Spring PropertyPlaceholder does not work on <httpj:engine> port attribute with custom prefix/suffix

Issue

We are trying to configure the "port" attribute on the <httpj:engine> element with Spring PropertyPlaceholder and custom prefix/suffix:

   <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
        <property name="location" value="classpath:port.properties"/>
         <property name="placeholderPrefix" value="{{"/>
         <property name="placeholderSuffix" value="}}"/>
    </bean>

    <httpj:engine-factory>
        <httpj:engine port="{{ssl.port}}">
            <httpj:tlsServerParameters secureSocketProtocol="TLSv1">
                <sec:keyManagers keyPassword="{{ssl.keystore.keyPassword}}">
                    <sec:keyStore file="{{ssl.keystore}}" password="{{ssl.keystore.password}}" type="JKS"/>
                </sec:keyManagers>
            <sec:trustManagers>
               <sec:keyStore file="{{ssl.truststore}}" password="{{ssl.truststore.password}}" type="JKS"/>
          </sec:trustManagers>
      <sec:cipherSuitesFilter>
              <sec:include>.*_EXPORT_.*</sec:include>
              <sec:include>.*_EXPORT1024_.*</sec:include>
              <sec:include>.*_WITH_DES_.*</sec:include>
              <sec:include>.*_WITH_AES_.*</sec:include>
              <sec:include>.*_WITH_NULL_.*</sec:include>
              <sec:exclude>.*_DH_anon_.*</sec:exclude>
          </sec:cipherSuitesFilter>
          <sec:clientAuthentication want="true" required="true"/>
        </httpj:tlsServerParameters>
       </httpj:engine>
      </httpj:engine-factory>

However, we get the following exception :

org.springframework.beans.factory.xml.XmlBeanDefinitionStoreException: Line 38 in XML document from URL [bundle://287.0:0/META-INF/spring/camel-context.xml] is invalid; nested exception is org.xml.sax.SAXParseException: cvc-datatype-valid.1.2.3: '{{ssl.port}}' is not a valid value of union type 'ParameterizedInt'.

Is it possible to have the "port" as a property? Is there any workaround?