Rather than hardcoding some properties that are also specified in your project’s build configuration, you can automatically expand them by instead using the existing build configuration. This is possible in both Maven and Gradle.

74.1.1 Automatic Property Expansion Using Maven

You can automatically expand properties from the Maven project by using resource filtering. If you use the spring-boot-starter-parent, you can then refer to your Maven ‘project properties’ with @..@ placeholders, as shown in the following example:

app.encoding[email protected]@
app.java.version[email protected]@

	Note
	Only production configuration is filtered that way (in other words, no filtering is applied on src/test/resources).

	Tip
	If you enable the addResources flag, the spring-boot:run goal can add src/main/resources directly to the classpath (for hot reloading purposes). Doing so circumvents the resource filtering and this feature. Instead, you can use the exec:java goal or customize the plugin’s configuration. See the plugin usage page for more details.

If you do not use the starter parent, you need to include the following element inside the <build/> element of your pom.xml:

<resources>
	<resource>
		<directory>src/main/resources</directory>
		<filtering>true</filtering>
	</resource>
</resources>

You also need to include the following element inside <plugins/>:

<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-resources-plugin</artifactId>
	<version>2.7</version>
	<configuration>
		<delimiters>
			<delimiter>@</delimiter>
		</delimiters>
		<useDefaultDelimiters>false</useDefaultDelimiters>
	</configuration>
</plugin>

	Note
	The useDefaultDelimiters property is important if you use standard Spring placeholders (such as ${placeholder}) in your configuration. If that property is not set to false, these may be expanded by the build.

74.1.2 Automatic Property Expansion Using Gradle

You can automatically expand properties from the Gradle project by configuring the Java plugin’s processResources task to do so, as shown in the following example:

processResources {
	expand(project.properties)
}

You can then refer to your Gradle project’s properties by using placeholders, as shown in the following example:

app.name=${name}
app.description=${description}

	Note
	Gradle’s expand method uses Groovy’s SimpleTemplateEngine, which transforms ${..} tokens. The ${..} style conflicts with Spring’s own property placeholder mechanism. To use Spring property placeholders together with automatic expansion, escape the Spring property placeholders as follows: \${..}.

A SpringApplication has bean properties (mainly setters), so you can use its Java API as you create the application to modify its behavior. Alternatively, you can externalize the configuration by setting properties in spring.main.*. For example, in application.properties, you might have the following settings:

spring.main.web-environment=false
spring.main.banner-mode=off

Then the Spring Boot banner is not printed on startup, and the application is not a web application.

	Note
	The preceding example also demonstrates how flexible binding allows the use of underscores (_) as well as dashes (-) in property names.

Properties defined in external configuration override the values specified with the Java API, with the notable exception of the sources used to create the ApplicationContext. Consider the following application:

new SpringApplicationBuilder()
	.bannerMode(Banner.Mode.OFF)
	.sources(demo.MyApp.class)
	.run(args);

Now consider the following configuration:

spring.main.sources=com.acme.Config,com.acme.ExtraConfig
spring.main.banner-mode=console

The actual application now shows the banner (as overridden by configuration) and uses three sources for the ApplicationContext (in the following order): demo.MyApp, com.acme.Config, and com.acme.ExtraConfig.

By default, properties from different sources are added to the Spring Environment in a defined order (see “Chapter 24, Externalized Configuration” in the ‘Spring Boot features’ section for the exact order).

A nice way to augment and modify this ordering is to add @PropertySource annotations to your application sources. Classes passed to the SpringApplication static convenience methods and those added using setSources() are inspected to see if they have @PropertySources. If they do, those properties are added to the Environment early enough to be used in all phases of the ApplicationContext lifecycle. Properties added in this way have lower priority than any added by using the default locations (such as application.properties), system properties, environment variables, or the command line.

You can also provide the following System properties (or environment variables) to change the behavior:

No matter what you set in the environment, Spring Boot always loads application.properties as described above. By default, if YAML is used, then files with the ‘.yml’ extension are also added to the list.

