如何在grails服务类中使用g.formatNumber

我想在服务中使用g.formatNumber,我尝试了以下方法,我上网了.这不起作用,它给我错误"Cannot invoke method formatNumber() on null object",代码如下

    import org.springframework.beans.factory.InitializingBean

    class MyService implements InitializingBean {
        boolean transactional = false
        def gspTagLibraryLookup  // being automatically injected by spring
        def g

        public void afterPropertiesSet() {
            g = gspTagLibraryLookup.lookupNamespaceDispatcher("g")
            assert g
        }

      def getFormattedNumber(){
       def number = g.formatNumber(number: 5000,234 , type: "number" , maxFractionDigits: 2)
       return number
     }
}

这该怎么做.