Why Can a Spring Boot JAR Run Directly?

Executable JAR produced by Spring Boot

Self‑contained JAR

Spring Boot packages all compile‑time and runtime dependencies, including third‑party libraries, into a single JAR. The packaging is driven by Maven or Gradle together with the Spring Boot plugin. Adding the starter dependency causes the plugin to copy every required artifact into the final archive:

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

Embedded web server

The built JAR contains an embedded servlet container (Tomcat, Jetty or Undertow). At runtime the container is started from within the application, eliminating the need for an external server deployment.

Main class as entry point

Each application defines a class annotated with @SpringBootApplication and a public static void main(String[] args) method that invokes SpringApplication.run() to bootstrap the Spring context.

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

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

Running the JAR

After mvn package (or the Gradle equivalent) produces my-spring-boot-app.jar, the application starts with a single command:

java -jar my-spring-boot-app.jar

The command launches the embedded server, which by default listens on port 8080, allowing the application to accept HTTP requests immediately.

Spring Boot starters

Starters aggregate commonly used dependencies (e.g., spring-boot-starter-web, spring-boot-starter-data-jpa). Declaring a starter eliminates manual dependency enumeration and configuration, further simplifying project setup.