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]

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]

'dw' simply executes the DataWeave code you pass as an argument and returns the transformation’s result

'split()' executes the code you pass as an argument and returns an iterator that allows you to process each instance of the output as a separate message.

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.

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