How Does Spring Boot Handle Multiple External Configuration Files to Override Default Properties?

Spring Boot and Multiple External Configuration Files

Spring Boot allows developers to load external configuration files to customize application settings. This can be useful for overriding default configurations or specifying environment-specific properties. However, a common challenge is ensuring that external configurations override default ones, especially when multiple files are involved.

The Order of Property Resolution

Spring Boot follows a specific order when resolving property values:

1. Command line arguments
2. System properties
3. Environment variables
4. JNDI attributes
5. Default properties
6. Application properties outside the JAR
7. Application properties inside the JAR
8. @PropertySource annotations
9. Spring Boot property sources (e.g., application.properties)

Overriding Default Properties with External Files

To override default properties with external files, use the spring.config.location property. This property takes a comma-separated list of file paths or URLs. You can specify files within the classpath or the external configuration folder.

For example, to override the default properties with external files located in the current working directory, use:

-Dspring.config.location=file:./config

However, in earlier versions of Spring Boot (prior to 2.0), spring.config.location had a different behavior. It would only override application.properties and not other property files.

Using spring.config.additional-location

In Spring Boot 2.0 and later, spring.config.location overrides the default properties instead of adding to them. To maintain the default properties while also including additional ones, use spring.config.additional-location.

For example, to keep the default properties and add a file from the classpath, use:

-Dspring.config.location=file:./config
-Dspring.config.additional-location=classpath:job1.properties,classpath:job2.properties

By specifying both spring.config.location and spring.config.additional-location, you can ensure that external files override the default ones, while also preserving any custom properties defined within the default configuration.

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