public abstract class AbstractApplicationContext extends DefaultResourceLoader
        implements ConfigurableApplicationContext {
    /**
     * Tell the subclass to refresh the internal bean factory.
     * @return the fresh BeanFactory instance
     * @see #refreshBeanFactory()
     * @see #getBeanFactory()
     */
    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        return getBeanFactory();
    }
    /**
     * Subclasses must implement this method to perform the actual configuration load.
     * The method is invoked by {@link #refresh()} before any other initialization work.
     * <p>A subclass will either create a new bean factory and hold a reference to it,
     * or return a single BeanFactory instance that it holds. In the latter case, it will
     * usually throw an IllegalStateException if refreshing the context more than once.
     * @throws BeansException if initialization of the bean factory failed
     * @throws IllegalStateException if already initialized and multiple refresh
     * attempts are not supported
     */
    protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;
    /**
     * Subclasses must return their internal bean factory here. They should implement the
     * lookup efficiently, so that it can be called repeatedly without a performance penalty.
     * <p>Note: Subclasses should check whether the context is still active before
     * returning the internal bean factory. The internal factory should generally be
     * considered unavailable once the context has been closed.
     * @return this application context's internal bean factory (never {@code null})
     * @throws IllegalStateException if the context does not hold an internal bean factory yet
     * (usually if {@link #refresh()} has never been called) or if the context has been
     * closed already
     * @see #refreshBeanFactory()
     * @see #closeBeanFactory()
     */
    @Override
    public abstract ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;
}

public class GenericApplicationContext extends AbstractApplicationContext implements BeanDefinitionRegistry {
    // 和 BeanDefinitionRegistry 相關的方法都委托給了 `beanFactory` 這個成員變量
    private final DefaultListableBeanFactory beanFactory;
    // 狀態(tài)位: 當前容器是不是已經(jīng) `刷新`過了, 也就是說 GenericApplicationContext 是不支持多次刷新操作的
    private final AtomicBoolean refreshed = new AtomicBoolean();
    /**
     * Do nothing: We hold a single internal BeanFactory and rely on callers
     * to register beans through our public methods (or the BeanFactory's).
     * @see #registerBeanDefinition
     */
    @Override
    protected final void refreshBeanFactory() throws IllegalStateException {
        if (!this.refreshed.compareAndSet(false, true)) {
            throw new IllegalStateException(
                    "GenericApplicationContext does not support multiple refresh attempts: just call 'refresh' once");
        }
        this.beanFactory.setSerializationId(getId());
    }
    @Override
    public final ConfigurableListableBeanFactory getBeanFactory() {
        return this.beanFactory;
    }
}

