstract class AbstractRefreshableWebApplicationContext extends AbstractRefreshableConfigApplicationContext
		implements ConfigurableWebApplicationContext, ThemeSource {
	/**
	 * Register request/session scopes, a {@link ServletContextAwareProcessor}, etc.
	 */
	@Override
	protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));
		beanFactory.ignoreDependencyInterface(ServletContextAware.class);
		beanFactory.ignoreDependencyInterface(ServletConfigAware.class);

		//注册web应用的所有域
		WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);
		WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);
	}
}

public abstract class WebApplicationContextUtils {
	/**
	 * Register web-specific scopes ("request", "session", "globalSession", "application")
	 * with the given BeanFactory, as used by the WebApplicationContext.
	 * @param beanFactory the BeanFactory to configure
	 * @param sc the ServletContext that we're running within
	 */
	public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, ServletContext sc) {
		beanFactory.registerScope(WebApplicationContext.SCOPE_REQUEST, new RequestScope());
		beanFactory.registerScope(WebApplicationContext.SCOPE_SESSION, new SessionScope(false));
		beanFactory.registerScope(WebApplicationContext.SCOPE_GLOBAL_SESSION, new SessionScope(true));
		if (sc != null) {
			ServletContextScope appScope = new ServletContextScope(sc);
			beanFactory.registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
			// Register as ServletContext attribute, for ContextCleanupListener to detect it.
			sc.setAttribute(ServletContextScope.class.getName(), appScope);
		}

		//注册ServletRequest bean！
		beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
		beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
		beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
		beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
		if (jsfPresent) {
			FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
		}
	}
}

public abstract class WebApplicationContextUtils {
	/**
	 * Factory that exposes the current request object on demand.
	 */
	@SuppressWarnings("serial")
	private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable {

		@Override
		public ServletRequest getObject() {
			return currentRequestAttributes().getRequest();
		}

		@Override
		public String toString() {
			return "Current HttpServletRequest";
		}
	}
}

Caused by: java.lang.IllegalArgumentException: Cannot load an ApplicationContext with a NULL 'contextLoader'. Consider annotating your test class with @ContextConfiguration or @ContextHierarchy.
	at org.springframework.util.Assert.notNull(Assert.java:112)
	at org.springframework.test.context.CacheAwareContextLoaderDelegate.loadContextInternal(CacheAwareContextLoaderDelegate.java:57)
	at org.springframework.test.context.CacheAwareContextLoaderDelegate.loadContext(CacheAwareContextLoaderDelegate.java:91)
	... 26 more