在正常情况下，当应用程序启动时，在main方法中调用springapplication的run方法，并将main方法所在的class对象作为参数传递。

public static void main(String[] args) {
       SpringApplication.run(BootApplication.class, args);
}

但是最近当我研究springboot启动过程时，我看到springapplication示例化的最后一步是通过以下方式获得应用程序主启动类的类示例

@SuppressWarnings({ "unchecked", "rawtypes" })
    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        // deduce main Class
        this.mainApplicationClass = deduceMainApplicationClass();
    }

为什么spring不能通过类似于下面判断的方法直接获取主启动类，那么类对象示例呢？

if (primarySources.size == 1) {
    this.mainApplicationClass = primarySources.get(0);
}else {
    this.mainApplicationClass = deduceMainApplicationClass();
}