Salient Soft

Update 30/1/2012

After further testing, I have found more issues trying to use a parameterised concrete class in a JPA constructor expression. EclipseLink was throwing MethodNotFoundException complaining that it could not find the constructor method for the class, due to the use of the parameterised type in one of the constructor arguments. The precise circumstances where this does and does not work are not clear.

My solution has been to not use a parameterised class at all in a JPA constructor expression. To avoid most of the code duplication, what I did was to subclass the parameterised class for each concrete type I was using, i.e. I created a concrete subclass which only contained a constructor calling the superclass constructor. This then allowed me to use a typed query in JPA, as the returned class was now non-parameterised and so I could create a class literal for it.

Original Post

Whilst in some situations this may be an issue, it is also a statement of fact!

This is due to type erasure at run time, and cannot be done.

It means, for example, that in some cases it is not possible to completely eliminate unchecked warnings.

One example of mine was the use of a parameterised type in a constructor expression class in JPA. I had a requirement to do fetches which involved returning an entity plus another column that could not be handled as a relationship/JPA entity property. The property in question was the count of children for a tree node entity. As I was using path enumeration to describe the tree, I could not use JPA relationships to handle it, and so had to return the child count separately. I had more than one type of tree, all handled polymorphically, and so wanted to re-use the constructor class by making it parameterised.

This all worked fine but the class could not have its class literal passed when created a TypedQuery to fetch the results back. The solution was to use some casting isolated in a specific method, and to suppress the generic warnings via @SuppressWarnings("unchecked").

The following posts detail this issue:-

http://stackoverflow.com/questions/2390662/java-how-do-i-get-a-class-literal-from-a-generic-type

http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#Why is there no class literal for the concrete instantiation of a parameterized type?

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It is a design feature of servlet sessions that if you open the same application in multiple windows in the same browser type, the session data is shared across them all. This post on Stack Overflow discusses the issue.

Therefore, if for example you need multiple sessions present in order to test Optimistic Lock collisions, you need a way around this. There are 2 solutions :-

1. Use a different browser type in each window. For example, if you use Firefox and Chrome as a pair, or Firefox and Opera for example, you will in fact get separate sessions as there is no sharing of cookies and other related data across different browser types.
2. Log in from another computer, for example by using an RDP session to another computer on the same desktop and running the application from there. In this case, both browser types can be the same.

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I use the click event to validate a navigation, returning true to allow it and false to cancel it.
On some occasions, the toolbar buttons (which are based internally on h:link with pre-emptive Get style navigation based on an outcome) do not navigate event when the click event correctly returns true.

Some toolbar buttons always did this, others always worked. No explanation could be found for the failure to navigate, even after extensive testing and online searching for answers.

The solution to this was to return cancel in the click event, but to have the click event explicitly do the navigation itself via Javascript. The following code fragment illustrates the problem:-

<h:link id="link" disabled="#{disabled}" outcome="#{outcome}" href="#{href}" title="#{title}" tabindex="#{tabIndex}"
      onclick="if (#{empty onNavigate ? ‘true’ : el:format1(onNavigate, el:concat3(‘\”, component.clientId, ‘\”))})
      window.location.href=this.href; return false;">

The if test checks if the navigation is require, then if so, window.location.href is set to the target href in the component. False is always returned so that standard navigation is disabled.

This solved the problem consistently. (This has also been logged as Mantis issue 104).

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This is a tricky issue to control correctly. The following points have been found during testing (Mojarra 2.0.4 FCS / Primefaces 2.2.1 / Glassfish 3.0.1)

