根据条件在按钮单击上显示确认弹出窗口

单击时有一个p:commandButton，我需要在列表中添加一些值.在我的托管bean中，我正在验证必须添加的值，如果该值验证为false，则必须显示一个确认弹出窗口.这是我的代码-

I have a p:commandButton on click of which I need to add a few values to a list. In my managed bean, I'm validating the value that has to be added, and if it validates to false, I have to display a confirmation popup. This is my code -

<p:commandButton id="add" value="Add" type="submit"                                      action="#{bean.doAdd}" ajax="false"
update=":List"/>

然后在bean中，单击添加"按钮，

And in the bean, on click of the "Add" button,

public String doAdd() throws Exception {
        if(response != null) {
            if(keyList.contains(response))  {
                if(!responseList.contains(response)) {
                    responseList.add(response);
                } 
            } else {
               //Have to display confirmation popup.
            }
            response = "";
        }

        return response;
    }

我正在使用jsf 2.0和primefaces 3.0.有人可以告诉我如何显示Bean的弹出窗口吗?

I'm using jsf 2.0 and primefaces 3.0. Can someone please tell me how to display the popup from the bean?