activiti - Assigning a task to a group or a user

I'm having a bit of trouble figuring out how to assign a simple task to a person OR a group based on the value they select in a group/people picker.

So in my UI a user can select either a person or group using the relevant person/group picker controls.

Using the activiti assignment properties, I set the Assignee as ${selectedPerson} and Candidate Groups as ${selectedGroup}. These variables relate to the values of the picker controls mentioned above:

The overall goal is that the user should be able to assign the task to either a person or a group, however the design above presents the following problems:

• When a user selects a group, the task is assigned successfully to a group, presumably the Assignee value is nullable.
• However, when a user selects a person but not a group, Activiti throws the following exception as it expects a group;

org.activiti.engine.ActivitiIllegalArgumentException: Expression did not resolve to a string or collection of strings

What is the correct way to do this?

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Full Stacktrace

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.activiti.engine.ActivitiIllegalArgumentException: Expression did not resolve to a string or collection of strings
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978)
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:868)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:650)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
    com.codahale.metrics.servlet.AbstractInstrumentedFilter.doFilter(AbstractInstrumentedFilter.java:97)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:316)
    org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:126)
    org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:90)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:114)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:122)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:111)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:149)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:168)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:48)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:205)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:205)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:120)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    com.activiti.security.CustomStatelessCSRFFilter.doFilterInternal(CustomStatelessCSRFFilter.java:68)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:64)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:91)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:53)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330)
    org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:213)
    org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:176)
    org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:344)
    org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:261)

This is because Activiti accepts Candidate Group's ID as a string or comma-separated values of string. but the form element (as you have selected) stores the group ID of the selected group as a Long, hence the IllegalArgumentException. try storing the variable as string after it is selected by the user, then pass this string as the Candidate group.

The method that you are using will only satisfy if both ${selectedPerson} and ${selectedGroup} has non null values.

Alternatively, you can create a Task Listener on the task to be assigned and execute the logic.

Something like:

public class CustomTaskListener implements TaskListener {
    @Override
    public void notify(DelegateTask delegateTask) {
    // get required variables
        String selectedPerson = (String) delegateTask.getVariable("selectedPerson");
        String selectedGroup = (String) delegateTask.getVariable("selectedGroup");
    // check for the condition
        if (null != selectedPerson && !selectedPerson.isEmpty()) {
    // set values
            delegateTask.setAssignee(selectedPerson);
        } else {
            delegateTask.addCandidateGroup(selectedGroup);
        }
    }
}