spring-boot/spring-boot

Spring Boot - Core

This module provides the core 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(MySpringConfiguration.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(MySpringConfiguration.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 convert any command line option arguments (starting with '--', e.g. --server.port=9000) to a PropertySource and add it to the Spring Environment with highest priority (taking precedence and overriding values from other sources). Properties in the Environment (including System properties and OS environment variables) can always be injected into Spring components using @Value with placeholders, e.g.

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"

	// ...	
}

CommandLineRunner beans

If you want 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 this 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 wish to return a specific exit code when the application ends.

Externalized Configuration

A SpringApplication will load properties from application.properties in the root of your classpath and add them to the Spring Environment. The actual search path for the files is:

1. classpath root
2. current directory
3. classpath /config package
4. /config subdir of the current directory.

The list is ordered by decreasing precedence (so properties can be overridden by others with the same name defined in later locations). In addition, profile specific properties can also be defined using the naming convention application-{profile}.properties (properties from these files override the default ones).

The values in application.properties are filtered through the existing Environment when they are used so you can refer back to previously defined values (e.g. from System properties).

app.name: MyApp
app.description: ${app.name} is a Spring Boot application

If you don't like application.properties as the configuration file name you can switch to another by specifying spring.config.name environment property. You can also refer to an explicit location using the spring.config.location environment property.

$ java -jar myproject.jar --spring.config.name=myproject

Note: You can also use '.yaml' files as an alternative to '.properties' (see below)_

Setting the Default Spring Profile

Spring Profiles are a way to segregate parts of the application configuration and make it only available in certain environments. Any @Component that is marked with @Profile will only be loaded in the profile specified by the latter annotation.

A SpringApplication takes this a stage further, in that you can use a spring.profiles.active Environment property to specify which profiles are active. You can specify the property in any of the usual ways, for example you could include it in your application.properties:

spring.profiles.active=dev,hsqldb

or specify on the command line using the switch --spring.profiles.active=dev,hsqldb.

Application Context Initializers

Spring provides a convenient ApplicationContextInitializer interface that can be used to customize an ApplicationContext before it is used. If you need to use an initializer with your SpringApplication you can use the addInitializers method.

You can also specify initializers by setting comma-delimited list of class names to the Environment property context.initializer.classes or by using Spring's SpringFactoriesLoader mechanism.

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 int 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 both containers.

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

@Bean
public EmbeddedServletContainerFactory servletContainer() {
	TomcatEmbeddedServletContainerFactory factory = new TomcatEmbeddedServletContainerFactory();
	factory.setPort(9000);
	factory.setSessionTimeout(10, TimeUnit.MINUTES);
	factory.addErrorPages(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 3.0 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.

Using YAML instead of Properties

YAML is a superset of JSON, and as such is a very convenient format for specifying hierarchical configuration data. The SpringApplication class will automatically support YAML as an alternative to properties whenever you have the SnakeYAML library on your classpath.

Loading YAML

Spring Boot provides two convenient classes that can be used to load YAML documents. The YamlPropertiesFactoryBean will load YAML as Properties and the YamlMapFactoryBean will load YAML as a Map.

For example, the following YAML document:

dev:
	url: http://dev.bar.com
	name: Developer Setup
prod:
	url: http://foo.bar.com
	name: My Cool App

Would be transformed into these properties:

environments.dev.url=http://dev.bar.com
environments.dev.name=Developer Setup
environments.prod.url=http://foo.bar.com
environments.prod.name=My Cool App

YAML lists are represented as comma-separated values (useful for simple String values) and also as property keys with [index] dereferencers, for example this YAML:

servers:
	- dev.bar.com
	- foo.bar.com

Would be transformed into these properties:

servers=dev.bar.com,foo.bar.com
servers[0]=dev.bar.com
servers[1]=foo.bar.com

Exposing YAML as properties in the Spring Environment.

The YamlPropertySourceLoader class can be used to expose YAML as a PropertySource in the Spring Environment. This allows you to the familiar @Value with placeholders syntax to access YAML properties.

You can also specify multiple profile-specific YAML document in a single file by by using a spring.profiles key to indicate when the document applies. For example:

server:
	address: 192.168.1.100
---
spring:
	profiles: production
server:
	address: 192.168.1.120

Typesafe Configuration Properties

Use the @Value("${property}") annotation to inject configuration properties can sometimes be cumbersome, especially if you are working with multiple properties or your data is hierarchical in nature. Spring Boot provides an alternative method of working with properties that allows strongly typed beans to govern and validate the configuration of your application. For example:

@Component
@ConfigurationProperties(name="connection")
public class ConnectionSettings {

	private String username;
	
	private InetAddress remoteAddress;

	// ... getters and setters

}

When the @EnableConfigurationProperties annotation is applied to your @Configuration, any beans annotated with @ConfigurationProperties will automatically be configured from the Environment properties. This style of configuration works particularly well with the SpringApplication external YAML configuration:

# application.yml

connection:
	username: admin
	remoteAddress: 192.168.1.1

# additional configuration as required

To work with @ConfigurationProperties beans you can just inject them in the same way as any other bean.

