java - Check if Cookie exists with JSP EL

Question

I am trying to check if a cookie exists on a JSP page using the Expression Language.

I have a cookie called persist which is set to either empty string or "checked".

If would like to check if the persist cookie exists.

I have tried the following:

<c:if test="${cookie.persist == null}">

<c:if test="${empty cookie.persist}">

Both of the above statements are true when the value of the persist cookie is the empty string and false when the value of the cookie is checked.

How do I distinguish between a cookie with the empty string as its value, and a cookie that does not exist.

(Note: I can easily work around this problem by assigning a non empty value to the cookie instead of the empty string.)

Answer 1

Closest what you can get is to check the cookie name in the request cookie header.

<c:if test="${fn:contains(header.cookie, 'persist=')}">

However, when there's another cookie with name foopersist, it fails.

If your container supports EL 2.2 (all Servlet 3.0 containers like Tomcat 7, Glassfish 3, etc do) then you could just use Map#containsKey().

<c:if test="${cookie.containsKey('persist')}">

If yours doesn't, best what you can do is to create an EL function (more concrete declaration example can be found somewhere near bottom of this answer):

<c:if test="${util:mapContainsKey(cookie, 'persist')}">

with

public static boolean mapContainsKey(Map<String, Object> map, String key) {
    return map.containsKey(key);
}