json-path/JsonPath · error · JsonPathException

length operation can not applied to null

Error message

length operation can not applied to null

What it means

JacksonJsonNodeJsonProvider.length throws JsonPathException when the target object is not a type that supports length() (Array, Map, Collection, TextNode). The message "length operation can not applied to null" (or a class name) means the JsonPath length() function was evaluated on an object that is null or of an unsupported node type (e.g. IntNode, BooleanNode).

Source

Thrown at json-path/src/main/java/com/jayway/jsonpath/spi/json/JacksonJsonNodeJsonProvider.java:225

        while (iter.hasNext()){
            keys.add(iter.next());
        }
        return keys;
    }

    @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 TextNode) {
                TextNode element = (TextNode) 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() {

View on GitHub (pinned to 62a4c9f0f6)

Solutions

1. Make the path definite (e.g. $.items[*]) or check the node for null/exists before applying length()
2. Add a filter guard like ?(@.items != null) or use a default value so length() only runs on present arrays/strings
3. Verify the JSON shape: if the value is a scalar number, compute length differently or fix the schema/producer
4. Catch JsonPathException around the length evaluation and treat it as 'length undefined'

Example fix

// before
Integer n = JsonPath.read(json, "$.maybeArray.length()");
// after
List<Object> arr = JsonPath.read(json, "$.maybeArray");
Integer n = (arr == null) ? 0 : arr.size();

Object node = JsonPath.read(json, "$.items");
boolean lengthApplies = node != null
    && (node instanceof List || node instanceof Map || node instanceof String || node instanceof TextNode);
if (!lengthApplies) throw new IllegalStateException("length() not applicable to: " + node);

static boolean supportsLength(Object o) {
    return o != null && (o instanceof List || o instanceof Map
        || o instanceof Collection || o instanceof CharSequence
        || o instanceof com.fasterxml.jackson.databind.node.TextNode);
}

try {
    int n = JsonPath.read(json, "$.items.length()");
} catch (JsonPathException e) {
    if (e.getMessage() != null && e.getMessage().startsWith("length operation can not applied")) {
        n = 0; // treat undefined length as empty
    } else throw e;
}

When it happens

Trigger: Evaluating a filter or length() function such as $.items.length() where the path resolves to null (missing property), a numeric/boolean/object node, or a non-TextNode scalar instead of an array/map/string/collection.

Common situations: Optional JSON fields missing in some documents so the path yields null; expecting a string but receiving a number node; calling length() on an object node instead of an array; defensive length checks in filters over heterogeneous documents.

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 cannot be applied to
• length operation can not applied to
• length operation can not applied to null
• length operation can not applied to null
• Put can not be performed to multiple properties

AI-assisted analysis of json-path/JsonPath@62a4c9f0f6 (2026-09-11). Data as JSON: /api/errors/72bde993a64a74bd. Report an issue: GitHub.