jstl - how do i pass an expression language variable to a javascript alert?

Question

I'm trying to make an admin page connected to a servlet, where I can insert some elements into a database. It's a demo of a concert ticket booking website and I use a form to input data, and the servlet to validate it.

If the servlet finds invalid data in one or more of the form's fields, it returns a custom message as a formatted string named mismatchOut - one invalidity message per line, eg:

Concert date must be in yyyy-mm-dd format!
start time must be in hh:mm:ss format!

I pass this in my jsp as:

<c:set var="invalidConcertInputMessage" value="${mismatchOut}" scope="page" />

What I want to do is pass this message to an alert, which will show the message to the admin user.

I've tried:

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        alert("${invalidConcertInputMessage}");
    </script>
</c:if>

,

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        alert(${invalidConcertInputMessage});
    </script>
</c:if>

,

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        alert(<c:out value="${invalidConcertInputMessage}");
    </script>
</c:if>

,

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        alert("<c:out value="${invalidConcertInputMessage}" />");
    </script>
</c:if>

and:

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        alert('<c:out value="${invalidConcertInputMessage}" />');
    </script>
</c:if>

and nothing seems to work. I'm probably missing the sintax here, as I'm a total newbie when it comes tu UI.

This works and prints out an alert saying "some text":

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        alert("some text");
    </script>
</c:if>

Please tell me how I can pass my custom message to the alert. Or is this even possible?

Answer 1

It seems the problem was that I was posting newline characters to my custom message string as \\n , but java parsed them as its own newlines, instead of just sending \\n to java script, which resulted in some form of invalid string that it wouldn't parse.

I fixed the problem by replacing \\n with \\n` in the custom string sent by the servlet.

And for anyone else in this predicament, this is the syntax that I got it working with:

<c:if test="${not empty invalidConcertInputMessage}">
    <script>
        var message = "${invalidConcertInputMessage}";
        alert(message);
    </script>
</c:if>