@Service
public class MyService {

	@Autowired
	private ConnectionSettings connection; 

 	//...
 	
	@PostConstruct
	public void openConnection() {
		Server server = new Server();
		this.connection.configure(server);
	}
}

It is also possible to shortcut the registration of @ConfigurationProperties bean definitions by simply listing the properties classes directly in the @EnableConfigurationProperties annotation:

@Configuration
@EnableConfigurationProperties(ConnectionSettings.class)
public class MyConfiguration {
}

Relaxed binding

Spring Boot uses some relaxed rules for binding Environment properties to @ConfigurationProperties beans, so there doesn't need to be an exact match between the Environment property name and the bean property name. Common examples where this is useful include underscore separated (e.g. context_path binds to contextPath), and capitalized (e.g. PORT binds to port) environment properties.

Spring will attempt to coerce the external application properties to the right type when it binds to the @ConfigurationProperties beans. If you need custom type conversion you can provide a ConversionService bean (with bean id conversionService) or custom property editors (via a CustomEditorConfigurer bean).

@ConfigurationProperties Validation

Spring Boot will attempt to validate external configuration, by default using JSR-303 (if it is on the classpath). You can simply add JSR-303 javax.valididation constraint annotations to your @ConfigurationProperties class:

@Component
@ConfigurationProperties(name="connection")
public class ConnectionSettings {

	@NotNull
	private InetAddress remoteAddress;

	// ... getters and setters

}

You can also add a custom Spring Validator by creating a bean definition called configurationPropertiesValidator.

Using Project Lombok

You can safely use Project Lombok to generate getters and setters for your @ConfigurationProperties. Refer to the documentation on the Lombok for how to enable it in your compiler or IDE.

External EmbeddedServletContainerFactory configuration

Spring Boot includes a @ConfigurationProperties annotated class called ServerProperties that can be used to configure the EmbeddedServletContainerFactory.

When registered as a bean, the ServerProperties can be used to specify:

• The port that the application listens on for its endpoints (server.port defaults to 8080)
• The address that the application endpoints are available on (server.address defaults to all local addresses, making it available to connections from all clients).
• The context root of the application endpoints (server.context_path defaults to '/')

If you are using Tomcat as you embedded container then, in addition to the generic ServerProperties, you can also bind server.tomcat.* properties to specify:

• The Tomcat access log pattern (server.tomcat.accessLogPattern)
• The remote IP and protocol headers (server.tomcat.protocolHeader, server.tomcat.remoteIpHeader)
• The Tomcat base directory (server.tomcat.basedir)

Customizing Logging

Spring Boot uses Commons Logging for all internal logging, but leaves the underlying log implementation open. Default configurations are provided for Java Util Logging, Log4J and Logback. In each case there is console output and file output (rotating, 10MB file size).

The various logging systems can be activated by including the appropriate libraries on the classpath, and further customized by supported by providing a suitable configuration file in the root of the classpath, or in a location specified by the Spring Environment property logging.config.

Depending on your logging system, the following files will be loaded:

Logging System	Customization
Logback	logback.xml
Log4j	log4j.properties or log4j.xml
JDK	logging.properties

To help with the customization some other properties are transferred from the Spring Environment to System properties:

Environment	System Property	Comments
logging.file	LOG_FILE	Used in default log configuration if defined
logging.path	LOG_PATH	Used in default log configuration if defined
PID	PID	The current process ID is discovered if possible and not already provided

All the logging systems supported can consult System properties when parsing their configuration files. See the default configurations in spring-boot.jar for examples.

Cloud Foundry Support

When a SpringApplication is deployed to Cloud Foundry appropriate meta-data will be exposed as Environemnt properties. All Cloud Foundry properties are prefixed vcap. You can use vcap properties to access application information (such as the public URL of the application) and service information (such as database credentials). See ApplicationContextInitializer Javdoc for complete details.

Further Reading

For more information about any of the classes or interfaces discussed in the document please refer to the extensive project Javadoc. If looking to reduce the amount of configuration required for your application you should consider spring-boot-autoconfigure. For operational concerns see spring-boot-actuator. For details on how to package your application into a single executable JAR file take a look at spring-boot-loader.