json-path/JsonPath · error · JsonPathException

Aggregation function attempted to calculate value using empt

Error message

Aggregation function attempted to calculate value using empty array

What it means

Same invoke() flow as the sequence functions' index error: if the object list is empty, there is nothing to select and the function throws JsonPathException('Aggregation function attempted to calculate value using empty array'). This branch fires before any index math when objectList has no elements.

Source

Thrown at json-path/src/main/java/com/jayway/jsonpath/internal/function/sequence/AbstractSequenceAggregation.java:40

    public Object invoke(String currentPath, PathRef parent, Object model, EvaluationContext ctx, List<Parameter> parameters) {
        if(ctx.configuration().jsonProvider().isArray(model)){

            Iterable<?> objects = ctx.configuration().jsonProvider().toIterable(model);
            List<Object> objectList = new ArrayList<>();
            objects.forEach(objectList::add);
            int targetIndex = this.targetIndex(ctx, parameters);
            if (targetIndex >= 0) {
                return objectList.get(targetIndex);
            } else {
                int realIndex = objectList.size() + targetIndex;
                if (realIndex > 0) {
                    return objectList.get(realIndex);
                } else {
                    throw new JsonPathException("Target index:" + targetIndex + " larger than object count:" + objectList.size());
                }
            }
        }
        throw new JsonPathException("Aggregation function attempted to calculate value using empty array");
    }
    
    protected int getIndexFromParameters(EvaluationContext ctx, List<Parameter> parameters) {
        List<Number> numbers = Parameter.toList(Number.class, ctx, parameters);
        return numbers.get(0).intValue();
    }
}

View on GitHub (pinned to 62a4c9f0f6)

Solutions

Check the array is non-empty before invoking first()/last()/index().
Catch JsonPathException and return null/default instead of the first element.
Prefer application-side access (list.get(0)) with an isEmpty() guard for clearer control flow.
Use Option.SUPPRESS_EXCEPTIONS where the library supports suppressing such errors.

Example fix

// before
Object first = JsonPath.read(json, "$.results.first()"); // results = []
// after
List<Object> r = JsonPath.read(json, "$.results");
Object first = (r != null && !r.isEmpty()) ? r.get(0) : null;

Defensive patterns

Strategy: validation

Validate before calling

List<?> r = JsonPath.read(json, "$.results");
Object first = (r != null && !r.isEmpty()) ? r.get(0) : null;

Try / catch

try {
    return JsonPath.read(json, "$.results.first()");
} catch (JsonPathException e) {
    if (e.getMessage().contains("empty array")) return null;
    throw e;
}

Prevention

Guard first()/last() calls with isEmpty() checks
Default to null rather than throwing when collections may be empty
Use SUPPRESS_EXCEPTIONS where acceptable and null-check results

When it happens

Trigger: Calling first(), last(), index(n), or slice() on an empty array, e.g. $.items.first() where items is [], via JsonPath.read()/compile().

Common situations: Taking 'the first' element of collections that can legitimately be empty (no search results, no notifications), and empty fixture data in tests.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

AI-assisted analysis of json-path/JsonPath@62a4c9f0f6 (2026-09-11). Data as JSON: /api/errors/c3271260473342ce. Report an issue: GitHub.