SpringApplication

1. What is SpringApplication?

In a normal Java program, the JVM starts from the main() method. In Spring Boot, that method commonly delegates startup work to SpringApplication.run().

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

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

Think of SpringApplication as the starter/manager of your Spring Boot application.

2. What Happens When You Call run()?

1. Spring Boot starts the application.
2. It prepares the environment.
3. It creates and prepares the appropriate ApplicationContext.
4. It loads bean definitions and refreshes the context.
5. It determines application availability.
6. It runs ApplicationRunner and CommandLineRunner components.
7. It marks the application ready to accept traffic.

3. Startup Failure

When startup fails, Spring Boot can use FailureAnalyzer implementations to provide a useful description and suggested action.

***************************
APPLICATION FAILED TO START
***************************

Description:
Embedded servlet container failed to start.
Port 8080 was already in use.

Action:
Identify and stop the process listening on port 8080
or configure the application to use another port.

If no failure analyzer handles the problem, enable the conditions report with:

java -jar myproject-0.0.1-SNAPSHOT.jar --debug

4. Lazy Initialization

Lazy initialization means beans are created when they are needed instead of all being created during startup.

spring.main.lazy-initialization=true

SpringApplication application = new SpringApplication(MyApplication.class);
application.setLazyInitialization(true);
application.run(args);

You can opt a particular bean out with @Lazy(false).

5. Customizing the Banner

Spring Boot can print a startup banner. Customize it with banner.txt on the classpath or spring.banner.location.

src/main/resources/banner.txt

Banner output can be controlled with:

spring.main.banner-mode=off

Supported modes are console, log, and off.

6. Customizing SpringApplication

SpringApplication application =
        new SpringApplication(MyApplication.class);

application.setBannerMode(Banner.Mode.OFF);
application.setLazyInitialization(true);

application.run(args);

The constructor arguments are configuration sources for Spring beans. Usually these are configuration classes, but they can also be component classes.

7. SpringApplicationBuilder

Use SpringApplicationBuilder when you want a fluent API or an ApplicationContext hierarchy.

new SpringApplicationBuilder()
        .sources(Parent.class)
        .child(Application.class)
        .bannerMode(Banner.Mode.OFF)
        .run(args);

Web components should be contained in the child context, and parent and child use the same Environment.

8. Application Availability

9. ApplicationAvailability

Application components can inject ApplicationAvailability to retrieve the current availability state.

You can also listen for changes using AvailabilityChangeEvent.

@Component
public class MyReadinessStateExporter {

    @EventListener
    public void onStateChange(
            AvailabilityChangeEvent<ReadinessState> event) {

        switch (event.getState()) {
            case ACCEPTING_TRAFFIC -> {
                // application can receive traffic
            }
            case REFUSING_TRAFFIC -> {
                // application should not receive traffic
            }
        }
    }
}

10. Application Events and Listeners

Other events can appear in the lifecycle, including WebServerInitializedEvent and ContextRefreshedEvent.

11. Web Environment Detection

If MVC and WebFlux are both present, MVC is used by default.

SpringApplication application =
        new SpringApplication(MyApplication.class);

application.setWebApplicationType(WebApplicationType.NONE);
application.run(args);

WebApplicationType.NONE is useful when you want a non-web application or need this behavior in a test.

12. Application Arguments

Arguments passed to SpringApplication.run() can be accessed through ApplicationArguments.

java -jar app.jar --debug logfile.txt

@Component
public class MyBean {

    public MyBean(ApplicationArguments args) {
        boolean debug = args.containsOption("debug");
        List<String> files = args.getNonOptionArgs();

        if (debug) {
            System.out.println(files);
        }
    }
}

For this example, files contains logfile.txt. Spring Boot also registers a CommandLinePropertySource in the Environment.

13. ApplicationRunner vs CommandLineRunner

CommandLineRunner

@Component
public class MyCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) {
        System.out.println("Application started!");
    }
}

ApplicationRunner

@Component
public class MyApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) {
        if (args.containsOption("debug")) {
            System.out.println("Debug mode enabled");
        }
    }
}

If multiple runners must execute in a specific order, use Ordered or @Order.

14. Application Exit

Each SpringApplication registers a JVM shutdown hook so the ApplicationContext can close gracefully.

You can provide custom process exit codes with ExitCodeGenerator.

@Bean
public ExitCodeGenerator exitCodeGenerator() {
    return () -> 42;
}

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

Exceptions may also implement ExitCodeGenerator. With multiple generators, the first non-zero generated exit code is used; ordering can be controlled with Ordered or @Order.

15. Admin Features

spring.application.admin.enabled=true

This exposes SpringApplicationAdminMXBean through the platform MBeanServer for remote administration and service-wrapper scenarios.

The property local.server.port can be used to find the HTTP port in applicable situations.

16. Application Startup Tracking

ApplicationStartup and StartupStep can help track and understand application startup.

SpringApplication application =
        new SpringApplication(MyApplication.class);

application.setApplicationStartup(
        new BufferingApplicationStartup(2048));

application.run(args);

BufferingApplicationStartup buffers startup steps. FlightRecorderApplicationStartup integrates Spring startup information with Java Flight Recorder for profiling.

java -XX:StartFlightRecording:filename=recording.jfr,duration=10s -jar demo.jar

Spring Boot can also expose startup information through the startup endpoint when configured.

17. Virtual Threads

Virtual threads require Java 21 or later. The current documentation strongly recommends Java 24 or later for the best experience.

spring.threads.virtual.enabled=true

Virtual threads are daemon threads. This can matter when using @Scheduled tasks because scheduler threads may not keep the JVM alive.

spring.main.keep-alive=true

18. Complete Startup Mental Model

19. Common Beginner Mistakes

20. Interview Questions

1. What is SpringApplication?
2. What does SpringApplication.run() do?
3. What is the role of ApplicationContext?
4. How does Spring Boot choose the web application type?
5. What is lazy initialization?
6. Why is lazy initialization not enabled by default?
7. How do you disable the Spring Boot banner?
8. What is SpringApplicationBuilder?
9. What is the difference between liveness and readiness?
10. Why should liveness generally not depend on external systems?
11. What is ApplicationAvailability?
12. Name the main Spring Boot startup events in order.
13. Difference between CommandLineRunner and ApplicationRunner?
14. How do you control runner ordering?
15. How does Spring Boot handle graceful shutdown?
16. What is ExitCodeGenerator?
17. What is ApplicationStartup used for?
18. What should you consider when enabling virtual threads?

21. Practice Exercises

22. Quick Cheat Sheet

23. Final Takeaway