Create Your First Spring Boot Application

Generate a Maven project with Java 21 and the Spring Web starter. Keep the generated wrapper in version control so developers and CI use the same build tool. Put the application class in a root package above feature packages so default component scanning can discover them.

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package com.example.tasks;

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

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

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package com.example.tasks.hello;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
class HelloController {
    @GetMapping("/hello")
    Greeting hello() {
        return new Greeting("Spring Boot is running");
    }
}

record Greeting(String message) {}

1. 01Run ./mvnw spring-boot:run and locate the log line naming the listening port.
2. 02Call curl -i http://localhost:8080/hello so status, Content-Type, and body are visible.
3. 03Confirm Jackson converted Greeting into JSON; the controller did not build a JSON string.
4. 04Run with --server.port=8081 and repeat the request without recompiling.

HTTP/1.1 200
Content-Type: application/json

{"message":"Spring Boot is running"}

SpringApplication.run(TaskApplication.class, args) is the boundary between the JVM entry point and a running Boot application. It prepares the environment, creates the appropriate ApplicationContext, loads configuration, refreshes the context, starts the embedded web server, runs startup callbacks, and publishes the ready event. The exact order matters: a readiness hook must not advertise success before its required initialization has completed.

1. 01The JVM calls main() and constructs SpringApplication metadata for TaskApplication.
2. 02Boot prepares profiles, properties, listeners, and the web application type.
3. 03The ApplicationContext is created and refreshed; definitions are registered and beans are instantiated.
4. 04The embedded server is started and the application publishes startup callbacks.
5. 05ApplicationReadyEvent is published only after runners complete successfully.

@SpringBootApplication is a convenience composition, not a new container. It combines @SpringBootConfiguration (the application configuration source), @EnableAutoConfiguration (conditional Boot infrastructure), and @ComponentScan (discovery of application components below the package). Each part has a separate responsibility.

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@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan
public class TaskApplication {
    public static void main(String[] args) {
        SpringApplication.run(TaskApplication.class, args);
    }
}

Move TaskApplication into com.example.tasks.bootstrap while leaving the controller in the sibling package com.example.tasks.hello. The process still starts, but GET /hello returns 404 because the default scan begins below com.example.tasks.bootstrap. Restore the application class to the common root package and the handler returns.