Mule Expression Language Tips

As a shortcut, Mule accepts an expression such as #[payload], rather than #[message.payload] because it knows to automatically evaluate the expression for the message context object. This shortcut only applies with the payload field.

Mule assumes that any unquoted string contained in a Mule Expression is a flow variable. To set the value of flow variable 'foo' to 'bar', the expression #[foo == bar] actually evaluates as:

#[flowVars.foo == flowVars.bar]

To access the flow variable, use one of the following expressions:

#[flowVars.foo]

#[foo]

To create an integer:

#[12]

To create a string:

#['mystring']

Return true if the lastname query string parameter from an HTTP listener is not null:

#[message.inboundProperties.'http.query.params'.lastname != null]

Return the number of elements in http.method:

#[message.inboundProperties.'http.query.params'.size()]

Return true if the number of elements in the map http.query.params is greater than 50:

#[message.inboundProperties.'http.query.params'.size() > 50]

Testing for Emptiness: The special literal empty tests the emptiness of a value. It returns an empty value depending on context. empty evaluates to:

Testing for NullPayload: Return true if message payload is null:

#[payload == null]

To create a session variable named sessionId whose value is the concatenation of the current message ID, "@" and the Mule instance node ID, use either set-session-variable or MEL code:

<set-session-variable variableName="sessionId" value="#[message.id+'@'+mule.nodeId]" />
<expression-component>sessionVars.sessionId = message.id+'@'+mule.nodeId;</expression-component>

The following shows how a bean can be dynamically retrieved from the registry, based on a name created by taking a value from a bean payload with a getTargetService() accessor:

<set-variable variableName="beanName" value="#[message.payload.targetService+'Processor']" />
<set-variable variableName="bean" value="#[app.registry.beanName]" />

To copy the current payload in a flow variable named originalPayload then restore it:

<set-variable variableName="originalPayload" value="#[message.payload]" />
<set-payload value="#[originalPayload]" />

To retrieve the message payload in a particular format, using Mule’s auto-transformation capability, use payloadAs:

<logger message="#[message.payloadAs(java.lang.String)]" />

To extract all *.txt and *.xml attachments, use a filtered projection:

<expression-transformer expression="#[($.value in message.inboundAttachments.entrySet() if $.key ~= '(.*\\.txt|.*\\.xml)')]" />

To ask for a null payload:

#[payload is NullPayload]

Assign to variable lastname the value of the message inbound property lastname:

#[lastname = message.inboundProperties.lastname]

Append a string to the message payload:

#[message.payload + 'mystring']

Call a static method:

#[java.net.URLEncoder.encode()]

Create a hash map:

#[new java.util.HashMap()]

Create a directory named target in the system’s temporary directory and set it as the current payload:

<expression-component>
    targetDir = new java.io.File(server.tmpDir, 'target');
    targetDir.mkdir();
    payload = targetDir
</expression-component>

Set the username and password for an HTTP request at runtime based on inbound message properties:

<http:request-config name="HTTP_Request_Configuration" host="api.acme.com/v1" port="8081" doc:name="HTTP">
    <http:basic-authentication username="#[message.inboundProperties.username]" password="#[message.inboundProperties.password]"/>
    </http:request-config>

    <flow>
        ...
        <http:request config-ref="request-config" path="users" doc:name="HTTP Connector"/>
        ...
    </flow>

Java interoperability, for example to create a random UUID and use it as an XSL-T parameter:

<mulexml:context-property key="transactionId"
         value="#[java.util.UUID.randomUUID().toString()]" />

Retrieve fullName only if the name object is not null:

<set-variable variableName="fullName" value="#[payload.name ? payload.name.fullName : otherCondition]"/>

Local variable assignment, as in this splitter expression that splits a multi-line payload in rows and drops the first row:

splitter expression='#[rows=StringUtils.split(message.payload,'\n\r');
         ArrayUtil.subarray(rows,1,rows.size())]" />

Elvis operator - Returns the first non-null value of a list of values:

#[message.payload.userName or message.payload.userId]

For the complete MEL reference, including lists of operators, imported Java classes, context objects, etc. see Mule Expression Language Reference.