Allow @ConfigurationProperties binding on interfaces

[philwebb]

It would be useful to allow configuration binding to work directly on fields if the user doesn't want to expose public setters.

[snicoll]

That would completely compromise work on #1001 that rely on the presence of a standard javabean property.

I also find this useful but we need to chose our camp. If we support it, we need a way to exclude unwanted fields/properties through annotations.

We also could add a mode to the annotations that would provide a hint to the processor. In that mode only the field would be exposed (same for metadata harvesting)

[philwebb]

What about if we support field based binding if there is a getter but no setter. Or another easier option might be to allow protected setters. What I wanted was a quick way to expose configuration properties in a read-only form.

Yet another option might be to allow @ConfigurationProperties on an interface, generating an implementation proxy class.

@ConfigurationProperties
public interface SomeProperties {
    String getDateToolAttribute();
    String getNumberToolAttribute();
    getProperties();
    String getResourceLoaderPath();
    String getToolboxConfigLocation();
}

[wilkinsona]

RJo on StackOverflow is interested in this functionality:

If the setters were not visible, the configuration properties would be unmodifiable without resorting to violence :)

[snicoll]

I am fine either way but we have to understand the consequence for the annotation processor. We can go with any crazy option as long as it's explicit (i.e. you have to enable a flag on the @ConfigurationProperties annotation). If we don't, then we'll have to update the algorithm auto-detection and potentially expose thing we don't want in the metadata

(reminder: we have no way to exclude a property)

[fromi]

I would be happy to be able to use @ConfigurationProperties with constructor injection, so I can create immutable and encapsulated classes.

[mwisnicki]

I, for one, would like to have it working with plain fields.

[mwisnicki]

Maybe use transient or @Transient to exclude properties from serialization/metadata ?

[mwisnicki]

BTW spring-boot documentation has an example that shows binding to field which I guess is a mistake.

[philwebb]

@mwisnicki Good spot on the docs. I've raised #2813 to address this.

[michaeltecourt]

In order to have an "immutable" configuration properties object, I currently use a workaround with an interface, whose implementation is a "Java Bean" annotated with @ConfigurationProperties. Any shortcut would be appreciated. By the way, enabling field based injection would make a lot of sense, as the behavior would be more consistent with @Value. Having to add both getters/setters can be counter-intuitive the first time around.

[jordanjennings]

@michaeltecourt Thanks for the suggestion. Doing this in combination with Lombok's @DaTa annotation on the implementation class makes this fairly painless.

[snicoll]

@michaeltecourt that's not the same thing at all. With @Value you're giving the full id in a regular Spring Framework annotation (i.e. that you can put on field, setter or constructor argument). @ConfigurationProperties is a strongly-typed binder on a regular class where properties are discovered.

If we were enabling field access by default, all the fields of your class would be exposed which isn't what you want. The presence of accessors defines if it should be exposed or not.

I think our best shot at it is to add a flag to enable field support explicitly. In that case, you're at least aware they are all exposed.

[michaeltecourt]

@snicoll By "all the fields of your class would be exposed" do you mean "injectable/bindable" ?

We mostly use @ConfigurationProperties like a bulk @Value.
The semantic and precise use cases might be different, but isn't the purpose the same? binding properties from the Spring context to Spring beans in a type-safe manner?

In the end, what I find slightly confusing as a user is that you can do this :

/** Works on fields, constructor, etc */
@Component
class Props {
  @Value("${cfg.props.a}") String a;
  @Value("${cfg.props.b}") BigDecimal b;

  public Props(a, b) { this.a = a; this.b = b; }
  public String getA() { return this.a; }
  public BigDecimal getB() { return this.b; }
 }

and not this :

@ConfigurationProperties("cfg.props")
class Props {
  String a;
  BigDecimal b;

  public Props(a, b) { this.a = a; this.b = b; }
  public String getA() { return this.a; }
  public BigDecimal getB() { return this.b; }
  // SETTERS NEEDED
}