How to check for the presence of a session variable in Mule MEL?

Question

I need to check if a session variable exists. I can check if it is present and set to a variable:

<when expression="#[sessionVars['foo'] == 'true']">

But I want something like:

<when expression="#[sessionVars['foo']]">
-- or --
<when expression="#[Exists(sessionVars['foo'])]">

Since, sometimes 'foo' is not assigned a variable.

Any ideas?

Answer 1

Testing for nullity should do the trick:

<when expression="#[sessionVars['foo'] == null]">

Or, as suggested by Anton below:

<when expression="#[sessionVars.containsKey('foo')]">

Answer 2

Suppose the session variable exist request_id = #[sessionVars.ses_request_id]

when you type request_id = #[sessionVars.ses_re] it will automatically shows the mule expression.