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Mule Expression Language Tips

This page provides quick tips for using Mule Expression Language (MEL), including information contained in the MEL Reference Card.


This document assumes you are familiar with Mule Expression Language (MEL), and that you are using Mule 3.4.0 or later. 

Variables, Integers, and Strings

  • 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:

  • To create an integer:

  • To create a string:



  • Return true if the lastname query string parameter from an HTTP inbound endpoint is not null:

    #[message.inboundProperties['http.query.params']['lastname'] != null]
  • Return the number of elements in http.method:

  • 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:

    • null

    • boolean false

    • empty strings or strings containing only white space

    • zero

    • empty collections

      The expression #[foo == empty] evaluates to true if the value if foo satisfies any of the requirements for emptiness.

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

    #[message.payload == null]

MEL and DataMapper

In versions prior to Mule 3.4.0, when mapping a null value to a map output, DataMapper neither creates the keys nor copies the values. This leads to exceptions when you try to access the resulting map using the non-existent keys.

  • To access a possible null value, use one of the following equivalent MEL expressions:

  • Setting Null from Within DataMapper: In DataMapper, you can also use the isnull() function to preserve the key in the map and convert the null value to a non-null value, as shown in the code below.

    (isnull($in.0.someNullableElement) ? '' : $in.0.someNullableElement);

(In Mule 3.4.0 and later, you can use the Set Null Values feature by clicking the Properties icon of the Output pane of the DataMapper view.)

Chained Elements

Given chained methods or properties, the expression #[a['b']['c'] == 'foo']  evaluates correctly even in the case that ' c ' is a null value. But if ' b ' is a null value, you will get a NullPointer exception.

For example, the expression evaluates correctly and returns true or false according to the value of Host in http.headers

message.inboundProperties['http.headers']['Host'] == 'localhost'

Attempting to reference the value of a non-existent key in http.headers also returns null, as in the expression below.

payload['http.headers']['non-existent'] == 'localhost'

However, the the following expression returns a NullPointer exception:

payload['non-existent']['Host'] == 'localhost'

Flow and Session Variables

Flow variables and session variables are available as top level variables.

  • 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, both shown hereafter:

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

Payload and Attachments

  • 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]

Regex Support

Regular expression helper functions retrieve null, a single value or an array of values, depending on matches. The forms that take a mel Expression argument apply the regex to the result of its evaluation instead of message.payload.

regex(regularExpression [, melExpression [, matchFlags]])

For example to select all the lines of the payload that begin with To:From:, or Cc: use:


XPath Support

XPath helper functions return DOM4J nodes. By default the XPath expression is evaluated on message.payload unless an xmlElement is specified:

xpath(xPathExpression [, xmlElement])

To get the text content of an element or an attribute:


JSON Processing

MEL has no direct support for JSON. The json-to-object-transformer can turn a JSON payload into a hierarchy of simple data structures that are easily parsed with MEL. For example, the following uses a filtered projection to build the equivalent of the $..[? (@.title=='Moby Dick')].price JSON path expression:

<json:json-to-object-transformer returnClass="java.lang.Object" />
    expression='#[($.price in message.payload if $.title =='Moby Dick')[0]]" />

Miscellaneous Operations

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

  • Append a string to the message payload:

    #[message.payload + 'mystring']
  • Call a static method:

  • Create a hash map:

    #[new java.util.HashMap()]

Cheat Sheet Examples

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

        targetDir = new, 'target');
        payload = targetDir    
  • Set the username and password for an HTTP endpoint at runtime based on inbound message properties:

    <https:outbound-endpoint address="https://#[message.inboundProperties.username]:#[message.inboundProperties.password]" />
  • 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()]" />
  • Safe bean property navigation,_ _for example to retrieve fullName only if the name object is not null:

    <set-variable variableName="fullName" value="#[message.payload.?name.fullName]" />
  • 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, to return the first non-null value of a list of values:

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

Global Configuration

Define global imports, aliases and global functions in the global configuration element. Global functions can be loaded from the file system, a URL, or a classpath resource.

  <expression-language autoResolveVariables="false">
    <import class="org.mule.util.StringUtils" />
    <import name="rsu" class="org.apache.commons.lang.RandomStringUtils" />
    <alias name="appName" expression="" />
    <global-functions file="extraFunctions.mvel">
      def reversePayload() { StringUtils.reverse(payload) }
      def randomString(size) { rsu.randomAlphanumeric(size) }

Advanced Tips

Accessing the Cache

You can access the Mule cache through the object store that serves as the cache repository. Depending on the nature of the object store, you can count, list, remove or perform other operations on entries.

The code below shows the XML representation of a cache scope that uses a custom object store class.

<ee:object-store-caching-strategy name="CachingStrategy">
  <custom-object-storeclass="" />

The object store above is an implementation of a ListableObjectStore, which allows you to obtain lists of the entries it contains. You can access the contents of the cache by invoking the getStore method on the CachingStrategy property of app.registry.

The expression below obtains the size of the cache by invoking allKeys(), which returns an iterable list.


If you need to manipulate the registry in a Java class, you can access it through muleContext.getRegistry().

Boolean Operations Gotchas

  • Boolean evaluations sometimes return unexpected responses, particularly when the value of a variable contains "garbage". See tables below.

    Expression When value of var1 is…​ …​ the expression evaluates to…​

    #[var1 == true]



    #[var1 == true]



    #[var1 == true]



    Expression When the value of something is…​ …​ and the value of abc is…​ …​ MEL successfully evaluates the expression.

    #[ == 'b']



    #[ == 'b']



    produces a NullPointer exception 

See Also