Mule MEL usage difference

Question

I have been using different forms of Mule's Expression language. I couldn't figure out the difference between

#[flowVars.myVariable]  

and

#[flowVars['myVariable']]

They both give the result when there is a variable. But why do they behave differently when the variable is not present?

Like if the variable being called is not available, then the first expression would result in a exception. Whereas the second expression just gives out a warning or prints out as is, if in a logger message.

Why is this difference?

Also when going through the documentation for Mule 3.6 I found that the second expression is not longer shown in the documentation.

Is the expression #[flowVars['myVariable']] being deprecated?

Answer 1

The difference comes from the way MVEL deals with these two different ways of accessing map entries.

• #[flowVars['myVariable']] is equivalent to flowVars.get('myVariable') , which does not fail if the flowVars map does not contain the 'myVariable' entry,
• #[flowVars.myVariable] treats the flowVars map as a virtual object, leading to an exception if the 'myVariable' entry is missing because in this case it doesn't resolve to a map get but instead to directly using an object member (either a field or a method), which must exist before being accessed.

I don't think #[flowVars['myVariable']] could be deprecated since it's a core feature provided by MVEL.

Reference: http://mvel.codehaus.org/MVEL+2.0+Property+Navigation#MVEL2.0PropertyNavigation-MapAccess

Answer 2

David has given a nice explanation around your question. To extend that explanation I would just like to add that you can use #[flowVars.?myVariable] to make your code null safe. This is equivalent to #[flowVars['myVariable']].

Regarding #[header:originalFilename], as David said this is not MEL. You can get a list of non-mel expressions which are commonly used in Mule applications in the following link.

http://www.mulesoft.org/documentation/display/current/Non-MEL+Expressions+Configuration+Reference