Auto-Configuration and the Main Class

@SpringBootApplication

@SpringBootApplication combines configuration, component scanning of its package and below, and auto-configuration. Auto-configuration classes create beans conditionally: with the JPA starter and H2 on the classpath, Boot configures a DataSource, an EntityManagerFactory and transactions; with webmvc it starts Tomcat and configures JSON with Jackson. Your own beans take precedence. Run with --debug (or the conditions report) to see which auto-configurations applied and why.

The application class

This file is from a demo "shop" service generated by start.spring.io for Spring Boot 4.1.1 and Java 21 (Temurin 21.0.12), built with Maven 3.9.16; the project compiled and its tests passed. Typed properties are enabled here; seeding lives in a separate component (see the testing section for why).

package com.example.shop;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.EnableConfigurationProperties;

@SpringBootApplication
@EnableConfigurationProperties(ShopProperties.class)
public class ShopApplication {

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

Startup log lines, run

I packaged the demo with mvn package, started it with java -jar on port 8085 and called it with curl; the output is copied from that run. The embedded Tomcat started on the chosen port and the context was ready in about 5 seconds.

java -jar target/shop-0.0.1-SNAPSHOT.jar --server.port=8085

Output:

Tomcat started on port 8085 (http) with context path '/'
Started ShopApplication in 5.003 seconds (process running for 5.452)

Quick check: What decides which auto-configurations apply?

• Only the main method arguments
• Alphabetical order
• A random choice
• Conditions such as classes on the classpath, existing beans and properties