为什么带有内部枚举的表达式语言不会返回值？

Question

我正在使用支持EL 3.0的Wildfly 10.1，因此支持EL中的常量（How to reference constants in EL?）。我有一个内部枚举的课程。

package com.abc.def.helpers;

import java.util.HashMap;
import java.util.Map;

public class MyConstants {
    public enum BusinessObject {
        CALENDAR ("Calendar"),
        PROJECT ("Project");

        private final String friendlyName;
        private static final Map<String, BusinessObject> friendlyNameToEnum = new HashMap<>();

        private BusinessObject(String value) {
            friendlyName = value;
        }

        static {
            for (BusinessObject businessObject : values()) {
                friendlyNameToEnum.put(businessObject.toString(), businessObject);
            }
        }

        @Override
        public String toString() {
            return friendlyName;
        }

        public static BusinessObject fromString(String friendlyName) {
            return friendlyNameToEnum.get(friendlyName);
        }
    }
}

我有一个包含以下内容的JSP文件。

<%@ page import="com.abc.def.helpers.MyConstants.*" %>
Constant scriplet: [<%= BusinessObject.CALENDAR.toString() %>]<br>
Constant EL: [${BusinessObject.CALENDAR}]<br>

结果是：

Constant scriplet: [Calendar]
Constant EL: []

如何通过EL检索值？