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

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