Add support for configuration property fallback

[philwebb]

Some way to specify the default value for interfaces and a different fallback property.

[snicoll]

An example

@ConfigurationProperties("foo.bar")
public class BarProperties {

    private int timeout 1000;

    @ConfigurationPropertyValue(fallback = "foo.default.timeout")
    public int getTimeout() { return timeout; }
  
   public void setTimeout(int timeout) { ... }  

}

The same annotation can be used to provide default values for interface-based configuration properties, something like

@ConfigurationProperties("foo.bar")
public interface BarProperties {

    @ConfigurationPropertyValue(fallback = "foo.default.timeout", default = "1000")
    int getTimeout();

}

[afranken]

instead of (or in addition to) adding an annotation that works for interfaces for properties classes, you should add an annotation for fields of properties classes (much like the @Autowired annotation) and not force users to add public getter and setter methods to properties classes.

[wilkinsona]

@afranken Please see #1254

[thombergs]

In addition to defining a fallback, I would like to define whether it's allowed for a configuration property to be missing. Most of the times, startup of the application should fail when an expected property is missing. With @ConfigurationProperties alone, I currently see no possibility to fail on startup if a property is missing (other than creating a @PostConstruct method or something similar that does manual validation). It will just be assigned NULL or if it's a primitive the default value of that primitive type.

So, essentially, it would be great to be able to define exceptionIfMissing (in the style of exceptionIfInvalid on @ConfigurationProperties) on the annotation proposed by @snicoll like this:

@ConfigurationProperties("foo.bar")
public class BarProperties {

   private int optionalTimeout;
   private int mandatoryTimeout;

   @ConfigurationPropertyValue(exceptionIfMissing=false)
   public int getOptionalTimeout() { return optionalTimeout; }
  
   @ConfigurationPropertyValue(exceptionIfMissing=true)
   public int getMandatoryTimeout() { return mandatoryTimeout; }
  
  ...
}

What are your thoughts on this?

[afranken]

@thombergs I would expect that JSR-303 validation will keep working as it does right now:
https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html#boot-features-external-config-validation

In your example, you would annotate your Integer fields with for example @Max or @Min, and @NotNull if you wanted an exception to be thrown during startup...

[thombergs]

Ah, I didn't read the docs careful enough. Bean Validation works fine for my proposed use case. Thanks for the pointer.