springboot official document

https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#_working_with_spring_boot

maven dependence

spring-boot-starter-parent

<?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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.example</groupId>
	<artifactId>myproject</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.1.0.RELEASE</version>
	</parent>

	<!-- Additional lines to be added here... -->

</project>

web dependence

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

Startup classes and interfaces

@RestController
@EnableAutoConfiguration
public class HelloWorldApp {

    @RequestMapping("/")
    String home() {
        return "Hello World!";
    }

    public static void main(String[] args) throws Exception {
        SpringApplication.run(HelloWorldApp.class, args);
    }
}

Explanatory notes

@RestController

@RestController = @Controller + @ResponseBody

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Controller //Annotations to spring MVC
@ResponseBody //Annotations to spring MVC
public @interface RestController {
    @AliasFor(
        annotation = Controller.class
    )
    String value() default "";
}

Official Document Interpretation

The @RestController and @RequestMapping annotations are Spring MVC annotations. (They are not specific to Spring Boot.)
Explanation: @RestController and @RequestMapping are annotations for spring mvc, with no specificity

@EnableAutoConfiguration

This annotation tells Spring Boot to "guess" how you want to configure Spring, based on the jar dependencies that you have added. Since spring-boot-starter-web added Tomcat and Spring MVC, the auto-configuration assumes that you are developing a web application and sets up Spring accordingly.
@EnableAutoConfiguration Conjecturally configure the developer springmvc,tomcat,spring Configuration of aspects. That is to say, it will be automatically configured. springmvc,tomcat,spring Some configurations

Start service and access interface

Start main method, console print:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.1.0.RELEASE)

2018-11-21 15:03:33.622  INFO 75693 --- [           main] com.yimingkeji.helloworld.HelloWorldApp  : Starting HelloWorldApp on yangzhenlongdeMacBook-Pro.local with PID 75693 (/Users/yangzhenlong/projs/my/yimingkeji/springboot/helloword/target/classes started by yangzhenlong in /Users/yangzhenlong/projs/my/yimingkeji/springboot)
2018-11-21 15:03:33.626  INFO 75693 --- [           main] com.yimingkeji.helloworld.HelloWorldApp  : No active profile set, falling back to default profiles: default
2018-11-21 15:03:34.499  INFO 75693 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2018-11-21 15:03:34.516  INFO 75693 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2018-11-21 15:03:34.516  INFO 75693 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/9.0.12
2018-11-21 15:03:34.522  INFO 75693 --- [           main] o.a.catalina.core.AprLifecycleListener   : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/yangzhenlong/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]
2018-11-21 15:03:34.581  INFO 75693 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2018-11-21 15:03:34.582  INFO 75693 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 917 ms
2018-11-21 15:03:34.604  INFO 75693 --- [           main] o.s.b.w.servlet.ServletRegistrationBean  : Servlet dispatcherServlet mapped to [/]
2018-11-21 15:03:34.607  INFO 75693 --- [           main] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'characterEncodingFilter' to: [/*]
2018-11-21 15:03:34.608  INFO 75693 --- [           main] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
2018-11-21 15:03:34.608  INFO 75693 --- [           main] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'formContentFilter' to: [/*]
2018-11-21 15:03:34.608  INFO 75693 --- [           main] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'requestContextFilter' to: [/*]
2018-11-21 15:03:34.763  INFO 75693 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2018-11-21 15:03:34.933  INFO 75693 --- [           main]
o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2018-11-21 15:03:34.936  INFO 75693 --- [           main] com.yimingkeji.helloworld.HelloWorldApp  : Started HelloWorldApp in 1.727 seconds (JVM running for 2.849)

You can see in the log that the project startup port (prot) is 8080 and the startup root path (path) is''

Browsers or http clients (such as postman) access localhost:8080

Hello World!

Start with a jar package

Add spring-boot-maven-plugin plug-in to pom.xml dependencies

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

Execute commands in the Terminal shell or Terminal:

$ mvn package

The jar package is generated in the project target directory:

Start jar package

$ java -jar target/helloword-1.0.0-SNAPSHOT.jar 
2018-11-21 15:12:35.077  INFO 75841 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2018-11-21 15:12:35.355  INFO 75841 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2018-11-21 15:12:35.359  INFO 75841 --- [           main] com.yimingkeji.helloworld.HelloWorldApp  : Started HelloWorldApp in 2.382 seconds (JVM running for 2.838)

Visit localhost:8080

Hello World!

Press Ctrl + C to end the service.

Start with mvn

$ mvn spring-boot:run