FreeMarker如何取得字符串的长度?

<#--
/*
* $Id: checkboxlist.ftl 720258 2008-11-24 19:05:16Z musachy $
*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*/
-->
<#assign itemCount = 0/>
<#if parameters.list??>
<@s.iterator value="parameters.list">
<#assign itemCount = itemCount + 1/>
<#if parameters.listKey??>
<#assign itemKey = stack.findValue(parameters.listKey)/>
<#else>
<#assign itemKey = stack.findValue('top')/>

<#if parameters.listValue??>
<#assign itemValue = stack.findString(parameters.listValue)?default("")/>
<#else>
<#assign itemValue = stack.findString('top')/>

<#assign itemKeyStr=itemKey.toString() />

<#if tag.contains(parameters.nameValue, itemKey)>
checked="checked"<#rt/>

<#if parameters.disabled?default(false)>
disabled="disabled"<#rt/>

<#if parameters.title??>
title="${parameters.title?html}"<#rt/>

<#include "/${parameters.templateDir}/simple/scripting-events.ftl" />
<#include "/${parameters.templateDir}/simple/common-attributes.ftl" />
/>
${itemValue?html}


<#else>