import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

/**
 * Hello world!
 *
 */
@SpringBootApplication
public class App 
{
    public static void main( String[] args )
    {

        System.out.println( "Hello World!" );

        SpringApplication.run(App.class,args);
    }
}

一、Spring  SPI机制，自动装配：

启动类使用@SpringApplication注解，看一下注解代码：

代码中使用@EnableAutoConfiguration以及@ComponentScan自动装配

其中注解@EnableAutoConfiguration使用了@Import加载，最后使用了SpringFactoriesLoader反射出maven中META-INF下spring.factories。

public @interface SpringBootApplication {
    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )
    Class<?>[] exclude() default {};

    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )
    String[] excludeName() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackages"
    )
    String[] scanBasePackages() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackageClasses"
    )
    Class<?>[] scanBasePackageClasses() default {};
}

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Import;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}


//
public class AutoConfigurationImportSelector ...{

...
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {

//加载类路径下面  META-INF/spring.factories
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());

        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

}


public abstract class SpringFactoriesLoader {
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

.....

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

                while(urls.hasMoreElements()) {
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
                        Entry<?, ?> entry = (Entry)var6.next();
                        List<String> factoryClassNames = Arrays.asList(StringUtils.commaDelimitedListToStringArray((String)entry.getValue()));
                        result.addAll((String)entry.getKey(), factoryClassNames);
                    }
                }

                cache.put(classLoader, result);
                return result;
            } catch (IOException var9) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var9);
            }
        }
    }

....
}

感兴趣的老铁，可以看一下java 的SPI和Spring的SPI，自动装配机制

二、SpringBoot启动时通过执行main方法中的SpringApplication.run方法去启动，在run方法中调用了SpringApplication的构造方法，在该构造方法中加载了META-INFA\spring.factories文件配置的ApplicationContextInitializer的实现类和ApplicationListenerr的实现类： 

 SpringApplication.run(..,..)

 

 

二、ApplicationContextInitializer 这个类当springboot上下文Context初始化完成后会调用。                   ApplicationListener当springboot启动时事件change后都会触发。

三、SpringApplication实例构造完之后会调用它的run方法，在run方法中作了以下几步重要操作：

1. 获取事件监听器SpringApplicationRunListener类型，并且执行starting()方法

2. 准备环境，并且把环境跟spring上下文绑定好，并且执行environmentPrepared()方法

3. 创建上下文，根据项目类型创建上下文

4. 执行spring的启动流程扫描并且初始化单实列bean

四、通过@SpringBootApplication注解将ClassPath路径下所有的META-INF\spring.factories文件中的EnableAutoConfiguration实例注入到IOC容器中