json-path/JsonPath · error · JsonPathException

length operation can not applied to null

Error message

length operation can not applied to null

What it means

Jackson3JsonNodeJsonProvider.length returns the size of ArrayNodes/ObjectNodes and StringNode values. When the node is none of these — most commonly null, but also numeric/boolean nodes — the provider falls through to the final throw, reporting 'length operation can not applied to null' (or the class name).

Source

Thrown at json-path/src/main/java/com/jayway/jsonpath/spi/json/Jackson3JsonNodeJsonProvider.java:217

    @Override
    public Collection<String> getPropertyKeys(Object obj) {
        return toJsonObject(obj).propertyNames();
    }

    @Override
    public int length(Object obj) {
        if (isArray(obj)) {
            return toJsonArray(obj).size();
        } else if (isMap(obj)) {
            return toJsonObject(obj).size();
        } else {
            if (obj instanceof StringNode) {
                StringNode element = (StringNode) obj;
                return element.size();
            }
        }
        throw new JsonPathException("length operation can not applied to " + (obj != null ? obj.getClass().getName() : "null"));
    }

    @Override
    public Iterable<?> toIterable(Object obj) {
        ArrayNode arr = toJsonArray(obj);
        Iterator<?> iterator = arr.iterator();
        return new Iterable<Object>() {
            @Override
            public Iterator<Object> iterator() {
                return new Iterator<Object>() {
                    @Override
                    public boolean hasNext() {
                        return iterator.hasNext();
                    }

                    @Override
                    public Object next() {
                        return unwrap(iterator.next());

View on GitHub (pinned to 62a4c9f0f6)

Solutions

1. Verify the node is an array/object/string node before applying length() (node.isArray() || node.isObject() || node.isStringNode())
2. Null-check the read result and supply a default size (0) when null
3. Adjust the path to target an existing container node
4. Use Option.SUPPRESS_EXCEPTIONS or read-with-default helpers for optional fields

Example fix

// before
int n = JsonPath.read(json, "$.items.length()"); // throws when items is null
// after
Object items = JsonPath.read(json, "$.items");
int n = (items instanceof List) ? ((List<?>) items).size() : 0;

Object v = JsonPath.read(json, "$.items");
if (v == null) throw new IllegalStateException("$.items is null; cannot take length");

boolean hasLength(Object o) { return o != null && (o instanceof List || o instanceof Map || o instanceof String); }

try {
    return JsonPath.<Integer>read(json, "$.items.length()");
} catch (PathNotFoundException | JsonPathException e) {
    return 0;
}

When it happens

Trigger: Evaluating $.field.length() where field is a null JSON node or a number/boolean; calling length(obj) directly on a null model; length() paths against missing fields that resolve to null nodes.

Common situations: Counting elements of optional array fields; Jackson3 JsonNode provider configs where absent fields produce NullNode; paths written against an older API shape where the field was always an array.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• length operation can not applied to null
• length operation can not applied to null
• length operation cannot be applied to
• length operation can not applied to
• Not a JSON Node

AI-assisted analysis of json-path/JsonPath@62a4c9f0f6 (2026-09-11). Data as JSON: /api/errors/4278754cf5ac340e. Report an issue: GitHub.