Spring Boot: using SpEL collection selection from YAML in @ConditionalOnExpression-CodePudding

I have a Spring Boot application with a YAML configuration that contains a feature list like this:

features:
  - key: feature1
    enabled: true
    description: First feature
  - key: feature2
    enabled: false
    description: Second feature
...

I would like to use @ConditionalOnExpression to conditionally initialize beans related to those features, identifying them by keys. Since "features" property is a list, it seems I need collection selection to do this. I have tried these two options for the annotation's value:

@ConditionalOnExpression("${features.?[key == 'feature1'][0].enabled}")

@ConditionalOnExpression("${features}.?[key == 'feature1'][0].enabled")

But both give the same error on startup:

org.springframework.expression.spel.SpelParseException: EL1041E: After parsing a valid expression, there is still more data in the expression: 'lcurly({)'

If I pass the expression (without ${}) to SpelExpressionParser.parseExpression() and then evaluate it (against a list of feature objects built programmatically), it works as expected and returns the value of "enabled" property. So the expression's structure seems to be OK, and the problem is how I use it in @ConditionalOnExpression. What exactly can I be doing wrong?

CodePudding user response：

${...} has nothing to do with SpEL; that syntax is for property placeholders in Spring.

e.g. @Value("${property.foo:default}" String someProperty.

This will set someProperty to the value of property.foo if present, or default otherwise. (the :... part can be omitted to force the property to exist).

CodePudding user response：

Spring SpEL supports only simple property placeholders inside the properties file.

If your properties file and condition code are like this, your SpEL will work:

features:
  feature1:
    enabled: true
    description: First feature.
  feature2:
    enabled: false
    description: Second feature.

@ConditionalOnExpression(
        value = "#{'${features.feature1.enabled}'.equals('true')}"
)

But if you want to load it as a map feature and use the map SpEL features;

The most elegant way is writing the key-values as an inline JSON (use ' and " chars to avoid cumbersome escaping) and parsing it using SpEL:

features: '{
"feature1": {"enabled":"true", "description":"..."},
"feature2": {"enabled":"false", "description":"..."}
}'

@ConditionalOnExpression(
        value = "#{${features}['feature1']['enabled'].equals('true')}"
)

If you had used the selection method it would have looked like this;

@ConditionalOnExpression(
        value = "#{${features}.?[key=='feature1']['feature1']['enabled'].equals('true')}"
)