Set empty string as null in JSTL/JSP

Question

I'm developing a web application using JSP/Servlet/JSTL in Eclipse Juno and GlassFish 3.1.2.1. What I need to achieve is to find a way to set the empty strings as null, E.g. Receive a parameter from another page that is set to an empty string and detect it as null. I have been searching and I found a parameter in JSF that I put in the web.xml .

<context-param>
    <param-name>
        javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL
    </param-name>
    <param-value>true</param-value>
</context-param>

But It didn't work. So what am I missing in those lines? How can I set the empty strings as null?.

As an extra note I need to set this because the system will be working in a Websphere where the empty strings are treated as null.

Thanks in advance.

UPDATE

Another example could be like this:

OnePage.jsp

<form action="anotherPage.jsp">
   <input type="text" name="foo" id="idFoo" value="" />
</form>

anotherPage.jsp

<c:out value="${param.foo}"/>

In GlassFish 3.1.2.1 the c:out tag would print nothing as the value is set to empty string. In Websphere Application Server, the value would be null. I need that GlassFish display null values instead of empty strings, as Websphere does.

Not sure understand your question. You want to convert parameter values to NULL if they are empty using request.getParameter method? — Loc
I've added another example of what I need in the description, thanks :) — Jessai

Loc Loc · Accepted Answer · 2013-12-11T17:19:35

You are misunderstanding about how JSTL expression work. If the variable is NULL OR event Not exists c:out will produce NOTHING (Empty). This is a specification of JSTL expression. I don't understand why your WebSphere produce "null". You need to re-check.

For examples:

if foo == null --> Result is empty
if foo does not exists --> Result is empty ( No exception )
if foo is empty --> Of course the result is empty
if foo is "null" ( String ) ==> the result is null

For your case. If you want to display NULL instead of empty, you need to do like this:

<c:if test="${empty param.foo}">null
</c:if>
<c:if test="${not empty param.foo}">
    <c:out value="${param.foo}"/>
</c:if>

OR you can create JSTL function to handle this case:

<tf:outNullOrValue (param.foo) />

The function outNullOrValue like this:

public static String outNullOrValue(String s) {
    return s == null ? "null" : s;
}

Of course, you need to create TLD for the function and declare taglib directive in your JSP.

Set empty string as null in JSTL/JSP

1 Answers