SpringBoot ${VARIABLE} is put as string value in configuration properties

Introduction

@Value annotation can inject environment variable. For instance we can use ${VARIABLE_NAME} to get variable. If we want to get default value if variable is not defined we have to put this after : char e.g. ${VARIABLE_NAME:defaultValue}. If variable and default value is not defined it throws error.

Problem

The problem is with using application.yml.

Project Example

/resources/application.yml

app:
  bug:
    key: ${VARIABLE_NAME}

BugProperties.java

@ConfigurationProperties("app.bug")
public class BugProperties {
    private String key;
    // getter & setter
}

How it behaviour

In this example bugProperties.getKey() if variable is not defined it returns ${VARIABLE_NAME} instead throw error or set just null.

Solution

I think that there are two option to consider.

Be consistent with @Value(...) annotation

Then it should throw an exception to be consistent with @Value(...) annotation.

Personally Preferred Solution

Then value should be set as null, I think that it is the best option for configuration properties because it is very often that configuration properties are validated using jakarta annotations like @NotNull, @NotBlank, @Size(...), and so on.

I would love to do a PR which fixes this functionality.

Comment From: snicoll

This is working as designed, please see the reference documentation.