在SpringApplication的实例属性中有一个初始器的属性：List<ApplicationContextInitializer<?>> initializers ，这些初始化器(initializers)是Spring Boot通过读取每个jar包下的/META-INF/spring.factories文件中的配置获取的。每一个initailizer都是一个实现了ApplicationContextInitializer接口的实例。ApplicationContextInitializer是Spring IOC容器中提供的一个接口：

package org.springframework.context;

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {

    /**

     * Initialize the given application context.

     * @param applicationContext the application to configure

     */

    void initialize(C applicationContext);

}

ApplicationContextInitializer是一个回调接口，它会在ConfigurableApplicationContext的refresh()方法调用之前被调用,做一些容器的初始化工作。这一点我们也可以通过SpringApplication的实例run方法的实现代码得到验证，为了说明问题，再次贴一下这段代码,注意下标红的代码和注释就自然理解了。

public ConfigurableApplicationContext run(String... args) {

        StopWatch stopWatch = new StopWatch();

        stopWatch.start();

        ConfigurableApplicationContext context = null;

        FailureAnalyzers analyzers = null;

        configureHeadlessProperty();

        SpringApplicationRunListeners listeners = getRunListeners(args);

        listeners.starting();

        try {

            ApplicationArguments applicationArguments = new DefaultApplicationArguments(

                    args);

            ConfigurableEnvironment environment = prepareEnvironment(listeners,

                    applicationArguments);

            Banner printedBanner = printBanner(environment);

            context = createApplicationContext();

            analyzers = new FailureAnalyzers(context);

            prepareContext(context, environment, listeners, applicationArguments,

                    printedBanner); // prepareContext方法中将会执行每个initializers的逻辑

            refreshContext(context);  // 执行bean的创建和实例化

            afterRefresh(context, applicationArguments);

            listeners.finished(context, null);

            stopWatch.stop();

            if (this.logStartupInfo) {

                new StartupInfoLogger(this.mainApplicationClass)

                        .logStarted(getApplicationLog(), stopWatch);

            }

            return context;

        }

        catch (Throwable ex) {

            handleRunFailure(context, listeners, analyzers, ex);

            throw new IllegalStateException(ex);

        }

    }

SpringBoot默认情况下提供了6个initializer，分别由2个jar提供：

　spring-boot-1.5.2.RELEASE.jar

• 　　org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,
• 　　org.springframework.boot.context.ContextIdApplicationContextInitializer,
• 　　org.springframework.boot.context.config.DelegatingApplicationContextInitializer,
• 　　org.springframework.boot.context.embedded.ServerPortInfoApplicationContextInitializer

spring-boot-autoconfigure-1.5.2.RELEASE.jar

• 　　org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,
• 　　org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

Spring Boot对initializer的获取过程如下：

initialize(Object[] sources)
　　-->(Collection) getSpringFactoriesInstances( ApplicationContextInitializer.class ))  //获取initializer实例
　　　　　　-->SpringFactoriesLoader.loadFactoryNames(type, classLoader))
　　　　　　-->createSpringFactoriesInstances(type, parameterTypes,classLoader, args, names)
　　　　　　　　-->Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes)
　　　　　　　　-->T instance = (T) BeanUtils.instantiateClass(constructor, args)
　　　　　　-->AnnotationAwareOrderComparator.sort(instances)

--> setInitializers(Collection<? extends ApplicationContextInitializer<?>> initializers)

　　　-->this.initializers = new ArrayList<ApplicationContextInitializer<?>>();

　　　-->this.initializers.addAll(initializers);    //存入List<ApplicationContextInitializer<?>> initializers