f:viewParam with Converter and ViewScoped exception out when invoke the second ajax request

Question

I have this f:viewParam that I try to bind validate and convert a userId into Player, and I got an unexpected results.

<f:metadata>
    <f:viewParam name="userId" value="#{myBean.selectedPlayer}" converter="pConverter"
         converterMessage="Bad Request. Unknown User" required="true"
         requiredMessage="Bad Request. Please use a link from within the system" />

</f:metadata>
<h:body>
    <p:messages id="msgs"/>        
    <h:form>
        <ul>
            <li><a href="index2.xhtml?userId=1">Harry</a></li>
            <li><a href="index2.xhtml?userId=2">Tom</a></li>
            <li><a href="index2.xhtml?userId=3">Peter</a></li>
        </ul>            
    </h:form>
    <h:form>
        <h:panelGrid columns="2" rendered="#{not empty myBean.selectedPlayer}">

            <h:outputText value="Id: #{myBean.selectedPlayer.id}"/>

            <h:outputText value="Name: #{myBean.selectedPlayer.name}"/>

        </h:panelGrid>
    </h:form>
    <h:form id="testForm">
        <h:inputText value="#{myBean.text}"/>
        <p:commandButton value="Switch" update=":msgs testForm"/>
        <h:outputText value="#{myBean.text}" rendered="#{not empty myBean.text}"/>
    </h:form>    
</h:body>

My Converter look like this

@FacesConverter(value="pConverter")
public class PConverter implements Converter {
private static final List<Player> playerList;
static{
    playerList = new ArrayList<Player>();        
    playerList.add(new Player(1, "Harry"));
    playerList.add(new Player(2, "Tom"));
    playerList.add(new Player(3, "Peter"));
}

@Override
public Object getAsObject(FacesContext fc, UIComponent uic, String value) {
    if(value == null || !value.matches("\\d+")){
        return null;
    }
    long id = Long.parseLong(value);
    for(Player p : playerList){
        if(p.getId() == id){
            return p;
        }
    }
    throw new ConverterException(new FacesMessage("Unknown userId: " + value));

}

@Override
public String getAsString(FacesContext fc, UIComponent uic, Object value) {
    if(!(value instanceof Player) || value == null){
        return null;
    }
    return String.valueOf(((Player)value).getId());
}
}

As I click the three link (Harry, Tom, Peter), the converter work great. It converter the id and bind the player back to my managed bean. I then type something in the text box, then click Switch, the first time it work fine, what I typed appear next to the button, but then I change what I type, and click Switch again, then error message appear Bad Request. Please use a link from within the system, which is the error message for required for f:viewParam. If I took the f:viewParam out then everything work fine. Surprisingly, if I switch from f:viewParam to o:viewParam (OmniFaces), then it work great.

BalusC · Accepted Answer · 2014-06-04T10:05:24.633

That's because the <f:viewParam> runs on every single HTTP request, also on postbacks. It works in your case fine for plain GET links, because you're passing exactly that parameter in the links. It fails in your case for POST forms, because you aren't passing that parameter in the button. So it becomes null in the request parameter map and the required validator kicks in and hence this validation error.

To keep the <f:viewParam required="true"> happy on POST forms as well, you basically need to retain the initial request parameter by <f:param> in the command buttons/links.

<p:commandButton value="Switch" update=":msgs testForm">
    <f:param name="userId" value="#{param.userId}" />
</p:commandButton>

The OmniFaces <o:viewParam>, which is designed to be used in combination with view scoped beans, has an additional check in the isRequired() getter (source code here):

@Override
public boolean isRequired() {
    // The request parameter get lost on postbacks, however it's already present in the view scoped bean.
    // So we can safely skip the required validation on postbacks.
    return !FacesContext.getCurrentInstance().isPostback() && super.isRequired();
}

So, this skips the required validator on every postback (and additionally, it also skips setting the model value on every postback due to its stateless nature). That's why you don't see the validation error and you still have the proper model value (which isn't reset on every postback).

Thank you very much. Omnifaces is pretty awesome, BalusC. I have one question about `o:viewParam`, in your blog, you said that omnifaces will avoid invoke the `Converter` every time ajax request invoke, however, it seems that it always get inside method `getAsString()`. So I wonder if this behavior is correct. I know that `getAsObject` is what setting the model value, so not calling `getAsObject` is a great improvement. And I thank you for that. — Thang Pham, May 10 '12 at 17:23
The `getAsString()` will be invoked when the object needs to be converted to string (to present in generated HTML). But it should be a rather cheap job as it's supposed to be the string value of one of its properties already. You've the object already at your hands at that point, so no expensive DB job necessary. — BalusC, May 10 '12 at 17:29

f:viewParam with Converter and ViewScoped exception out when invoke the second ajax request

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