apache/pulsar · error · IllegalArgumentException

Function classname cannot be null

Error message

Function classname cannot be null

What it means

For Python runtime functions, doCommonChecks requires a className pointing to the Python class implementing the function (unlike Go, which needs none, and Java, whose className is checked separately in doJavaChecks). A null/empty className with Runtime.PYTHON throws this IllegalArgumentException.

Source

Thrown at pulsar-functions/utils/src/main/java/org/apache/pulsar/functions/utils/FunctionConfigUtils.java:802

                            "Output topic %s is also being used as an input topic (topics must be one or the other)",
                            outputTopic));
        }
    }

    public static void doCommonChecks(FunctionConfig functionConfig) {
        if (isEmpty(functionConfig.getTenant())) {
            throw new IllegalArgumentException("Function tenant cannot be null");
        }
        if (isEmpty(functionConfig.getNamespace())) {
            throw new IllegalArgumentException("Function namespace cannot be null");
        }
        if (isEmpty(functionConfig.getName())) {
            throw new IllegalArgumentException("Function name cannot be null");
        }
        // go doesn't need className. Java className is done in doJavaChecks.
        if (functionConfig.getRuntime() == FunctionConfig.Runtime.PYTHON) {
            if (isEmpty(functionConfig.getClassName())) {
                throw new IllegalArgumentException("Function classname cannot be null");
            }
        }

        Collection<String> allInputTopics = collectAllInputTopics(functionConfig);
        if (allInputTopics.isEmpty()) {
            throw new IllegalArgumentException("No input topic(s) specified for the function");
        }
        for (String topic : allInputTopics) {
            if (!TopicName.isValid(topic)) {
                throw new IllegalArgumentException(String.format("Input topic %s is invalid", topic));
            }
        }

        if (!isEmpty(functionConfig.getOutput())) {
            if (!TopicName.isValid(functionConfig.getOutput())) {
                throw new IllegalArgumentException(
                        String.format("Output topic %s is invalid", functionConfig.getOutput()));
            }

View on GitHub (pinned to 820761864e)

Solutions

1. Set the fully-qualified Python class: functionConfig.setClassName("my.module.MyFunction").
2. Add the 'className' key to the Python function's YAML/JSON config.
3. Ensure the class actually exists in the submitted Python payload/archive and matches the module path.
4. If the function is Go, confirm the runtime is set to GOLANG — then className is not required.

Example fix

// before
config.setRuntime(FunctionConfig.Runtime.PYTHON);
config.setTenant("public");
config.setNamespace("default");
// after
config.setRuntime(FunctionConfig.Runtime.PYTHON);
config.setClassName("examples.function.MyFunction");
config.setTenant("public");
config.setNamespace("default");

if (config.getRuntime() == FunctionConfig.Runtime.PYTHON
        && (config.getClassName() == null || config.getClassName().trim().isEmpty())) {
    throw new IllegalArgumentException("Python functions require FunctionConfig.className");
}

static boolean hasRequiredClassName(FunctionConfig c) {
    if (c == null || c.getRuntime() == null) return false;
    switch (c.getRuntime()) {
        case PYTHON: return c.getClassName() != null && !c.getClassName().trim().isEmpty();
        case GOLANG: return true; // className not required for Go
        default: return true; // Java checked separately in doJavaChecks
    }
}

try {
    FunctionConfigUtils.validateNonJavaFunction(functionConfig, null);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("classname")) {
        throw new IllegalStateException("Python functions need 'className' (e.g. mymodule.MyFunction)", e);
    }
    throw e;
}

When it happens

Trigger: Submitting a function with functionConfig.setRuntime(FunctionConfig.Runtime.PYTHON) but without setClassName("mymodule.MyFunction"), through validateNonJavaFunction, the REST API, or pulsar-admin; Go functions with no className do NOT trigger this — only PYTHON configs.

Common situations: Copying a Go function config template for a Python function (Go configs omit className legitimately); YAML config with 'runtime: PYTHON' but no 'className'; typos in the class name field key.

Understand the failure class

Background: "X is required", "field cannot be empty", error-the-field-is-required: missing required-field validation errors, explained — this error's family across 39 libraries.

Related errors

• When Guarantees == ATMOST_ONCE, autoAck must be equal to tru
• Function class name is not provided.
• Only one of schemaType or serdeClassName should be set in in
• Only one of outputSchemaType or outputSerdeClassName should
• Effectively-once processing guarantees not yet supported in

AI-assisted analysis of apache/pulsar@820761864e (2026-09-06). Data as JSON: /api/errors/8057170f2bc83d95. Report an issue: GitHub.