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The <o:importFunctions> allows the developer to have access to all functions of the given fully qualified name of a type in the Facelet scope using the usual EL functions syntax without the need to register them in .taglib.xml file. The functions are those public static methods with a non-void return type. For example:

<o:importFunctions type="java.lang.Math" var="m" />
<o:importFunctions type="org.omnifaces.util.Faces" />
...
#{m:abs(-10)}
#{m:max(bean.number1, bean.number2)}
...
<base href="#{Faces:getRequestBaseURL()}" />
                

The functions prefix becomes by default the simple name of the type. You can override this by explicitly specifying the var attribute. If there are multiple function methods with exactly the same name, then the one with the least amount of parameters will be used. If there are multiple function methods with exactly the same name and amount of parameters, then the choice is unspecified (technically, JVM-dependent) and should not be relied upon. So if you absolutely need to differentiate functions in such case, give them each a different name.

Note that the colon : operator to invoke the method is as required by EL functions spec. It's by design not easily possible to change it to the period . operator. Also note that in case of org.omnifaces.util.Faces it's considered poor practice if the same functionality is already available through the implicit EL variables #{facesContext}, #{view}, #{request}, etc such as #{request.contextPath} which should be preferred over #{Faces:getRequestContextPath()}.

The resolved functions are by reference stored in the cache to improve retrieving performance.