Customizing the calculated field evaluation context

A formula of a calculated field is evaluated as a spring expression language statement against a SpEL EvaluationContext.

Default evaluation context

The current document is always the root object against which expressions are evaluated.

In addition the default EvaluationContext maps all functions of DynamicDocumentFormulaFunctions as if they are functions directly on the root object. This allows short-hands like sum(1,2) instead of T(….DynamicDocumentFormulaFunctions).sum(1,2).

For security reasons, the default evaluation context does not allow access to components of the BeanFactory. Expressions like @myHelperBean.doSomething() will not work out of the box.

Customizing the evaluation context

Whenever an evaluation context for a document is created, a BuildDynamicDocumentFormulaEvaluationContext event is published. Listening for the event allows a developer to customize the evaluation context or even replace it entirely.

The event also provides information of the DynamicDocumentDefinition (document type) for which a context is being created. Allowing the use of a different evaluation context for different document types.

Example customizing an evaluation context

@Configuration
@RequiredArgsConstructor
static class CustomEvaluationContextConfiguration
{
    private final BeanFactory beanFactory;

    @EventListener(condition = "#document.definitionId == 'my-dataset:my-document'") (1)
    public void registerCustomEvaluationContext( BuildDynamicDocumentFormulaEvaluationContext document ) {
        StandardEvaluationContext ec = BuildDynamicDocumentFormulaEvaluationContext.createDefaultEvaluationContext();
        ec.setBeanResolver( new BeanFactoryResolver( beanFactory ) ); (2)
        document.setEvaluationContext( ec );
    }
}

1	Listen for event of type `BuildDynamicDocumentFormulaEvaluationContext`, but only if it is for a document of the given type (`condition` attribute).
2	Set a `BeanResolver` on the evaluation context, allowing direct access to the beans defined in the `BeanFactory`. This enables expressions like `@myHelperBean.doSomething($(myField))`.

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