偶遇到 java.util.ConcurrentModificationException 的异常

今天在调试程序 遇到了如此问题  贴上代码来看看
稍后分析

List<String> list = null;
boolean isUpdate = false;
try {
    list = JSON.parse(var.getValue(), ArrayList.class);
    if(CollectionUtils.isNotEmpty(list)){
        for (String workno : list) {
            if (cmd.getUserId().toString().equals(workno)) {
                isUpdate = true;
                list.remove(workno);
                //排除重复
                if (!list.contains(cmd.getTransferUserId().toString())) {
                    list.add(cmd.getTransferUserId().toString());
                }
            }
        }
        map.put(var.getName(), JSON.json(list));
    }
} catch (ParseException e) {
    log.warn("Admin|doTransPermissions|Exception", e);
    throw new UedException(e);
} catch (IOException e) {
    log.warn("Admin|doTransPermissions|Exception", e);
    throw new UedException(e);
}java.util.ConcurrentModificationException原因