@ConfigurationProperties should have an option to fail fast for unresolvable properties

[anno1985]

When using placeholders in application.yaml (property files/externalised configuration) together with the @ConfigurationProperties annotation, there should be an option to have Spring fail fast at startup when a defined property is not found.

Example:

example.key: ${MY_ENV_VAR}

@ConfigurationProperties(prefix="example")
public class AcmeProperties {
  public String key;
  // Getters, setters, constructors omitted... 
}

Precondition:
No environment variable MY_ENV_VAR defined.

Current behaviour:
key above is populated with the verbatim String ${MY_ENV_VAR}

Expected/desired behaviour:
An exception is thrown while the application is starting up.

Potential cause:
Hardcoded flag in https://github.com/spring-projects/spring-boot/blob/v2.2.0.RELEASE/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/PropertySourcesPlaceholdersResolver.java#L51

Further information:
https://stackoverflow.com/q/58622189/2018047

NB:
Validating the String with @Validated after (failed) resolution is not the same as checking if resolution fails. Also, when no fail-fast is active, it might potentially be better to mirror System.getenv("MY_ENV_VAR")'s behaviour, and return null instead of the actual placeholder, ${MY_ENV_VAR}, verbatim.

[philwebb]

I believe we intentionally set ignoreUnresolvablePlaceholders to true to keep back compatibility with previous releases. I agree it would be nice to have a fail fast option.

[philwebb]

See also #8693 and #13202

[philwebb]

@mbhave also tracked down some internal discussion we had about this last time around:

From @wilkinsona

Is it by design that the 2.0 binder doesn’t ignore placeholder resolution failures? Someone on Gitter is complaining and I can’t remember if it’s intentional or not. Here’s a little app that starts fine with 1.5.x but fails with 2.0.x:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.ConfigurableApplicationContext;

import com.example.demo.PlaceholderChangeApplication.ExampleProperties;

@SpringBootApplication
@EnableConfigurationProperties(ExampleProperties.class)
public class PlaceholderChangeApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context =
                SpringApplication.run(PlaceholderChangeApplication.class,
                        "--com.example.alpha=Value with ${placeholder}");
        System.out.println(context.getBean(ExampleProperties.class).getAlpha());
    }

    @ConfigurationProperties(prefix="com.example")
    static class ExampleProperties {

        private String alpha;

        public String getAlpha() {
            return alpha;
        }

        public void setAlpha(String alpha) {
            this.alpha = alpha;
        }

    }

}

It works in 1.5.x because of this change that was in the early days of Spring Boot: 8922a6b

[philwebb]

I think we intentionally rolled this back in 2.0 because the major upgrade was quite onerous already and we didn't want to add more friction.

[camhashemi]

we also desire this feature. it's easier to tell that a server won't start up if the process fails than if it goes into the current spring retry loop.

lacking this feature directly: are there any alternative paths to the same result?