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MEL Cheat Sheet

The following document reproduces the content from the original Mule Expression Language (MEL) Reference card. See also Mule Expression Language Reference.

Prerequisites

This document assumes you are familiar with Mule Expression Language (MEL).

Server, Mule, Application, and Message Variables

Overview of the global contexts and the variables to which they give access.

server mule app message

fileSeparator

clusterId

encoding

ID

host

home

name

rootId

ip

nodeId

standalone

correlationId

locale

version

workDir

correlationSequence

javaVersion

registry

correlationGroupSize

javaVendor

replyTo

osName

dataType

osArch

payload

osVersion

inboundProperties

systemProperties

inboundAttachments

timeZone

outboundProperties

tmpDir

outboundAttachments

userName

exception

userHome

userDir

Create a directory named target in the system’s temporary directory and set the payload to the variable storing the file object:

<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>

Variables, Integers, and Strings

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

Flow and Session Variables

Flow variables are available in the flowVars context or directly as top level variables (unless autoResolveVariables is false or their name violates the MEL naming conventions). Session variables are accessible across flows, applications and even servers. You can set a session variable using the session variable transformer.

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

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

<set-variable variableName="originalPayload" value="#[message.payload]" />
<set-payload value="#[flowVars['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 melExpression argument apply the regex to the result of its evaluation instead of message.payload. For more information, see MEL Regex.

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

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

#[regex('^(To|From|Cc):')]

XPath3 Support

XPath3 helper functions can return a DOM Node, or collection of DOM Nodes, or text. For more information, see XPath and MEL XPath. By default the XPath3 expression is evaluated on message.payload unless an xmlElement is specified:

xpath3(xPathExpression [, xmlElement])

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

#[xpath3('//title').text]
#[xpath3('//title/@id').value]

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 the equivalent of this JSON path expression:

$..[? (@.title=='Moby Dick')].price

The following uses a filtered projection:

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

Tests

  • 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:

    • 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:

      #[payload == null]

Chained Elements

For chained methods or properties, the expression #[[a.b.c] == 'foo'] evaluates correctly even in the case that c is a null value. However, if b is a null value, the expression throws a NullPointerException.

In this example, if a field named address is null, the expression throws a NullPointerException.

#[payload.address.zipcode]

To make this same expression null safe, use the .? operator.

#[payload.address.?zipcode]

Using this operator avoids a NullPointerException if address is an empty value, instead it returns null.

If you’d like the expression to return a different value if no address is defined, you can use a chained or operator.

#[payload.address.?zipcode or 'Zipcode not set']

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 (see extraFunctions.mvel in line 6 below). Flow variables auto-binding can be turned off via the autoResolveVariables attribute in line 2.

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

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="org.mule.util.store.SimpleMemoryObjectStore" />
</ee:object-store-caching-strategy>

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.

#[app.registry.CachingStrategy.getStore().allKeys().size()]"

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]

    'true'

    true

    #[var1 == true]

    'True' 'false'

    false

    #[var1 == true]

    'u5hsmg930'

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

    #[payload.something.abc == 'b']

    'something'

    'null'

    ✔

    #[payload.something.abc == 'b']

    'null'

    'abc'

    X
    Produces a NullPointer exception

    Note also that, if given the expression #[flowVars.abc.toString()] and the value of ‘abc’ is null, Mule throws a NullPointerException.

Miscellaneous Operations

  • 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()]

More MVEL Goodness

Quick access to the MVEL 2.0 Documentation is available from github in MVEL (MVFLEX Expression Language).

  • Java interoperability - Creates 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 - Retrieves fullName only if the name object is not null:

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

  • Local variable assignment - Works to split a multi-line payload into rows and drops the first row, as in this splitter expression:

    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]

    Note: Mule checks the operands for emptiness, but not when a value is set to null.

    For example:

    If you set myop = "", Mule detects the operand as null. However, if you set myop = null, Mule does not detect that myop is null.

See Also