Spring Boot

Spring Boot provides the central features for the other modules in the project. It is relatively unopinionated and it has minimal required dependencies which makes it usable as a stand-alone library for anyone whose tastes diverge from ours.

SpringApplication

The SpringApplication class provides a convenient way to bootstrap a Spring application that will be started from a main() method. In many situations you can just delegate to the static SpringApplication.run method:

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

When you application starts you should see something similar to the following:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::   v0.0.0.BUILD.SNAPSHOT

2013-07-31 00:08:16.117  INFO 56603 --- [           main] o.s.b.s.app.SampleApplication   : Starting SampleApplication v0.1.0 on mycomputer with PID 56603 (/apps/myapp.jar started by pwebb)
2013-07-31 00:08:16.166  INFO 56603 --- [           main] ationConfigEmbeddedWebApplicationContext : Refreshing org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@6e5a8246: startup date [Wed Jul 31 00:08:16 PDT 2013]; root of context hierarchy

By default INFO logging messages will shown, including some relevant startup details such as the user that launched the application.

Customizing SpringApplication

If the SpringApplication defaults aren't to your taste you can instead create a local instance and customize it. For example, to turn off the banner you would write:

public static void main(String[] args) {
	SpringApplication app = new SpringApplication(SpringConfiguration.class);
	app.setShowBanner(false);
	app.run(args);
}

Note that the constructor arguments passed to SpringApplication are configuration sources for spring beans. In most cases these will be references to @Configuration classes, but they could also be references to XML configuration or to packages that should be scanned.

See the SpringApplication Javadoc for a complete list of the configuration options

Accessing command line properties

By default SpringApplication will expose any command line arguments as Spring Properties. This allows you to easily access arguments by injecting them as @Values

import org.springframework.stereotype.*
import org.springframework.beans.factory.annotation.*

@Component
public class MyBean {
	
	@Value("${name}")
	private String name; 
	// Running 'java -jar myapp.jar --name=Spring' will set this to "Spring"

	// ...	
}

You can also use the special --spring.profiles.active argument to enable specific Spring profiles from the command line.

CommandLineRunner beans

If you wan't access to the raw command line argument, or you need to run some specific code once the SpringApplication has started you can implement the CommandLineRunner interface. The run(String... args) method will be called on all spring beans implementing the interface.

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

@Component
public class MyBean implements CommandLineRunner {

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

You can additionally implement the org.springframework.core.Ordered interface or use the org.springframework.core.annotation.Order annotation if several CommandLineRunner beans are defined that must be called in a specific order.

Application Exit

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

In addition, beans may implement the org.springframework.boot.ExitCodeGenerator interface if they with to return a specific exit code when the application ends.

Embedded Servlet Container Support

Spring Boot introduces a new type of Spring ApplicationContext that can be used to start an embedded servlet container. The EmbeddedWebApplicationContext is a special type of WebApplicationContext that starts the container by searching for a single EmbeddedServletContainerFactory bean contained within itself. We provide TomcatEmbeddedServletContainerFactory and JettyEmbeddedServletContainerFactory implementations for running embedded Tomcat or Jetty.

One advantage of using a Spring bean to define the embedded container is that you can use
all the standard Spring concepts. For example, it becomes trivial to define a Tomcat server that sets its port from an injected @Value:

@Configuration
public class MyConfiguration {
	
	@Value("${tomcatport:8080}")
	private String port; 

	@Bean
	public EmbeddedServletContainerFactory servletContainer() {
		return new TomcatEmbeddedServletContainerFactory(this.port);
	}

}

Customizing Servlet Containers

Both the Tomcat and Jetty factories extend from the base AbstractEmbeddedServletContainerFactory class. This provides a uniform way to configure your container regardless of which implementation you actually choose.

Settings that you traditionally configure in a web.xml or via an implementation specific configuration file can now be performed programmatically. For example:

@Bean
public EmbeddedServletContainerFactory servletContainer() {
	TomcatEmbeddedServletContainerFactory factory = new TomcatEmbeddedServletContainerFactory();
	factory.setPort(9000);
	factory.setSessionTimeout(10, TimeUnit.MINUTES);
	factory.addaddErrorPages(new ErrorPage(HttpStatus.404, "/notfound.html");
	return factory;
}

In addition, you can also add ServletContextInitializer implementations which allow you to customize the javax.servlet.ServletContext in the same way as any Servlet 3 environment.

Servlets and Filters

Servlets and Filters can be defined directly as beans with the EmbeddedWebApplicationContext. By default, if the context contains only a single Servlet it will be mapped to '/'. In the case of multiple Servlets beans the bean name will be used as a path prefix. Filters will map to '/*'.

If convention based mapping is not flexible enough you can use the ServletRegistrationBean and FilterRegistrationBean classes for complete control. You can also register items directly if your bean implements the ServletContextInitializer interface.

External Configuration

FIXME include loadcations that SpringApplication hits fact you can conf SpringApplication and propery bindings

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