Spring Boot SpringApplication Class with Examples - RameshMF/spring-boot-developers-guide GitHub Wiki

In this article, we will discuss usage and important of SpringApplication in Spring boot applications.

The SpringApplication class provides a convenient way to bootstrap a Spring application that is started from a main() method. By default SpringApplication class will perform the following steps to bootstrap your application:

1. Create an appropriate ApplicationContext instance (depending on your classpath)
2. Register a CommandLinePropertySource to expose command line arguments as Spring properties
3. Refresh the application context, loading all singleton beans
4. Trigger any CommandLineRunner beans In most circumstances the static run(Class, String[]) method can be called directly from your main method to bootstrap your application:

package net.guides.springboot2;

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

@SpringBootApplication
public class Application {

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

For more advanced configuration a SpringApplication instance can be created and customized before being run:

 public static void main(String[] args) {
   SpringApplication application = new SpringApplication(MyApplication.class);
   // ... customize application settings here
   application.run(args)
 }

Create ApplicationContext Instance

SpringApplication class automatically creates the ApplicationContext from the classpath, scan the configuration classes and launch the application. This class is very helpful in launching Spring MVC or Spring REST application using Spring Boot.

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

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

It also runs an embedded Tomcat server in Spring web application.

More Usage of SpringApplication Class

Let's discuss few important APIs/Methods of SpringApplication class.

Customizing the Banner

There are many options to change banner. The simplest way is by adding a banner.txt file to your classpath or by setting the spring.banner.location property to the location of such a file. If the file has an encoding other than UTF-8, you can set spring.banner.charset.

The SpringApplication.setBanner(…​) method can be used if you want to generate a banner programmatically. Use the org.springframework.boot.Banner interface and implement your own printBanner() method.

Customizing SpringApplication

If the SpringApplication defaults are not to your taste, you can instead create a local instance and customize it. For example, to turn off the banner, you could write:

public static void main(String[] args) {
	SpringApplication app = new SpringApplication(MySpringConfiguration.class);
	app.setBannerMode(Banner.Mode.OFF);
	app.run(args);
}

Note that you can customize defaults by providing appropriate properties to SpringApplication class.

Fluent Builder API

If you need to build an ApplicationContext hierarchy (multiple contexts with a parent/child relationship) or if you prefer using a “fluent” builder API, you can use the SpringApplicationBuilder.

The SpringApplicationBuilder lets you chain together multiple method calls and includes parent and child methods that let you create a hierarchy, as shown in the following example:

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

Web Environment

A SpringApplication attempts to create the right type of ApplicationContext on your behalf. The algorithm used to determine a WebApplicationType is fairly simple:

• If Spring MVC is present, an AnnotationConfigServletWebServerApplicationContext is used
• If Spring MVC is not present and Spring WebFlux is present, an AnnotationConfigReactiveWebServerApplicationContext is used
• Otherwise, AnnotationConfigApplicationContext is used

Using the ApplicationRunner or CommandLineRunner

If you need to run some specific code once the SpringApplication has started, you can implement the ApplicationRunner or CommandLineRunner interfaces. Both interfaces work in the same way and offer a single run method, which is called just before SpringApplication.run(…​) completes.

The following example shows a CommandLineRunner with a run method:

import org.springframework.boot.*;
import org.springframework.stereotype.*;

@Component
public class MyBean implements CommandLineRunner {

	public void run(String... args) {
		// Do something...
	}
}

Application Exit

Each SpringApplication registers a shutdown hook with the JVM to ensure that the ApplicationContext closes gracefully on exit. All the standard Spring lifecycle callbacks (such as the DisposableBean interface or the @PreDestroy annotation) can be used.

This exit code can then be passed to System.exit() to return it as a status code, as shown in the following example:

@SpringBootApplication
public class ExitCodeApplication {

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

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

There are more usage of SpringApplication class, you can check out official documentation at https://docs.spring.io/spring-boot/docs/2.0.8.RELEASE/reference/htmlsingle/#boot-features-spring-application