接口作用：该接口的实现类会在容器刷新之前执行
例：
定义该接口实现类
public class MyApplicationContextInitalizer1 implements ApplicationContextInitializer<ConfigurableApplicationContext>
启动类
SpringApplication springApplication=new SpringApplication(DemoApplication.class);
//MyApplicationContextInitalizer1实现了ApplicationContextInitializer接口
springApplication.addInitializers(new MyApplicationContextInitalizer1());
springApplication.addInitializers(new MyApplicationContextInitalizer2());
springApplication.run();