jsf 2 - How to validate particular fields in a form before saving the entire form in Primefaces -

i have jsf form save data,in form there fileuploaded component of primefaces, each , every file upload, has particular description. using event listener operation.now requirement want restrict user when upload file without adding description has display validation message "please add description" process should done before submit whole form.

source code:

<h:panelgrid id="ff" columns="2" cellpadding="8">                      <p:outputlabel for="file" value="description:" styleclass="label"                                 style="font-weight:bold" />                             <p:inputtextarea rows="5" cols="30" id="file"                                 value="#{manageproject.desc}" required="true"                                 requiredmessage="please enter file desccription" styleclass="input"                                 countertemplate="{0} characters remaining." autoresize="false" >                                 <p:ajax listener="#{manageproject.handlefiles()}"></p:ajax>                                 </p:inputtextarea>                     </h:panelgrid>                          <p:fileupload                              fileuploadlistener="#{manageproject.handlefileupload}"                             mode="advanced" dragdropsupport="false" multiple="false"  update="ff"                             sizelimit="5242880" filelimit="3" invalidsizemessage="maximum file size should 5mb"                              >                              </p:fileupload>  

beanclass:

public void handlefileupload(fileuploadevent event) {           docs=new docs();          try {                  byte[] data = new byte[(int) event.getfile().getsize()                                         ];             inputstreams = event.getfile().getinputstream();                                          inputstreams.read(data);                                         docs.setimage(data);                                         docs.setname(event.getfile().getfilename());                                         system.out.println("desc="+desc);                                         docs.setdesc(desc);                                         docses.add(docs);                                                                 facesmessage msg = new facesmessage(event.getfile().getfilename()+" "+"has been uploaded.");                                               facescontext.getcurrentinstance().addmessage(null, msg);          }