Unexpected behavior using Mule Expression Language

mulesoft-logoWhile developing flows with a recent Mule ESB there is a big chance you will make use of MEL in your configuration. Although this feature has added great benefits while developing Mule flows it sometimes drives me crazy. In this post I will show two examples which took me some time to get it working.
The first was when I was using an expression-transformer to get a part of an XML document as payload. The expression I tried first was:

  <return-argument evaluator="xpath" expression="//test" />

I used the following xml as an example input:


This results in an empty payload, not a NullPayload but an empty String as payload.
However if I define my transformer like the following:

 <expression-transformer expression="xpath('//test')"/>

it results in a Dom4J Object as payload with the following String content:


which was exacty what I needed. I haven’t checked the source code to see if it could be clarified or explained somehow but it is good to know this difference in behavior.

The other example I ran into had to do with checking if a message property existed in a choice expression. I wanted to see if a incoming message (SOAP/XML) had an attachment attached to it. This becomes visible in the Mule Message as a ‘cxf_attachments’ property at the INVOCATION scope. So I first tried it with the following MEL expression:

<when expression="#[header:INVOCATION:cxf_attachments?==null">

I also tried several alternatives of this expression but I couldn’t get it to work. So I used the ‘old-fashioned’ way that could be used before MEL was in place:

<when expression="message.getInvocationProperty('cxf_attachments') != null" evaluator="groovy">

This worked like a charm. So you see that although MEL does add nice features I sometimes find it difficult to determine how it behaves in certain situations.

About Pascal Alma

Pascal is a senior software developer and architect. Pascal has been designing and building applications since 2001. He is particularly interested in Open Source toolstack (Mule, Spring Framework, JBoss) and technologies like Web Services, SOA and Cloud technologies. Lately he is having great fun by building iOS apps with Swift. Specialties: JEE AWS XML/XSD/XSLT Web Services/SOA Mule ESB/ WSO2 ESB Maven Cloud Technology Swift/ iOS
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