Spring Boot logs the configuration files that are loaded at the DEBUG level and the candidates it has not found at TRACE level.

See ConfigFileApplicationListener for more detail.

Some people like to use (for example) --port=9000 instead of --server.port=9000 to set configuration properties on the command line. You can enable this behavior by using placeholders in application.properties, as shown in the following example:

server.port=${port:8080}

Tip

If you inherit from the spring-boot-starter-parent POM, the default filter token of the maven-resources-plugins has been changed from ${*} to @ (that is, @maven.token@ instead of ${maven.token}) to prevent conflicts with Spring-style placeholders. If you have enabled Maven filtering for the application.properties directly, you may want to also change the default filter token to use other delimiters.

	Note
	In this specific case, the port binding works in a PaaS environment such as Heroku or Cloud Foundry. In those two platforms, the PORT environment variable is set automatically and Spring can bind to capitalized synonyms for Environment properties.

YAML is a superset of JSON and, as such, is a convenient syntax for storing external properties in a hierarchical format, as shown in the following example:

spring:
	application:
		name: cruncher
	datasource:
		driverClassName: com.mysql.jdbc.Driver
		url: jdbc:mysql://localhost/test
server:
	port: 9000

Create a file called application.yml and put it in the root of your classpath. Then add snakeyaml to your dependencies (Maven coordinates org.yaml:snakeyaml, already included if you use the spring-boot-starter). A YAML file is parsed to a Java Map<String,Object> (like a JSON object), and Spring Boot flattens the map so that it is one level deep and has period-separated keys, as many people are used to with Properties files in Java.

The preceding example YAML corresponds to the following application.properties file:

spring.application.name=cruncher
spring.datasource.driverClassName=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost/test
server.port=9000

See “Section 24.6, “Using YAML Instead of Properties”” in the ‘Spring Boot features’ section for more information about YAML.

The Spring Environment has an API for this, but you would normally set a System property (spring.profiles.active) or an OS environment variable (SPRING_PROFILES_ACTIVE). Also, you can launch your application with a -D argument (remember to put it before the main class or jar archive), as follows:

$ java -jar -Dspring.profiles.active=production demo-0.0.1-SNAPSHOT.jar

In Spring Boot, you can also set the active profile in application.properties, as shown in the following example:

spring.profiles.active=production

A value set this way is replaced by the System property or environment variable setting but not by the SpringApplicationBuilder.profiles() method. Thus, the latter Java API can be used to augment the profiles without changing the defaults.

See “Chapter 25, Profiles” in the “Spring Boot features” section for more information.

A YAML file is actually a sequence of documents separated by --- lines, and each document is parsed separately to a flattened map.

If a YAML document contains a spring.profiles key, then the profiles value (a comma-separated list of profiles) is fed into the Spring Environment.acceptsProfiles() method. If any of those profiles is active, that document is included in the final merge (otherwise, it is not), as shown in the following example:

server:
	port: 9000
---

spring:
	profiles: development
server:
	port: 9001

---

spring:
	profiles: production
server:
	port: 0

In the preceding example, the default port is 9000. However, if the Spring profile called ‘development’ is active, then the port is 9001. If ‘production’ is active, then the port is 0.

	Note
	The YAML documents are merged in the order in which they are encountered. Later values override earlier values.

To do the same thing with properties files, you can use application-${profile}.properties to specify profile-specific values.

Spring Boot binds external properties from application.properties (or .yml files and other places) into an application at runtime. There is not (and technically cannot be) an exhaustive list of all supported properties in a single location, because contributions can come from additional jar files on your classpath.

A running application with the Actuator features has a configprops endpoint that shows all the bound and bindable properties available through @ConfigurationProperties.

The appendix includes an application.properties example with a list of the most common properties supported by Spring Boot. The definitive list comes from searching the source code for @ConfigurationProperties and @Value annotations as well as the occasional use of Binder. For more about the exact ordering of loading properties, see "Chapter 24, Externalized Configuration".