struts2中级联菜单标签<s:doubleselect>的若干常见问题(二级菜单不显示，FreeMarker问题)

初学struts2没多久，在用到级联菜单时遇到以下两个问题，相信很多人也会遇到。

一、FreeMarker问题
例如下面的代码：

<s:set name="foobar"
        value="#{'Java': {'Spring', 'Hibernate', 'Struts 2'}, '.Net': {'Linq', ' ASP.NET 2.0'}, 
        'Database': {'Oracle', 'SQL Server', 'DB2', 'MySQL'}}" />
<s:doubleselect list="#foobar.keySet()"
        doubleName="technology"
        doubleList="#foobar[top]"
        label="Technology"/>

这个代码没有在一个<s:form>标签里，执行之后，显示就会出现以下错误：

freemarker.core.InvalidReferenceException: Expression parameters.formName is undefined on line 98, column 43 in template/simple/doubleselect.ftl.
    at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:124)
    at freemarker.core.Expression.getStringValue(Expression.java:118)
    at freemarker.core.Expression.getStringValue(Expression.java:93)
    at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
    at freemarker.core.Environment.visit(Environment.java:196)
    at freemarker.core.MixedContent.accept(MixedContent.java:92)
    at freemarker.core.Environment.visit(Environment.java:196)
    at freemarker.core.Environment.include(Environment.java:1375)
    at freemarker.core.Include.accept(Include.java:155)
    at freemarker.core.Environment.visit(Environment.java:196)
    at freemarker.core.MixedContent.accept(MixedContent.java:92)
    at freemarker.core.Environment.visit(Environment.java:196)
    at freemarker.core.Environment.process(Environment.java:176)
    at freemarker.template.Template.process(Template.java:232)
    at org.apache.struts2.components.template.FreemarkerTemplateEngine.renderTemplate(FreemarkerTemplateEngine.java:168)
    at org.apache.struts2.components.UIBean.mergeTemplate(UIBean.java:530)
    at org.apache.struts2.components.UIBean.end(UIBean.java:484)
    at org.apache.struts2.views.jsp.ComponentTagSupport.doEndTag(ComponentTagSupport.java:43)
    at org.apache.jsp.hello_jsp._jspx_meth_s_005fdoubleselect_005f1(hello_jsp.java:613)
    at org.apache.jsp.hello_jsp._jspx_meth_s_005fform_005f0(hello_jsp.java:185)
    at org.apache.jsp.hello_jsp._jspService(hello_jsp.java:104)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:388)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:413)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.yuqiaotech.simplejee.webapp.servlet.SampleFilter.doFilter(SampleFilter.java:46)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.yuqiaotech.simplejee.webapp.servlet.SimpleClickstreamFilter.doFilter(SimpleClickstreamFilter.java:51)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:291)
    at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:877)
    at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:594)
    at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1675)
    at java.lang.Thread.run(Thread.java:619)

解决办法很简单，只要给这个标签的formName属性设置上值即可，如下

<s:set name="foobar" 
        value="#{'Java': {'Spring', 'Hibernate', 'Struts 2'}, '.Net': {'Linq', ' ASP.NET 2.0'}, 
        'Database': {'Oracle', 'SQL Server', 'DB2', 'MySQL'}}" />
<s:doubleselect list="#foobar.keySet()"
        doubleName="technology" 
        doubleList="#foobar[top]" 
        label="Technology" 
        formName="aaa"/>

这样就不会出现上述错误，但紧接着有第二个问题。

二、二级菜单没有值，不显示

这个问题非常好解决，但我也在网上找了很久，不过我也小白了一把，相信很多人在真正用时，都会把这个<s:doubleselect>放在一个FORM里，不然它也就没有意义了。

所以做如下改动就可以了，并且第一个问题中提到要加formName属性也可以去掉了，如下：

<s:form action="Menu" name="aaa">
<s:set name="foobar" 
        value="#{'Java': {'Spring', 'Hibernate', 'Struts 2'}, '.Net': {'Linq', ' ASP.NET 2.0'}, 
        'Database': {'Oracle', 'SQL Server', 'DB2', 'MySQL'}}" />
<s:doubleselect list="#foobar.keySet()"
        doubleName="technology" 
        doubleList="#foobar[top]" 
        label="Technology"/>
</s:form>