Application configuration

On this page, you will:

Get to know the application.yml file.
Use the @ConfigurationProperties annotation.
Use profiles to simulate different application environments.

application.yml

We will start this example by adding an entry to the application.yml file. This YAML file holds all configuration values for a specific application.

Place the following snippet in the file.

applicationKey: 73RNNJIJFN83N3R3NKFNK

We are defining a configuration item with name applicationKey and value 73RNNJIJFN83N3R3NKFNK.

To use this configuration item, we can use the @Value annotation in our class. Modify the StartupLogger class to the one below:

A component which displays a configuration value!

@Component
@Slf4j
public class StartupLogger {

    @Value( "${applicationKey}" ) (1)
    private String applicationKey;

    @Bean
    public Void dummyBean() {
        LOG.info( "Your application key is: {}", applicationKey);
        return null;
    }
}

1	The `@Value` annotation can be used to perform expression based dependency injection. In combination with the property syntax or the Spring Expression Language (SpEL), it can be used to fetch properties from a configuration file or beans.

Run the application.

$ mvn spring-boot:run

In the console you should see the following:

INFO --- [ost-startStop-1] c.e.demo.application.StartupLogger       : Your application key is: 73RNNJIJFN83N3R3NKFNK

Next up, you will see how we can use @ConfigurationProperties. This is a more structured way of defining your configuration values.

@ConfigurationProperties

@ConfigurationProperties can be used to map configuration values onto a strongly typed configuration class. To demonstrate this we will modify the example from above to use @ConfigurationProperties instead.

Create a configuration class ApplicationProperties.

A configuration class with one configuration value.

@ConfigurationProperties (1)
@Component
@Data (2)
public class ApplicationProperties {
    private String applicationKey;  (3)
}

1	The `@ConfigurationProperties` annotation specifies that this is a configuration class.
2	We use this Lombok annotation to generate getters and setters which Spring Boot require for setting the field appropriately.
3	This field will map to the entry from the YAML file. The field name and the YAML key must match.

Modify your StartupLogger class to the one below.

A component which displays a configuration value from a configuration class.

@Component
@Slf4j
@RequiredArgsConstructor
public class StartupLogger {

    private final ApplicationProperties applicationProperties; (1)

    @Bean
    public Void dummyBean() {
        LOG.info( "Your application key is: {}", applicationProperties.getApplicationKey());  (2)
        return null;
    }
}

1	The configuration class is wired.
2	The `applicationKey` YAML entry is now automatically mapped onto the configuration class by Spring and can be retrieved through the getter.

The advantages of doing this are:

Configuration values belonging to the same 'object' can be bundled. For example, you can bundle FTP properties like hostname, username and password into an FtpProperties configuration class.
Instead of having multiple @Value annotations, a single configuration class has to be mapped, making your code less verbose.
You can take advantage of Spring validation, to validate configuration values at application startup.

Have a look at Spring Boot Externalized Configuration for more information.

In the next section we will learn how to specify different values per configuration item, for example to differentiate between development and production environments.

Application profiles

In the previous example we created a configuration item applicationKey which holds the value 73RNNJIJFN83N3R3NKFNK.

But what needs to happen when this value is different between environments? For example in development you might want to use DEV:487FEF7E8F7E8F7E8 whereas in production you want the value PROD:FEU4349323FJ83NF.

To achieve this, you can use profiles and the @Profile annotation.

Place the following configuration items anywhere in the application-dev.yml and application-prod.yml files.

application-dev.yml

applicationKey: 'DEV:487FEF7E8F7E8F7E8'

application-prod.yml

applicationKey: 'PROD:FEU4349323FJ83NF'

Run the application.

$ mvn spring-boot:run

You should see the following in your console:

INFO --- [ost-startStop-1] c.e.demo.application.StartupLogger       : Your application key is: 73RNNJIJFN83N3R3NKFNK

Why is this?

You specified configuration values in several configuration files, but you did not specify which profile needs to be active. This can be done by specifying the spring.profiles.active property when starting the application.

A hint was also in the console:

INFO --- [  restartedMain] com.example.demo.DemoApplication         : Starting DemoApplication with PID 133348
INFO --- [  restartedMain] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default

Let’s fix this and start our application with the dev profile.

$ mvn spring-boot:run -Dspring-boot.run.profiles=dev

Because we are running the application with the spring-boot-maven-plugin, the parameter that should be passed is actually spring-boot.run.profiles.

Alterntatively, an environment variable could be used instead:

$ SPRING_PROFILES_ACTIVE=dev mvn spring-boot:run

You would get the same behaviour when using the following command line in case of an executable jar:

$ java -jar target/demo.jar -Dspring.profiles.active=dev

In the console you should now see:

INFO --- [  restartedMain] com.example.demo.DemoApplication         : Starting DemoApplication with PID 126896
INFO --- [  restartedMain] com.example.demo.DemoApplication         : The following profiles are active: dev
...
INFO --- [ost-startStop-1] c.e.demo.application.StartupLogger       : Your application key is: DEV:487FEF7E8F7E8F7E8

For production, the command line would be:

$ java -jar target/demo.jar -Dspring.profiles.active=prod

Note that it’s also possible to combine profiles, the following would activate the dev profile as well as the test-data profile.

$ java -jar target/demo.jar -Dspring.profiles.active=dev,test-data

Read the Spring Boot Profiles documentation for more configuration options.

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