Spring实现内置监听器

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>5.2.5.RELEASE</version>
</dependency>

<!--注册监听器 ContextLoaderListener-->
<!--
    监听器被创建对象后，会读取/WEB-INF/applicationContext.xml
    可以修改默认的文件位置，使用context-param重新指定文件位置
-->
<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:applicationContext.xml</param-value>
</context-param>
<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

try {
    if (this.context == null) {
        this.context = this.createWebApplicationContext(servletContext);
    }
    if (this.context instanceof ConfigurableWebApplicationContext) {
        ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)this.context;
        if (!cwac.isActive()) {
            if (cwac.getParent() == null) {
                ApplicationContext parent = this.loadParentContext(servletContext);
                cwac.setParent(parent);
            }
            this.configureAndRefreshWebApplicationContext(cwac, servletContext);
        }
    }
    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

WebApplicationContext context = null;
Object attr = getServletContext().getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
if (attr != null){
    context = (WebApplicationContext)attr;
}

public static WebApplicationContext getRequiredWebApplicationContext(ServletContext sc) throws IllegalStateException {
    WebApplicationContext wac = getWebApplicationContext(sc);
    if (wac == null) {
        throw new IllegalStateException("No WebApplicationContext found: no ContextLoaderListener registered?");
    } else {
        return wac;
    }
}
@Nullable
public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
    return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}
@Nullable
public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
    Assert.notNull(sc, "ServletContext must not be null");
    Object attr = sc.getAttribute(attrName);

ServletContext sc = getServletContext();
WebApplicationContext context = WebApplicationContextUtils.getWebApplicationContext(sc);