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】

// @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};
    }
    ...
}

/demo_war_war：Root WebApplicationContext
/demo_war_war：WebApplicationContext for namespace 'dispatcher-servlet'

public class SpringApplication {

	// 对象初始化的时候 会从spring.factories里拿出来
	private void initialize(Object[] sources) {
		...
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		...
	}

	public ConfigurableApplicationContext run(String... args) {
		...
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
		...
	}

	private void prepareContext(ConfigurableApplicationContext context,
			ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments, Banner printedBanner) {
		...
		applyInitializers(context);
		...
	}

	// 执行处  把所有的遍历执行（已经排序）getInitializers里会根据order进行排序
	protected void applyInitializers(ConfigurableApplicationContext context) {
		for (ApplicationContextInitializer initializer : getInitializers()) {
			Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
					initializer.getClass(), ApplicationContextInitializer.class);
			Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
			initializer.initialize(context);
		}
	}
	
}

context.initializer.classes=你的实现类全类名

	public static void main(String[] args) {
		//type01
		SpringApplication springApplication = new SpringApplication(Application.class);
		// 添加进去
		springApplication.addInitializers(new Demo01ApplicationContextInitializer());
		springApplication.run(args);
 		//SpringApplication.run(InitializerDemoApplication.class,args);
	}