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

For example, consider the following YAML document:

environments:
	dev:
		url: http://dev.example.com
		name: Developer Setup
	prod:
		url: http://another.example.com
		name: My Cool App

The preceding example would be transformed into the following properties:

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

YAML lists are represented as property keys with [index] dereferencers. For example, consider the following YAML:

my:
servers:
	- dev.example.com
	- another.example.com

The preceding example would be transformed into these properties:

my.servers[0]=dev.example.com
my.servers[1]=another.example.com

To bind to properties like that by using the Spring DataBinder utilities (which is what @ConfigurationProperties does), you need to have a property in the target bean of type java.util.List (or Set) and you either need to provide a setter or initialize it with a mutable value. For example, the following example binds to the properties shown previously:

@ConfigurationProperties(prefix="my")
public class Config {

	private List<String> servers = new ArrayList<String>();

	public List<String> getServers() {
		return this.servers;
	}
}

	Note
	When lists are configured in more than one place, overriding works by replacing the entire list. In the preceding example, when my.servers is defined in several places, the entire list from the PropertySource with higher precedence overrides any other configuration for that list. Both comma-separated lists and YAML lists can be used for completely overriding the contents of the list.

The YamlPropertySourceLoader class can be used to expose YAML as a PropertySource in the Spring Environment. Doing so lets you use the @Value annotation with placeholders syntax to access YAML properties.

You can specify multiple profile-specific YAML documents in a single file by using a spring.profiles key to indicate when the document applies, as shown in the following example:

server:
	address: 192.168.1.100
---
spring:
	profiles: development
server:
	address: 127.0.0.1
---
spring:
	profiles: production
server:
	address: 192.168.1.120

In the preceding example, if the development profile is active, the server.address property is 127.0.0.1. Similarly, if the production profile is active, the server.address property is 192.168.1.120. If the development and production profiles are not enabled, then the value for the property is 192.168.1.100.

If none are explicitly active when the application context starts, the default profiles are activated. So, in the following YAML, we set a value for spring.security.user.password that is available only in the "default" profile:

server:
  port: 8000
---
spring:
  profiles: default
  security:
    user:
      password: weak

Whereas, in the following example, the password is always set because it is not attached to any profile, and it would have to be explicitly reset in all other profiles as necessary:

server:
  port: 8000
spring:
  security:
    user:
      password: weak

Spring profiles designated by using the spring.profiles element may optionally be negated by using the ! character. If both negated and non-negated profiles are specified for a single document, at least one non-negated profile must match, and no negated profiles may match.

YAML files cannot be loaded by using the @PropertySource annotation. So, in the case that you need to load values that way, you need to use a properties file.

As we showed earlier, any YAML content is ultimately transformed to properties. That process may be counter-intuitive when overriding “list” properties through a profile.

For example, assume a MyPojo object with name and description attributes that are null by default. The following example exposes a list of MyPojo objects from AcmeProperties:

@ConfigurationProperties("acme")
public class AcmeProperties {

	private final List<MyPojo> list = new ArrayList<>();

	public List<MyPojo> getList() {
		return this.list;
	}

}

Consider the following configuration:

acme:
  list:
    - name: my name
      description: my description
---
spring:
  profiles: dev
acme:
  list:
       - name: my another name

If the dev profile is not active, AcmeProperties.list contains one MyPojo entry, as previously defined. If the dev profile is enabled, however, the list still contains only one entry (with a name of my another name and a description of null). This configuration does not add a second MyPojo instance to the list, and it does not merge the items.

When a collection is specified in multiple profiles, the one with the highest priority (and only that one) is used. Consider the following example:

acme:
  list:
	- name: my name
	  description: my description
	- name: another name
	  description: another description
---
spring:
  profiles: dev
acme:
  list:
	 - name: my another name

In the preceding example, if the dev profile is active, AcmeProperties.list contains one MyPojo entry (with a name of my another name and a description of null).

As well as using @ConfigurationProperties to annotate a class, you can also use it on public @Bean methods. Doing so can be particularly useful when you want to bind properties to third-party components that are outside of your control.

To configure a bean from the Environment properties, add @ConfigurationProperties to its bean registration, as shown in the following example:

@ConfigurationProperties(prefix = "another")
@Bean
public AnotherComponent anotherComponent() {
	...
}

Any property defined with the another prefix is mapped onto that AnotherComponent bean in manner similar to the preceding AcmeProperties example.

Spring Boot uses some relaxed rules for binding Environment properties to @ConfigurationProperties beans, so there does not need to be an exact match between the Environment property name and the bean property name. Common examples where this is useful include dash-separated environment properties (for example, context-path binds to contextPath), and capitalized environment properties (for example, PORT binds to port).

For example, consider the following @ConfigurationProperties class:

@ConfigurationProperties(prefix="acme.my-project.person")
public class OwnerProperties {

	private String firstName;

	public String getFirstName() {
		return this.firstName;
	}

	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}

}

In the preceding example, the following properties names can all be used:

	Note
	The prefix value for the annotation must be in kebab case (lowercase and separated by -, such as acme.my-project.person).

	Tip
	We recommend that, when possible, properties are stored in lower-case kebab format, such as my.property-name=acme.

Spring attempts 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 a bean named conversionService) or custom property editors (through a CustomEditorConfigurer bean) or custom Converters (with bean definitions annotated as @ConfigurationPropertiesBinding).

Note

As this bean is requested very early during the application lifecycle, make sure to limit the dependencies that your ConversionService is using. Typically, any dependency that you require may not be fully initialized at creation time. You may want to rename your custom ConversionService if it is not required for configuration keys coercion and only rely on custom converters qualified with @ConfigurationPropertiesBinding.

Spring Boot attempts to validate @ConfigurationProperties classes whenever they are annotated with Spring’s @Validated annotation. You can use JSR-303 javax.validation constraint annotations directly on your configuration class. To do so, ensure that a compliant JSR-303 implementation is on your classpath and then add constraint annotations to your fields, as shown in the following example:

@ConfigurationProperties(prefix="acme")
@Validated
public class AcmeProperties {

	@NotNull
	private InetAddress remoteAddress;

	// ... getters and setters

}

In order to validate the values of nested properties, you must annotate the associated field as @Valid to trigger its validation. The following example builds on the preceding AcmeProperties example:

@ConfigurationProperties(prefix="acme")
@Validated
public class AcmeProperties {

	@NotNull
	private InetAddress remoteAddress;

	@Valid
	private final Security security = new Security();

	// ... getters and setters

	public static class Security {

		@NotEmpty
		public String username;

		// ... getters and setters

	}

}

You can also add a custom Spring Validator by creating a bean definition called configurationPropertiesValidator. The @Bean method should be declared static. The configuration properties validator is created very early in the application’s lifecycle, and declaring the @Bean method as static lets the bean be created without having to instantiate the @Configuration class. Doing so avoids any problems that may be caused by early instantiation. There is a property validation sample that shows how to set things up.

	Tip
	The spring-boot-actuator module includes an endpoint that exposes all @ConfigurationProperties beans. Point your web browser to /actuator/configprops or use the equivalent JMX endpoint. See the "Production ready features" section for details.

The @Value annotation is a core container feature, and it does not provide the same features as type-safe configuration properties. The following table summarizes the features that are supported by @ConfigurationProperties and @Value:

If you define a set of configuration keys for your own components, we recommend you group them in a POJO annotated with @ConfigurationProperties. You should also be aware that, since @Value does not support relaxed binding, it is not a good candidate if you need to provide the value by using environment variables.

Finally, while you can write a SpEL expression in @Value, such expressions are not processed from application property files.