Spring Boot Cli Hello World

Java has never had the easiest of Hello World programs. For the very simplest Hello World, we would have to create a class, define a static method within that class, and to display output, there’s a call to “System.out.println”:

    public class MyMain {
        public static void main(String args[]) {
            System.out.println("Hello World");
        }
    }

That’s without the use of packages (namespaces), Maven, or the Spring framework.

Place the .java file in src/main/java and creating a pom.xml file with the Spring Boot dependency (below):

    <?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>org.example</groupId>
        <artifactId>springBootTry</artifactId>
        <version>1.0-SNAPSHOT</version>

        <properties>
            <maven.compiler.source>18</maven.compiler.source>
            <maven.compiler.target>18</maven.compiler.target>
        </properties>

        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-test</artifactId>
                <version>2.6.5</version>
            </dependency>
        </dependencies>
    </project>

Add the @SpringBootApplication annotation to MyMain - this is a combination of the @Configuration, @EnableAutoConfiguration and @ComponentScan annotations.

To define the entry point, there are two approaches - implement the CommandLineRunner interface and override the run method, or create a method that returns a CommandLineRunner instance and annotate it with @Bean.

Approach 1:

    package com.pockettheories;

    import org.springframework.boot.CommandLineRunner;
    import org.springframework.boot.autoconfigure.SpringBootApplication;

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

        @Override
        public void run(String... args) throws Exception {
            System.out.println("Hello World");
        }
    }

Approach 2:

    package com.pockettheories;

    import org.springframework.boot.CommandLineRunner;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.annotation.Bean;

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

        @Bean
        public CommandLineRunner commandLineRunner(ApplicationContext ctx) {
            return args -> {
                System.out.println("Hello World");
            };
        }
    }