WMEntity activityEntity = admin.getActivityDefinitionInfo(sessionHandle, assignment.getProcessId(), assignment.getActivityId());
// get performer
WMFilter filter = new WMFilter("Name", WMFilter.EQ, "Performer");
filter.setFilterType(XPDLBrowser.SIMPLE_TYPE_XPDL);
WMAttribute[] performers = xpdlBrowser.listAttributes(sessionHandle, activityEntity, filter, true).getArray();
if (performers.length > 0) {
WMAttribute performer = performers[0];
// get participant
String performerId = performer.getValue().toString();