Where is 'param' variable defined in Spring MVC/Apache Tiles application

Question

A default startup Spring MVC with Apache Tiles application has a list of predefined tagx files. I find a variable ${param} is used everywhere but couldn't find there it defined and initiated, please anyone can give a hint about where does it come from? Thanks

For example, from language.tagx:

<jsp:directive.attribute name="locale" type="java.lang.String" required="true" rtexprvalue="true" description="The locale for the language to be added." />
<jsp:directive.attribute name="label" type="java.lang.String" required="true" rtexprvalue="true" description="The country label for the language to be added." />
<jsp:directive.attribute name="render" type="java.lang.Boolean" required="false" rtexprvalue="true" description="Indicate if the contents of this tag and all enclosed tags should be rendered (default 'true')" />

<c:if test="${empty render or render}">
<spring:url var="img" value="/images/${locale}.png" />
<spring:url var="url" value="">
  <c:if test="${null ne param.form}">
    <spring:param name="form" value="1" />
  </c:if>
  <c:if test="${not empty param.find}">
    <spring:param name="find" value="${param.find}" />
  </c:if>
  <spring:param name="lang" value="${locale}" />
  <c:if test="${not empty param.page}">
    <spring:param name="page" value="${param.page}" />
  </c:if>
  <c:if test="${not empty param.size}">
    <spring:param name="size" value="${param.size}" />
  </c:if>
</spring:url>
<spring:message code="global_language_switch" arguments="${label}" var="lang_label" htmlEscape="false" />
<a href="${url}" title="${fn:escapeXml(lang_label)}">
  <img class="flag" src="${fn:escapeXml(img)}" alt="${fn:escapeXml(lang_label)}" />
</a>
<c:out value=" " />
</c:if>

Answer 1

param is an implicit object provided by the Unified Expression Language. It allows you to access the values of request parameters.