在何处执行？

执行时机在SpringApplication类的prepareContenxt中，在run方法中可以找到prepareContext方法。

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments, Banner printedBanner) {...// 执行applyInitializersapplyInitializers(context);...
}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);}
}

何时初始化？

在SpringApplication构造函数中会初始化initializers。

@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {...// 初始化initializerssetInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));...
}private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {return getSpringFactoriesInstances(type, new Class<?>[] {});
}private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {ClassLoader classLoader = getClassLoader();// 在META-INF/spring.factories中寻找实现类。Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);AnnotationAwareOrderComparator.sort(instances);return instances;
}

自己写一个ApplicationContextInitializer

public class OrangeInitializer implements ApplicationContextInitializer {@Overridepublic void initialize(ConfigurableApplicationContext applicationContext) {System.out.println("my initializer");}
}

在resources的META-INF中创建一个spring.factories文件，下面是我的例子，具体的路径还是看自己的。

org.springframework.context.ApplicationContextInitializer=\com.example.spring.spring.listener.OrangeInitializer

可以看到启动SpringBoot之后，my initializer打印出来了。

那这个类的设计具体有什么作用呢？？

想要知道这里类的作用，切入点就是看看它的实现类都干了什么事情，下面八个类，有一个OrangeInitializer是自定义的，其他七个是SpringBoot启动就有的。当然也可以从名称上去猜测，他就是用来初始化ApplicationContext的。看一下它的默认实现都干了什么事情！

1. DelegatingApplicationContextInitializer

private static final String PROPERTY_NAME = "context.initializer.classes";@Override
public void initialize(ConfigurableApplicationContext context) {ConfigurableEnvironment environment = context.getEnvironment();// 获取对象List<Class<?>> initializerClasses = getInitializerClasses(environment);// 执行具体的ApplicationContextInitializerif (!initializerClasses.isEmpty()) {applyInitializerClasses(context, initializerClasses);}
}private List<Class<?>> getInitializerClasses(ConfigurableEnvironment env) {// 从Environment中拿context.initializer.classes。String classNames = env.getProperty(PROPERTY_NAME);List<Class<?>> classes = new ArrayList<>();if (StringUtils.hasLength(classNames)) {for (String className : StringUtils.tokenizeToStringArray(classNames, ",")) {classes.add(getInitializerClass(className));}}return classes;
}

通过配置文件去注入ApplicationContextInitializer。

public class EnvironmentInitializer implements ApplicationContextInitializer {@Overridepublic void initialize(ConfigurableApplicationContext applicationContext) {System.out.println("my environmentInitializer");}
}

application.yml如果自己有多个类，可以用英文逗号隔开

context:initializer:classes: com.example.spring.spring.initializer.EnvironmentInitializer

2. SharedMetadataReaderFactoryContextInitializer

这个就简单多了，就把一个BeanFactoryPostProcessor放入到ConfigurableApplicationContext中，会在AbstractApplicationContext的refresh的invokeBeanFactoryPostProcessors去执行。

@Override
public void initialize(ConfigurableApplicationContext applicationContext) {BeanFactoryPostProcessor postProcessor = new CachingMetadataReaderFactoryPostProcessor(applicationContext);applicationContext.addBeanFactoryPostProcessor(postProcessor);
}

3. ContextIdApplicationContextInitializer

也不知道作用是啥。

@Override
public void initialize(ConfigurableApplicationContext applicationContext) {ContextId contextId = getContextId(applicationContext);applicationContext.setId(contextId.getId());applicationContext.getBeanFactory().registerSingleton(ContextId.class.getName(), contextId);
}

4. ConfigurationWarningsApplicationContextInitializer

也是添加一个BeanFactoryPostProcessor处理器。

@Override
public void initialize(ConfigurableApplicationContext context) {context.addBeanFactoryPostProcessor(new ConfigurationWarningsPostProcessor(getChecks()));
}

5. RSocketPortInfoApplicationContextInitializer

不知道是干什么的

6. ServerPoerInfoApplicationContextInitializer

7. ConditionEvaluationReportLoggingListener

不知道

@Override
public void initialize(ConfigurableApplicationContext applicationContext) {this.applicationContext = applicationContext;applicationContext.addApplicationListener(new ConditionEvaluationReportListener());if (applicationContext instanceof GenericApplicationContext) {// Get the report early in case the context fails to loadthis.report = ConditionEvaluationReport.get(this.applicationContext.getBeanFactory());}
}

总结

1. ApplicationContextInitializer是用来填充ApplicationContext各种属性的
2. 在refresh之前执行
3. 有spring.factories / application.yml两种方法进行自定义扩展