flowable/flowable-engine · error · FlowableIllegalArgumentException

User or group are required.

Error message

User or group are required.

What it means

createIdentityLink requires an identity link body that carries at least a user or a group. If both RestIdentityLink.user and RestIdentityLink.group are null, FlowableIllegalArgumentException("User or group are required.") is thrown. The endpoint cannot create an identity link with no identity attached.

Source

Thrown at modules/flowable-cmmn-rest/src/main/java/org/flowable/cmmn/rest/service/api/runtime/caze/CaseInstanceIdentityLinkCollectionResource.java:76

        return restResponseFactory.createRestIdentityLinks(runtimeService.getIdentityLinksForCaseInstance(caseInstance.getId()));
    }

    @ApiOperation(value = "Add an involved user to a case instance", tags = {"Case Instance Identity Links" }, nickname = "createCaseInstanceIdentityLinks",
            notes = "Note that the groupId in Response Body will always be null, as it’s only possible to involve users with a case instance.",
        code = 201)
    @ApiResponses(value = {
            @ApiResponse(code = 201, message = "Indicates the case instance was found and the link is created."),
            @ApiResponse(code = 400, message = "Indicates the requested body did not contain a userId or a type."),
            @ApiResponse(code = 404, message = "Indicates the requested case instance was not found.")
    })
    @PostMapping(value = "/cmmn-runtime/case-instances/{caseInstanceId}/identitylinks", produces = "application/json")
    @ResponseStatus(HttpStatus.CREATED)
    public RestIdentityLink createIdentityLink(@ApiParam(name = "caseInstanceId") @PathVariable String caseInstanceId, @RequestBody RestIdentityLink identityLink) {

        CaseInstance caseInstance = getCaseInstanceFromRequestWithoutAccessCheck(caseInstanceId);

        if (identityLink.getGroup() == null && identityLink.getUser() == null) {
            throw new FlowableIllegalArgumentException("User or group are required.");
        }
        
        if (StringUtils.isEmpty(identityLink.getGroup()) && StringUtils.isEmpty(identityLink.getUser())) {
            throw new FlowableIllegalArgumentException("Only one value of user or group is supported.");
        }

        if (identityLink.getType() == null) {
            throw new FlowableIllegalArgumentException("The identity link type is required.");
        }

        if (restApiInterceptor != null) {
            restApiInterceptor.createCaseInstanceIdentityLink(caseInstance, identityLink);
        }

        if (StringUtils.isNotEmpty(identityLink.getGroup())) {
            runtimeService.addGroupIdentityLink(caseInstance.getId(), identityLink.getGroup(), identityLink.getType());
            
        } else {

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Include either "user" or "group" in the request body.
2. Validate the payload client-side before calling the endpoint.
3. Ensure field names match the RestIdentityLink schema (user, group, type).
4. Catch FlowableIllegalArgumentException and return a 400 to the caller with the message.

Example fix

// before
{"type": "participant"}
// after
{"type": "participant", "user": "kermit"}

// Java
if (link.getUser() == null && link.getGroup() == null) {
    throw new IllegalArgumentException("identity link requires user or group");
}

try { restTemplate.postForEntity(identityLinksUrl, link, RestIdentityLink.class); }
catch (HttpClientErrorException e) { /* 400: User or group are required */ }

When it happens

Trigger: POST /cmmn-runtime/case-instances/{id}/identitylinks with a JSON body lacking both "user" and "group" fields (e.g. only {"type":"participant"}).

Common situations: Malformed request payloads; client serializing only the type field; frontend forms submitted without selecting a user or group; wrong field names in the JSON body.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• The identity link type is required.
• Variable operation is missing for variable: ${variable.getNa
• Variable value is missing for variable: ${variable.getName()
• Variable operation is missing for variable:
• A group or a user is required to create an identity link.

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/ddebf49c6b17eea7. Report an issue: GitHub.