Spring Boot Properties as Environment variables

Question

I have come across some behaviour which seems inconsistent in how spring boot handles environment variables in application properties files vs configuration property classes. I am unsure whether this is a bug in spring or an error in my understanding of what "ought" to happen.

I have

@Data
@ConfigurationProperties("foo")
@Validated
public class ClientProperties {

    @NotBlank
    private String apiKey;

    @NotBlank
    private String uri;

}

In the application.properties file I have:

foo.baseUri=https://system.appspot.com
foo.uri=${foo.baseUri}/Valuation?apikey=${foo.apiKey}&bar={bar}

Setting Just FOO_APIKEY

If I run my app with:

export FOO_APIKEY=DEF

Then I get

APPLICATION FAILED TO START
***************************

Description:

Binding to target class ClientProperties(apiKey=null, uri=https://system.appspot.com/Valuation?apikey=DEF&bar={bar}) failed:

    Property: foo.apiKey
    Value: null
    Reason: may not be empty

Note that in the URI the api key is set as expected as well as the base URI

Setting Just FOO_API_KEY

Next, if instead, I try to set just this property (remove the old env var):

export FOO_API_KEY=ABC

Then my app starts, but the values are not as expected. My logs show:

API Key: ABC.
URI Property: ${foo.baseUri}/Insurance?apikey=${foo.apiKey}&bar={bar}.

Note that now the base uri also disappeared as well as the API key being missing.

Setting Both Properties FOO_API_KEY and FOO_APIKEY

When I set both environment variables the app starts but the apiKey property of ClientProperties holds the value of the FOO_APIKEY export, where as the uri property of ClientProperties holds the value of the FOO_API_KEY export.

API KEY IS: ABC.
URI IS: https://system.appspot.com/Insurance?apikey=DEF&bar={bar}.

Notes

I actually don't need the value from ClientProperties.apiKey. It's only ever used in the app via the ClientProperties.uri which is already being resolved in application.properties. However, I specify the property so that I can have validation to ensure the value gets set. I could remove the value from my class and everything would be ok - expect then I lose my validation.

Spring boot version is: 1.5.10.RELEASE