详解Spring Framework提供的扩展点：ApplicationContextInitializer应用上下文初始化器，以及它在SpringBoot中的应用【享学Spring】（上）

// @since 3.1
public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
  // Initialize the given application context.
  void initialize(C applicationContext);
}

public class ContextLoader {
  ...
  // 它是个list，也就是说可以向此容器注册N上下文初始化器
  private final List<ApplicationContextInitializer<ConfigurableApplicationContext>> contextInitializers = new ArrayList<>();
  // 为啥不用addAll？  因为此处泛型需要强转~
  public void setContextInitializers(@Nullable ApplicationContextInitializer<?>... initializers) {
    if (initializers != null) {
      for (ApplicationContextInitializer<?> initializer : initializers) {
        this.contextInitializers.add((ApplicationContextInitializer<ConfigurableApplicationContext>) initializer);
      }
    }
  }
  // ======================它的执行时机如下;======================
  public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
    ...
    configureAndRefreshWebApplicationContext(cwac, servletContext);
    ...
  }
  protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
    ...
    customizeContext(sc, wac);
    wac.refresh();
    ...
  }
  // 由此可见所有的ApplicationContextInitializer的执行，都发生在wac.refresh();之前
  protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
    //这里是从ServletContext 读取配置
    //因为我们可以这么配置的globalInitializerClasses：ApplicationContextInitializer实现类的全类名（可以逗号分隔配置多个）
    // 或者key是它：contextInitializerClasses
    // 拿到配置的这些全类名后，下面都要反射创建对象的~~~ 
    List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses = determineContextInitializerClasses(sc);
    for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
      Class<?> initializerContextClass = GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
      if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) {
        throw new ApplicationContextException(String.format(
            "Could not apply context initializer [%s] since its generic parameter [%s] " +
            "is not assignable from the type of application context used by this " +
            "context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(),
            wac.getClass().getName()));
      }
      // 创建好实例对象后，添加进全局的list里面（默认使用空构造函数）
      this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
    }
    // 采用order排序后，再分别执行~~~~
    AnnotationAwareOrderComparator.sort(this.contextInitializers);
    for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
      initializer.initialize(wac);
    }
  }
  ...
}

public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {
  private final List<ApplicationContextInitializer<ConfigurableApplicationContext>> contextInitializers = new ArrayList<>();
  // 添加的代码和上面一模一样~
  public void setContextInitializers(@Nullable ApplicationContextInitializer<?>... initializers) {
    if (initializers != null) {
      for (ApplicationContextInitializer<?> initializer : initializers) {
        this.contextInitializers.add((ApplicationContextInitializer<ConfigurableApplicationContext>) initializer);
      }
    }
  }
  // 执行时机
  @Override
  protected final void initServletBean() throws ServletException {
    ...
      this.webApplicationContext = initWebApplicationContext();
      initFrameworkServlet();
    ...
  }
  protected WebApplicationContext initWebApplicationContext() {
    ...
    configureAndRefreshWebApplicationContext(cwac);
    ...
  }
  protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
    ...
    postProcessWebApplicationContext(wac);
    applyInitializers(wac);
    wac.refresh();  
  }
  protected void applyInitializers(ConfigurableApplicationContext wac) {
    // 此处可以看到，只会拿globalInitializerClasses这个key配置的了~~~~ 只有全局的此处才会继续有用
    String globalClassNames = getServletContext().getInitParameter(ContextLoader.GLOBAL_INITIALIZER_CLASSES_PARAM);
    if (globalClassNames != null) {
      for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
        this.contextInitializers.add(loadInitializer(className, wac));
      }
    }
    // 本Servlet里不仅仅可以直接set进来，也可以contextInitializerClasses直接配置全类名
    if (this.contextInitializerClasses != null) {
      for (String className : StringUtils.tokenizeToStringArray(this.contextInitializerClasses, INIT_PARAM_DELIMITERS)) {
        this.contextInitializers.add(loadInitializer(className, wac));
      }
    }
    // 最终排序、执行~~~
    AnnotationAwareOrderComparator.sort(this.contextInitializers);
    for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
      initializer.initialize(wac);
    }
  }
}

// @since 3.1
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
  ...
}

public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {
  // 它最终会被本类的此处调用：listener.setContextInitializers(getRootApplicationContextInitializers());
  // 我们知道ContextLoaderListener所有事情都是委托给了ContextLoader类去完成的~
  @Nullable
  protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
    return null;
  }
}
public abstract class AbstractDispatcherServletInitializer extends AbstractContextLoaderInitializer {
  // 它最终会本类的此处调用dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());
  @Nullable
  protected ApplicationContextInitializer<?>[] getServletApplicationContextInitializers() {
    return null;
  }
}

@Order(10)
public class MyApplicationContextInitializer implements ApplicationContextInitializer {
    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {
        // 该方法此处不能用 因为还没初始化会报错：call 'refresh' before accessing beans via the ApplicationContext
        //int beanDefinitionCount = applicationContext.getBeanDefinitionCount();
        System.out.println(applicationContext.getApplicationName() + "：" + applicationContext.getDisplayName());
    }
}

/**
 * 自己实现 基于注解驱动的ServletInitializer来初始化DispatcherServlet
 */
public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
    @Override
    protected ApplicationContextInitializer<?>[] getServletApplicationContextInitializers() {
        return new ApplicationContextInitializer[]{new MyApplicationContextInitializer()};
    }
    @Override
    protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
        return new ApplicationContextInitializer[]{new MyApplicationContextInitializer()};
    }
    /**
     * 根容器的配置类；（Spring的配置文件）   父容器；
     */
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[]{RootConfig.class, JdbcConfig.class, AsyncConfig.class, ScheduldConfig.class};
    }
    /**
     * web容器的配置类（SpringMVC配置文件）  子容器；
     */
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[]{WebMvcConfig.class};
    }
    ...
}