1. One key issue is that to start a new conversation on a page, the navigation to that page must not propagate any previously active conversation. This will then start a new transient (i.e. not long-running/not started) conversation on the new page, and conversation.begin() can be used to make it long-running (i.e. start it).
2. When doing a standard JSF Ajax postback, the current conversation is always propagated, and it does not appear to be possible to prevent this/start a new conversation from a postback. JSF always adds the current CID as a querystring parameter
3. To prevent propagation and to allow a new page to start a new conversation, GET navigation needs to be performed. This fits in well with my main toolbar options – I use pre-emptive GET navigation for the main pages with full page refreshes.
4. Primefaces p:button allows the new JSF 2 outcome style GET navigation but cannot be stopped from propagating a conversation – it always adds a CID parameter. If you add one with an empty or null value, you still get a “cid=” attribute which CDI then spits out as an invalid conversation on the new page.
5. h:link can be stopped from propagating by using an explicit f:param for the cid, providing the param has no value or an explicit null value (an empty string will not work).
6. Trying to refer to another inactive conversation, e.g. by storing conversation scoped bean references in a session level map and looking them up, seriously screws up CDI with an extremely long/possibly recursive stack trace. (See Mantis issue xxx for the stack dump). This is likely to be due to the proxying mechanism used for conversations.
7. Therefore, it is not possible to invent say a ‘onPageLeave’ event for a previous page and call it after a new page has started and had its conversation activated.
8. If therefore I need to refresh all pages for the application, for example if an OptimisticLockException occurred, I use a central workspace session bean to manage all the pages. I set a refresh pending flag in a map for each page, and then when the page is next visited, the workspace bean calls a refresh event of my own for the page, and then clears the pending flag. When processing the current page after an OptimisticLockException, I can refresh directly if the user requests it from a lock dialog.
9. My workspace bean holds the CID for each page against the page name in a map, and uses this when rendering the Toolbar, adding the appropriate cid= parameter for each toolbar option. In fact I use an outer map keyed on page group name (where the page group corresponds to a top level toolbar option), with an inner map containing all the cids and refresh pending flags for the individual pages in that group. This allows for example a report toolbar option to be a menu of currently active reports, and allows new reports to be created dynamically as new conversations and added to the list under the report menu on the toolbar.
10. Conversations are started and ended via calls to the workspace bean, which can then add/remove entries from the maps to keep track of what pages/cids are currently active.
11. Regarding navigation and cid propagation, another option is to call JSF directly to obtain the navigation case for an outcome. ConfigurableNavigationHandler allows you to pass an outcome and get the actual navigation case, from which you can then get the URL. This should allow full control of the cid= propagation parameter on the querystring, and allow control for components which do not support JSF 2.0 style GET pre-emptive navigation directly from an outcome. The url can be passed to any component which accepts a url, and should then still perform the correct navigation and conversation control. See CoreJSF 3 page 333 for a code example on this. This method could prove useful for example with a Primefaces MenuItem, which does either JSF postback style navigation, or URL navigation, but does not directly do pre-emptive GET navigation by giving it an outcome.

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Initially this looked very strange. When navigating to a new form, the rendered= methods for the old form were being called again, and were failing due to scoping issues – a new request scope bean was being created causing a null pointer exception in the method as required state had been lost.

This post and also this post on StackOverflow explain the problem. Apparently JSF rechecks the rendered conditions after the submit as part of an attack safeguard. This is what caused recreation of a new bean and the failure.

This behaviour was certainly unexpected, and I have not seen it documented ‘up front’ previously – so it is certainly one to be aware of! This is not the first time I have had issues with the render= attribute.

In my case, resolving issues in creation of conversations solved the problem – once I had conversations propagating properly, everything worked correctly and the state was present for the extra calls to the rendered= methods.

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This post here discusses using an EL reference to specify a converter. This gets around the fact that you cannot use CDI beans in a converter class.

You can also do this in a validator, for example using the validator attribute on h:inputText. This is useful if you need access to other instance variables in order to do the validation. In my case, I needed access to a list for a table to verify uniqueness on string names in the list.

This is mentioned in Core JSF 3 on page 294:-

In the preceding section, you saw how to implement a validation class. However, you can also add the validation method to an existing class and invoke it through a method expression, like this:
<h:inputText id=”card” value=”#{payment.card}”
required=”true” validator=”#{payment.luhnCheck}”/>
The payment bean must then have a method with the exact same signature as the validate method of the Validator interface:

public class PaymentBean {
…
public void luhnCheck(FacesContext context, UIComponent component, Object value) {
… // same code as in the preceding example
}
}

Why would you want to do this? There is one major advantage. The validation method can access other instance variables of the class. You saw an example in the section, “Supplying Attributes to Converters” on page 289.

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