仅做源码解析，目前先省略掉搭建部分，互联网上有大量的搭建教程。当前springboot版本为2.7.x，其他版本可能稍有不同。
我们先来看一下核心的SpringApplication类，在java/org/springframework/boot/SpringApplication.java下，在类注释之上简要阐述了当前类的作用，以及最核心的SrpingApplication.run方法的使用。简单的翻译一下：

/**
* Class that can be used to bootstrap and launch a Spring application from a Java main
* method. By default class will perform the following steps to bootstrap your* application:
* 这个class的方法能够用于引导和启动Spring应用，
* 默认的class将会执行下面的步骤进行引导应用 *
* 

* 
Create an appropriate {@link ApplicationContext} instance (depending on your
* classpath)
* 
Register a {@link CommandLinePropertySource} to expose command line arguments as
* Spring properties
* 
Refresh the application context, loading all singleton beans
* 
Trigger any {@link CommandLineRunner} beans
* 
* 1。创建适当ApplicationContext的实例（依赖于你的classpath）
* 2。注册CommandLinePropertySource并添加命令行参数到spring属性中
* 3。刷新整个应用上下文，加载所有的单实例bean
* 4。触发所有的CommandLineRunner beans *
* In most circumstances the static {@link #run(Class, String[])} method can be called* directly from your {@literal main} method to bootstrap your application:
* 更多的情况在静态的方法run中可以看到，在你的应用中使用run来进行引导 * 
* @Configuration
* @EnableAutoConfiguration
* public class MyApplication  {
*
*   // ... Bean definitions
*
*   public static void main(String[] args) {
*     SpringApplication.run(MyApplication.class, args);
*   }
* }
* 
*
* 

* For more advanced configuration a {@link SpringApplication} instance can be created and* customized before being run:
* 更多的配置，你可以创建SpringApplication实例进行自定义 *
* 
* public static void main(String[] args) {
*   SpringApplication application = new SpringApplication(MyApplication.class);
*   // ... customize application settings here
*   application.run(args)
* }
* 
*
* {@link SpringApplication}s can read beans from a variety of different sources. It is
* generally recommended that a single {@code @Configuration} class is used to bootstrap
* your application, however, you may also set {@link #getSources() sources} from:
* 

* 
The fully qualified class name to be loaded by
* {@link AnnotatedBeanDefinitionReader}
* 
The location of an XML resource to be loaded by {@link XmlBeanDefinitionReader}, or
* a groovy script to be loaded by {@link GroovyBeanDefinitionReader}
* 
The name of a package to be scanned by {@link ClassPathBeanDefinitionScanner}
* 
*
* Configuration properties are also bound to the {@link SpringApplication}. This makes it
* possible to set {@link SpringApplication} properties dynamically, like additional
* sources ("spring.main.sources" - a CSV list) the flag to indicate a web environment
* ("spring.main.web-application-type=none") or the flag to switch off the banner
* ("spring.main.banner-mode=off").
*
* @author Phillip Webb
* @author Dave Syer
* @author Andy Wilkinson
* @author Christian Dupuis
* @author Stephane Nicoll
* @author Jeremy Rickard
* @author Craig Burke
* @author Michael Simons
* @author Madhura Bhave
* @author Brian Clozel
* @author Ethan Rubinson
* @author Chris Bono
* @since 1.0.0
* @see #run(Class, String[])
* @see #run(Class[], String[])
* @see #SpringApplication(Class...)*/

包含了大概介绍了启动流程springapplication启动流程，还有加载属性的加载器，最后提到了可以在启动的时候进行动态配置文件的配置。
启动流程比较重要贴在下面：

• 1.创建适当ApplicationContext的实例（依赖于你的classpath）
• 2.注册CommandLinePropertySource并添加命令行参数到spring属性中
• 3.刷新整个应用上下文，加载所有的单实例bean
• 4.触发所有的CommandLineRunner beans
  接下来转到注释中多次提到的SpringApplication.run方法

/**
* A basic main that can be used to launch an application. This method is useful when
* application sources are defined via a {@literal --spring.main.sources} command line
* argument.
* 这个基础的bean，可以启动一个应用，这个方法通常情况下应用的资源定义通过spring.main.sources命令行参数
* 

* Most developers will want to define their own main method and call the
* {@link #run(Class, String...) run} method instead.
* @param args command line arguments
* @throws Exception if the application cannot be started
* @see SpringApplication#run(Class[], String[])
* @see SpringApplication#run(Class, String...)
*/
public static void main(String[] args) throws Exception {SpringApplication.run(new Class[0], args);}
/**
* Static helper that can be used to run a {@link SpringApplication} from the
* specified sources using default settings and user supplied arguments.
* @param primarySources the primary sources to load
* @param args the application arguments (usually passed from a Java main method)
* @return the running {@link ApplicationContext}
*/
public static ConfigurableApplicationContext run(Class[] primarySources, String[] args) {return new SpringApplication(primarySources).run(args);}

注意看这里，写了theprimary sources to load，表示主要资源的加载位置
这时候我们发现，当前的SpringApplicaroin.run被拆分成了两个步骤，第一步new SpringApplication(primarySources)，在实例化后，在调用非静态的run(args)方法进行初始化，先看new SpringApplication(primarySources)

/**
* Create a new {@link SpringApplication} instance. The application context will load
* beans from the specified primary sources (see {@link SpringApplication class-level}
* documentation for details). The instance can be customized before calling
* {@link #run(String...)}.
* @param primarySources the primary bean sources
* @see #run(Class, String[])
* @see #SpringApplication(ResourceLoader, Class...)
* @see #setSources(Set)
*/
public SpringApplication(Class... primarySources) {this(null, primarySources);
}/**
* Create a new {@link SpringApplication} instance. The application context will load
* beans from the specified primary sources (see {@link SpringApplication class-level}
* documentation for details). The instance can be customized before calling
* {@link #run(String...)}.
* @param resourceLoader the resource loader to use
* @param primarySources the primary bean sources
* @see #run(Class, String[])
* @see #setSources(Set)
*/
@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {//默认是赋值为nullthis.resourceLoader = resourceLoader;//断言检测class是不是nullAssert.notNull(primarySources, "PrimarySources must not be null");//将class变成一个LinkedHashSetthis.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));//判断当前的web类型，正常来说一定返回SERVLETthis.webApplicationType = WebApplicationType.deduceFromClasspath();//创建BootstrapRegistryInitializerthis.bootstrapRegistryInitializers = new ArrayList<>(getSpringFactoriesInstances(BootstrapRegistryInitializer.class));//创建ApplicationContextInitializersetInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));//创建ApplicationListenersetListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//设置当前的main所在的classthis.mainApplicationClass = deduceMainApplicationClass();
}

我们可以看到只有两个有参数构造器，我们默认调用的是只有一个参数的构造器，最终调用的是SpringApplication(ResourceLoader resourceLoader, Class… primarySources)，第一参数可以自定义一个资源加载器，第二个参数为当前的启动类，