nesting of el expressions in JSF for resource API

问题

<p:graphicImage value="#{resource['images:primefaces-ui/#{car.manufacturer}.jpg']}"/>

I would like to calculate the inner el first and then calculate the path using resource API. I tried <c:set> tag but its variable is also an EL, so it does not make any difference.

回答1:

In the current EL 2.2 version, you cannot nest EL expressions nor String-concatenate EL variables that way. Use <c:set> to prepare the dynamic key before using it in another EL expression by simply inlining the EL expression in a string:

<c:set var="resourceName" value="images:primefaces-ui/#{car.manufacturer}.jpg" />
<p:graphicImage value="#{resource[resourceName]}"/>

An alternative is simply using library and name attributes instead of generating an URL based on those via #{resource} mapping:

<p:graphicImage library="images" name="primefaces-ui/#{car.manufacturer}.jpg" />

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Update: since EL 3.0, you can use the += operator to String-concatenate EL variables, if you really can't use library/name for some reason.

<p:graphicImage value="#{resource['images:primefaces-ui/' += car.manufacturer += '.jpg']}"/>

来源：https://stackoverflow.com/questions/7975089/nesting-of-el-expressions-in-jsf-for-resource-api