Support arithmetic expression evaluation in spring-boot-configuration-processor for default values

[cposhing]

Feature Request: Support Arithmetic Expression Evaluation in spring-boot-configuration-processor for Default Values

Description

The spring-boot-configuration-processor does not evaluate simple arithmetic expressions (e.g., 10 * 10) when generating metadata for @ConfigurationProperties fields, resulting in incorrect default values in spring-configuration-metadata.json. This is a limitation for developers who use expressions to define default values.

Steps to Reproduce

1. Create a Spring Boot project with spring-boot-configuration-processor dependency.
2. Define a @ConfigurationProperties class with a field initialized using an arithmetic expression:

   @ConfigurationProperties(prefix = "foo")
   public class FooProperties {
       private int bar = 10 * 10;
   }

3. Build the project to generate spring-configuration-metadata.json.
4. Check the generated metadata file in target/classes/META-INF/spring-configuration-metadata.json.

Expected Behavior

The metadata should reflect the computed default value:

{
  "properties": [
    {
      "name": "foo.bar",
      "type": "java.lang.Integer",
      "defaultValue": 100
    }
  ]
}

Actual Behavior

The metadata shows the Java default value for int (0) because the expression 10 * 10 is not evaluated:

{
  "properties": [
    {
      "name": "foo.bar",
      "type": "java.lang.Integer",
      "defaultValue": 0
    }
  ]
}

Environment

• Spring Boot Version: 3.3.4
• Operating System: Windows 11
• JDK Version: OpenJDK 17
• Build Tool: Maven 3.9.9

Additional Details

• The processor correctly handles static literals (e.g., private int bar = 100) and certain factory methods (e.g., Instant.parse("2019-02-12T11:49:22.455Z"), Duration.ofDays(1)) due to Spring’s ConversionService support.
• Arithmetic expressions like 10 * 10 are deterministic but not evaluated, forcing developers to use workarounds like static literals.
• This limitation affects IDE auto-completion and documentation, as the metadata does not reflect the intended default value.
• Proposed Enhancement: Add support for evaluating simple arithmetic expressions (e.g., addition, multiplication) in the configuration processor, possibly by integrating a lightweight expression evaluator or leveraging Spring Expression Language (SpEL) at compile time.
• Workaround: Using private int bar = 100 works but loses the expressiveness of the arithmetic expression.

[snicoll]

Workaround: Using private int bar = 100 works but loses the expressiveness of the arithmetic expression.

I don't thinks that's the only way to express this. You could define a constant that has whatever arithmetic you want and then use the constant as the default value.

Supporting arithmetic expression like that doesn't sound very realistic to me as we're at the level of structure parsing. Is there a problem defining a constant?

[spring-projects-issues]

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[snicoll]

If I've missed something with the proposal above, please comment. Regardless, we're not going to implement this. The next logical step would be to show 10 * 10 as the default value in your IDE because it's more expressive, and that's also something we can't do reliably.