1. 导入spring boot相关依赖

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.6.RELEASE</version>
    <relativePath/>
</parent>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

2. 编写一个主程序，启动SpringBoot应用

@SpringBootApplication
public class HelloWorld01QuickApplication {
    public static void main(String[] args) {
        SpringApplication.run(HelloWorld01QuickApplication.class, args);
    }

}

3. 编写一个Controller

@Controller
public class HelloWorldController {
    @ResponseBody
    @RequestMapping("/hello")
    public String hello(){
        return "Hello World!";
    }
}

再pom.xml文件中，我们可以看到HelloWorld引用了一个上层pom文件，他的上层文件spring-boot-starter-parent又引用了一个上层文件spring-boot-dependencies

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.6.RELEASE</version>
    <relativePath/>
</parent>

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.2.6.RELEASE</version>
    <relativePath>../../spring-boot-dependencies</relativePath>
</parent>

[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-X9X3h4ut-1588239225049)(C:\Users\lym\AppData\Roaming\Typora\typora-user-images\image-20200430161406719.png)]

spring-boot-dependencies`就是来管理SpringBoot应用里面所有依赖版本，是SpringBoot的版本管理中心，所以我们导出的包的版本不需要我们自己管理。

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

spring-boot-start-web:spring-boot-starter: spring-boot场景启动器；帮我们导入了web模块正常运行所依赖的组件；

SpringBoot将所有的功能场景都抽取出来，做成一个个的starters（启动器），只需要再项目里面引入这些starter相关的创景的所有依赖都会导入进来。要用什么功能就导入什么场景的启动器。

@SpringBootApplication
public class HelloWorld01QuickApplication {
    public static void main(String[] args) {
        SpringApplication.run(HelloWorld01QuickApplication.class, args);
    }
}

• @SpringBootApplication:SpringBoot应用标注再某某个类上说明这个类是SpringBoot的主配置类，SpringBoot就应该运行这个类的main方法来启动SpringBoot应用；

下面是@SpringBootApplication这个注解类的源码

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)

• @SpringBootConfiguration这个注解表示SpringBoot的配置类，标注再某个类上，表示这是一个SpringBoot的配置类。在@SpringBootConfiguration中我们可以看到@Configuration注解，表示这是一个配置类。
• @EnableAutoConfiguration:开启自动配置功能；

以前我们需要自己配置的东西，SpringBoot会帮我们自动配置，该注解告诉SpringBoot开启自动配置功能。

@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 {};
}

@AutoConfigurationPackage:

@Import({AutoConfigurationImportSelector.class})

在AutoConfigurationImportSelector类中有selectImports方法

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
        AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
}

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    } else {
        AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
        List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
        configurations = this.removeDuplicates(configurations);
        Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
        this.checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = this.filter(configurations, autoConfigurationMetadata);
        this.fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }
}

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    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 static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    String factoryTypeName = factoryType.getName();
    return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, 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();
                    String factoryTypeName = ((String)entry.getKey()).trim();
                    String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                    int var10 = var9.length;

                    for(int var11 = 0; var11 < var10; ++var11) {
                        String factoryImplementationName = var9[var11];
                        result.add(factoryTypeName, factoryImplementationName.trim());
                    }
                }
            }

            cache.put(classLoader, result);
            return result;
        } catch (IOException var13) {
            throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
        }
    }
}

classLoader.getResources(“META-INF/spring.factories”)由这行代码可知，最终是通过ClassLoader加载

META-INF/spring.factories文件中的类来确定自己加载的配置类，该文件中表明了需要导入组件的全类名，通过@Import这些组件，这些组件就会被添加到容器中，会给容器中导入非常多的自动配置类。