Using enum in JSTL

问题:

I am trying to do some website development using jstl and I run into the following problem:

Here I am trying to create a dropdown, where the displayed value is the country names, and the value is the country code. To do this I have the following enum in the backend java code:

public static enum CountryCodes implements EnumConstant {
       USA, CAN, AUS, GBR, DEU, ESP, GUM, IND, ISR, MEX, NZL, PAN, PRI;

       public final String toCountry(){
               switch(this){
               case USA:
                       return "United States";
               case CAN:
                       return "Canada";
               case AUS:
                       return "Australia";
               case GBR:
                       return "Great Britan";
               case DEU:
                       return "Germany";
               case ESP:
                       return "Spain";
               case GUM:
                       return "Guam";
               case IND:
                       return "India";
               case ISR:
                       return "Isreal";
               case MEX:
                       return "Mexico";
               case NZL:
                       return "New Zealand";
               case PAN:
                       return "Panama";
               case PRI:
                       return "Puerto Rico";

               }
               return this.toString();
       }
}

And the jsp code snippet is like the following:

<c:set var="countryCodes" value="<%=RequestConstants.CountryCodes.values()%>" />
<td>
    <select id="<%=RequestConstants.CLModifyPage.COUNTRY_CODE%>"
        name="<%=RequestConstants.CLModifyPage.COUNTRY_CODE%>">
        <c:forEach items="${countryCodes}" var="countryCode">
            <c:choose>
                <c:when
                    test="${sessionScope.CURRENT_INSTITUTION.countryCode == countryCode}">
                    <option value="${countryCode}" selected="selected">
                        ${countryCode.toCountry()}</option>
                </c:when>
                <c:otherwise>
                    <option value="${countryCode}">${countryCode.toCountry()}
                        </option>
                </c:otherwise>
            </c:choose>
        </c:forEach>
    </select>
</td>

But the above code has two problems:

1. countryCode.toCountry() doesn't really work... I am not sure what syntax it should be.

2. if "${sessionScope.CURRENT_INSTITUTION.countryCode}" is not a valid enum value, i.e, if it's something like "AAA", then the comparison fails and throws an java.lang.IllegalArgumentException: no enum const CountryCodes.AAA defined. How can I get around that?

回答1:

Your approach is too complicated.

Redesign your enum as follows:

public enum CountryCode {

    USA("United States"), 
    CAN("Canada"),
    AUS("Australia");
    // ...

    private String label;

    private CountryCode(String label) {
        this.label = label;
    }

    public String getLabel() {
        return label;
    }

}

(note that it has now a fullworthy and more efficient getter!)

Store the enum values in the application scope during servlet's init() method or, better, during ServletContextListener's contextInitialized() method:

servletContext.setAttribute("countryCodes", CountryCode.values());

Finally traverse it as follows:

<select name="countryCode">
    <c:forEach items="${countryCodes}" var="countryCode">
        <option value="${countryCode}" ${institution.countryCode == countryCode ? 'selected' : ''}>${countryCode.label}</option>
    </c:forEach>
</select>

回答2:

If you are using spring you can

<form:select path="_path" >
    <spring:eval expression="T(com.EnumName).values()" var="_enum"/>
    <c:forEach items="${_enum}" var="_value">
        <form:option value="${_value}" label="${_value.label}"/>
    </c:forEach>
</form:select>