json-path/JsonPath · error · InvalidPathException

Function with name: does not exist.

Error message

Function with name:  does not exist.

What it means

PathFunctionFactory.newFunction(name) looks up a registered function class by name (e.g. length(), sum(), concat()). If the name is not in the FUNCTIONS map, it throws InvalidPathException('Function with name: X does not exist.'). This guards the function-call syntax in paths like $.books.length().

Source

Thrown at json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java:79

    /**
     * Returns the function by name or throws InvalidPathException if function not found.
     *
     * @see #FUNCTIONS
     * @see PathFunction
     *
     * @param name
     *      The name of the function
     *
     * @return
     *      The implementation of a function
     *
     * @throws InvalidPathException
     */
    public static PathFunction newFunction(String name) throws InvalidPathException {
        Class functionClazz = FUNCTIONS.get(name);
        if(functionClazz == null){
            throw new InvalidPathException("Function with name: " + name + " does not exist.");
        } else {
            try {
                return (PathFunction)functionClazz.newInstance();
            } catch (Exception e) {
                throw new InvalidPathException("Function of name: " + name + " cannot be created", e);
            }
        }
    }
}

View on GitHub (pinned to 62a4c9f0f6)

Solutions

1. Use a supported built-in function name: length(), sum(), avg(), min(), max(), stddev(), concat(...), append(...).
2. For a custom function, register it first: PathFunctionFactory.registerFunction("myfunc", MyFunc.class).
3. Print the path and check the segment after '.' before '()' for typos.
4. Catch InvalidPathException and validate user-supplied paths against a whitelist of allowed functions.

Example fix

// before
Integer n = JsonPath.read(json, "$.books.size()"); // no such function
// after
Integer n = JsonPath.read(json, "$.books.length()");

Set<String> allowed = Set.of("length","sum","avg","min","max","stddev","concat","append");
for (String f : extractFunctionNames(path)) if (!allowed.contains(f)) throw new InvalidPathException("Unknown function: " + f);

try {
    return JsonPath.read(json, path);
} catch (InvalidPathException e) {
    if (e.getMessage().contains("does not exist")) { /* unknown function: report name */ }
    throw e;
}

When it happens

Trigger: A path uses function-call syntax with an unknown or misspelled function name — e.g. $.items.size() or $.arr.count() — passed to JsonPath.read()/compile(). Also occurs when a custom function is not registered via PathFunctionFactory.registerFunction().

Common situations: Confusing Java-ish names (size, count) with JsonPath functions (length, sum, avg, min, max, stddev, concat, append), using functions from non-default modules that were never registered, or typos in dynamic path strings.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• Arguments to function: '' are not closed properly.
• Could not find matching close quote for %s when parsing : %s
• Could not find matching close for %s when parsing regex in :
• Expected character: %c
• Filter must start with '[' and end with ']'.

AI-assisted analysis of json-path/JsonPath@62a4c9f0f6 (2026-09-11). Data as JSON: /api/errors/fbac2e39606b17a7. Report an issue: GitHub.