public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext {
    // 是否允許覆蓋重復的 Bean 定義信息
    @Nullable
    private Boolean allowBeanDefinitionOverriding;
    // 當前容器是不是要支持循環(huán)依賴(spring-boot-2.6中默認禁用)
    @Nullable
    private Boolean allowCircularReferences;
    // 刷新之前可能已經(jīng)存在的一個 beanFactory
    // 每次刷新都會將當前 beanFactory 銷毀重建
    @Nullable
    private volatile DefaultListableBeanFactory beanFactory;
    /**
     * This implementation performs an actual refresh of this context's underlying
     * bean factory, shutting down the previous bean factory (if any) and
     * initializing a fresh bean factory for the next phase of the context's lifecycle.
     */
    @Override
    protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) { // this.beanFactory != null: 刷新之前已經(jīng)有一個 beanFactory 
            // 銷毀舊的 beanFactory
            // 1. 調用的實際是: getBeanFactory().destroySingletons();
            destroyBeans();
            // 2. this.beanFactory = null;
            closeBeanFactory();
        }
        try {
            // 1. 重新創(chuàng)建一個 beanFactory
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            // 2.
            beanFactory.setSerializationId(getId());
            // 3. 實際上是給 allowBeanDefinitionOverriding 和 allowCircularReferences 賦值
            customizeBeanFactory(beanFactory);
            // 4. 這是一個抽象方法: 就是給新創(chuàng)建的 beanFactory 中加載 `BeanDefinition`
            // BeanDefinition 的加載一般都是在子類中委托給了各種 `BeanDefinitionReader`
            loadBeanDefinitions(beanFactory);
            this.beanFactory = beanFactory;
        } catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }
    @Override
    protected final void closeBeanFactory() {
        DefaultListableBeanFactory beanFactory = this.beanFactory;
        if (beanFactory != null) {
            beanFactory.setSerializationId(null);
            this.beanFactory = null;
        }
    }
    protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
        if (this.allowBeanDefinitionOverriding != null) {
            beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
        }
        if (this.allowCircularReferences != null) {
            beanFactory.setAllowCircularReferences(this.allowCircularReferences);
        }
    }
    protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
            throws BeansException, IOException;
    @Override
    public final ConfigurableListableBeanFactory getBeanFactory() {
        DefaultListableBeanFactory beanFactory = this.beanFactory;
        if (beanFactory == null) {
            throw new IllegalStateException("BeanFactory not initialized or already closed - " +
                    "call 'refresh' before accessing beans via the ApplicationContext");
        }
        return beanFactory;
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <context-param>
        <!-- 主要關注一下這個配置項, 如果不配置就從 spring-web.jar 的 `org.springframework.web.context.ContextLoader.ContextLoader.properties` 文件中獲取 -->
        <param-name>contextClass</param-name>
        <param-value>org.springframework.web.context.support.XmlWebApplicationContext</param-value>
    </context-param>
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
    <!-- 省略其他配置 -->
    <!-- 省略其他配置 -->
    <!-- 省略其他配置 -->
</web-app>

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    // 就是在這里初始化 ApplicationContext 的
    @Override
    public void contextInitialized(ServletContextEvent event) {
        // 父類 ContextLoader 中的方法
        initWebApplicationContext(event.getServletContext());
    }
}

public class ContextLoader {
    private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
    private static final Properties defaultStrategies;
    static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            // 去 classpath 下加載 `ContextLoader.properties` 
            // 這個文件在 spring-web.jar 的 `org.springframework.web.context.ContextLoader.ContextLoader.properties`
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        } catch (IOException ex) {
            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
        }
    }
    // 這里就是創(chuàng)建具體的 ApplicationContext 實例
    // 因為是 web 環(huán)境，所以創(chuàng)建的是 `WebApplicationContext` 的實現(xiàn)類的實例
    protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
        // 這里才是確定到底創(chuàng)建什么類型的 `WebApplicationContext`
        Class<?> contextClass = determineContextClass(sc);
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }
    protected Class<?> determineContextClass(ServletContext servletContext) {
        // CONTEXT_CLASS_PARAM常量值就是: contextClass(在 web.xml 中配置的那個)
        // 1. 如果你指定了 `contextClass` 就使用你指定的 `WebApplicationContext` 實現(xiàn)類
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            } catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load custom context class [" + contextClassName + "]", ex);
            }
        }
        // 2. 如果沒有指定 `contextClass` 配置就使用  `defaultStrategies` 來
        else {
            contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
            try {
                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
            } catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load default context class [" + contextClassName + "]", ex);
            }
        }
    }
}

# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.
# 指定默認的 `WebApplicationContext` 的實現(xiàn)類是： `XmlWebApplicationContext`
org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

@FunctionalInterface
public interface ApplicationContextFactory {
    // 省略幾個 default 方法
    /**
     * Creates the {@link ConfigurableApplicationContext application context} for a
     * {@link SpringApplication}, respecting the given {@code webApplicationType}.
     * @param webApplicationType the web application type
     * @return the newly created application context
     */
    ConfigurableApplicationContext create(WebApplicationType webApplicationType);
}