@Controller
public class HelloController {

    @RequestMapping("/hello")
    @ResponseBody
    public String hello(){
        return "hello world";
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>


    <!--引入父工程，spring-boot-dependencies核心依赖在父工程中-->
    <!--我们在引入或使用一些SpringBoot插件过程中不需要指定版本号，就是因为父工程中已经引入了对应的插件-->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.4.5</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <groupId>cn.ywrby</groupId>
    <artifactId>SpringBootTest</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>SpringBootTest</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>11</java.version>
    </properties>

    <dependencies>
        <!--启动器本质就是SpringBoot的启动场景，所有启动器都以spring-boot-starter为前缀
            这些启动器内部封装了实现该场景需要进行的全部操作
            SpringBoot存在很多开箱即用的Starter依赖，即SpringBoot会将所有功能场景变成一个个启动器方便使用
        -->
        <!--SpringBoot中的Web启动器，会帮项目导入Web环境所需的所有依赖-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!--SpringBoot单元测试启动器-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

// @SpringBootApplication注解表明该类是一个SpringBoot应用
@SpringBootApplication
public class SpringBootTestApplication {
    public static void main(String[] args) {
        //run方法 启动SpringBoot应用
        SpringApplication.run(SpringBootTestApplication.class, args);
    }
}

/**
 * Static helper that can be used to run a {@link SpringApplication} from the
 * specified source using default settings.
 * @param primarySource the primary source to load
 * @param args the application arguments (usually passed from a Java main method)
 * @return the running {@link ApplicationContext}
 */
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
	return run(new Class<?>[] { primarySource }, args);
}

@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}
)}
)
public @interface SpringBootApplication {...}

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

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
...
...