Anatomy of a Spring Boot application

The anatomy of a Spring Boot application will be that it will be inheriting from a spring-boot-starter-parent project that will in return have all the common dependencies available for a Spring Boot application. Apart from this, there will be one or more spring-boot-starter POM such as spring-boot-starter-web, spring-boot-starter-jpa, and so on. The following example excerpt from pom.xml shows the basic dependencies of a Spring Boot application:

<parent>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-parent</artifactId>
   <version>1.5.9.RELEASE</version>
   <relativePath/> <!-- lookup parent from repository -->
</parent>

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

The minimum bootstrapping point of a Spring Boot application will be a class with a main method that will be annotated with a @SpringBootApplication annotation along with the main method body, which calls the SpringApplication.run static method, for which a configuration class (a class with @Configuration annotation—the @SpringBootApplication annotation transitively has one) needs to be passed, along with a String array of arguments. The following code shows the minimum bootstrapping point of a Spring Boot application:

import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class SpringBootIntroApplication {

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

   @Bean
   public ApplicationRunner applicationRunner() {
      return args -> {
         System.out.println("Hello, World!");
      };
   }
}

By running the preceding class, a Spring Boot application can be provisioned and executed. There are several ways to run a Spring Boot application; some of them are mentioned here:

• Running the Spring Boot application main class using an IDE.
• Building a JAR or WAR file using the following Maven command and then running:

$ mvn clean install
$ java -jar target/<package-name>.[jar|war]

• Run this using the Spring Boot Maven plugin:

$ mvn clean spring-boot:run

The @SpringBootApplication annotation comprises of @EnableAutoConfiguration and @ComponentScan annotations that do the heavy lifting of auto-configuring the Spring Boot application with the default settings and scanning the packages for any Spring-specific components such as services, components, repositories, and so on.