Clearly document the limitations of @Value with respect to relaxed binding

[johanhaleby]

Suppose I have a property like this:

@Value("${something.with-dash}")
private String mySomething;

It's hard to make Spring map hyphens like with-dash from an environment variable. Let's say I've built my spring onejar then I can't just provide an environment variable like this (bash):

$ SOMETHING_WITH-DASH=Something java -jar my-spring-onejar.jar

You need to use a work-around like this:

$ env 'SOMETHING_WITH-DASH=Something' java -jar my-spring-onejar.jar

But in certain frameworks such as Kubernetes you cannot use a hyphen in environment variables since they use the C_IDENTIFIER format as the lowest common denominator. I've aded an issue for this in Kubernetes but I'm not sure if they're willing to change it.

So I wonder if instead Spring could enhance its way that hyphens are treated in environment variables when matching the @Value annotation? For example if SOMETHING_WITH_DASH could map to something.with-dash?

[snicoll]

Are you aware that we are already supporting this with @ConfigurationProperties? I'd argue that it is far more superior than the basic @Value approach so you might just as well give that a try. @Value is a core framework feature and the relaxed binding is not available there.

[johanhaleby]

Hmm I think at least some of the relaxed binding properties work for @Value as well. But why shouldn't the same relaxed binding properties work for both @ConfigurationProperties and @Value?

[snicoll]

Yes, there are some basic relaxed binding in the core framework as well.

Advanced relaxed binding is a spring boot feature and is quite opinionated so it does not belong in the core framework IMO. @Value is a core framework feature. I think we could improve the core feature (to some extend) but regardless of that I'd strongly advice not to use @Value with Spring Boot as a better solution exists already (see link in my first answer).

WDYT @dsyer @wilkinsona ?

[johanhaleby]

Ah I see. Perhaps the documentation can be even more clear that the same rules/properties does not apply to the @Value annotation since it's in core? I think that for a casual user like myself it can be a bit confusing to have a different set of rules for relaxed binding. It's sometimes hard to know what's in "core" and what's in "spring boot" since you typically just include a spring-boot-starter-x in your pom/build.gradle file. But I do understand that you want to have a clean separation between core and boot as well.

[snicoll]

You need to use a work-around like this:

I've just built a small example with SOMETHING_WITH-DASH and your code above with @Value breaks for me.

Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'something.with-dash' in string value "${something.with-dash}"

On a second though and checking the core framework again we don't have any such transformation of the key. I am busy documenting the differences between @Value and @ConfigurationProperties in #4892 anyway