Iteration-scoped variable in JSF / Richfaces iteration?

Question

OK, this should be an interesting one I think. I want to minimize the number of invocations to my Seam component inside an iteration. I am using Seam 2.2.1.CR1, Richfaces 3.3.3.Final, JSF 1.2 and Facelets.

Please take a look the following Facelet snippet:

<rich:datatable value="#{myBean.products}" var="prod">

<rich:column rowspan="#{rowspan.calcHomePageProductRowspan(prod)}">
    #{prod.name}
</rich:column>

<rich:column rowspan="#{rowspan.calcHomePageProductRowspan(prod)}">
    #{prod.number}
</rich:column>

...

<rich:column rowspan="#{rowspan.calcHomePageProductRowspan(prod)}">
    #{prod.somethingElse1}
</rich:column>

<rich:column rowspan="#{rowspan.calcHomePageProductRowspan(prod)}">
    #{prod.somethingElse2}
</rich:column>

...

<rich:column rowspan="#{rowspan.calcHomePageProductRowspan(prod)}">
    #{prod.somethingElse3}
</rich:column>

</rich:datatable>

In the above code I am computing an attribute (here the rowspan, but that doesn't really matter, it could be any attribute or value at all, so please don't focus on that) by evaluating an EL expression. As you can see, the method that calculates the value takes the current prod as an argument.

I have made an internal optimization in the rowspan Seam component, and it keeps in a HashMap all the already computed values for products. So, when the the EL expression is evaluated at the second rich:column, the rowspan first looks up in the HashMap the already computed value, and returns that, instead of re-computing all over again.

Although this is better that re-computing all over again, I still have to make an invocation to a Seam component. Is there a way to somehow invoke the Seam component only once, and somehow retain the computed value for the current iteration?

The analogous to Java would be to define a variable inside the loop at each iteration, and reuse it throughout the iteration.

Note: I have already made other Seam-oriented optimizations such as @BypassInterceptors, the Seam component is in the EVENT scope, so no hierarchical lookups take place etc.

JSP or Facelets? If JSP, it ends here (no, JSTL <c:set> can't help here inside an UIData component). If Facelets, use <ui:param>. — BalusC

Shervin Asgari Shervin Asgari · Accepted Answer · 2011-01-27T16:25:35

Is there a way to somehow invoke the Seam component only once, and somehow retain the computed value for the current iteration?

Sure, in theory.

But I am not sure I fully understand your question. Which seam component are you talking about? rowspan?

If that is the case, then yeah, its invoked each time you call it, which makes sense. You are looping through a dataTable, and for each row you call it.

Without knowing more details about what you are trying to do, its difficult to suggest an answer. Is the code slow? Is that why you need to optimize further?

Update

Try this, though I am not sure if it works

<rich:dataTable value="#{myBean.products}" var="prod">
   <ui:param name="myrowspan" value="#{rowspan.calcHomePageProductRowspan(prod)}"/>
   <rich:column rowspan="#{myrowspan}">
     #{prod.name}
   </rich:column>
</rich:dataTable>

Second update

So if you don't change your code to be a @Out, @Factory, @Unwrap or similar, then this will always be evaluated each time it runs. This is just how JSF works.
That's why they say that you should do this in your getters, because JSF will call this for each JSF Phase.

public List<Foo> getFoo() {
  if(this.field != null) { 
    field = (List)entityManager.createQuery("from Foo").getResultList();
  } 
  return this.field; 
}

If you wouldn't have cached the list, and checking for null, JSF would hit the database for each phase and for each row in the data table.

Thus this is just something you have to live with.

Iteration-scoped variable in JSF / Richfaces iteration?

1 Answers