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Migrating MEL to DataWeave

In Mule 4, the DataWeave expression language replaces Mule Expression Language (MEL). This section provides guidance on migrating MEL expressions in your Mule apps to DataWeave (2.0 and later).

DataWeave at a Glance

DataWeave is a Mule-specific language that can be used lightly as an expression language to simply reference values and evaluate conditions, or as a scripting language to construct complex data transformations that include functions and recursion.

Unlike MEL, it supports different data types out of the box, like JSON, XML, or CSV, which means data no longer needs to be transformed before expressions can be evaluated. It also supports advanced use cases like mapping functions or filtering data.

It’s important to notice that the main data selector in DataWeave is the . operator. Since it shares most keywords with MEL, the syntax for most scenarios is identical. For example, #[message.payload.name] is the same in both languages.

The main difference between MEL and DataWeave is that DataWeave expressions have no side effects. You can use DataWeave to extract or generate data but not to modify it. The Java Integration examples explain how to deal with those use cases.

MEL Usages

The next sections show how to adapt some uses of MEL to Mule 4.

Accessing Context Variables

Except for the following changes, context variables (also called Mule Runtime variables) remain the same in DataWeave:

MEL DataWeave

Flow variables

flowVars

  • Example: #[flowVars.myVar]

Mule Event Variables

vars

  • Example: #[vars.myVar]

Exception

exception

Mule Error:

error

Inbound Attachments

message.inboundAttachments

Multipart Data Formats

See:

Inbound Properties

message.inboundProperties

  • Examples:

    #[inboundProperties.'http.query.params']
#[inboundProperties]

Mule Message Attributes (metadata)

attributes

  • Examples:

    #[attributes.queryParams]
#[attributes.headers]

Inbound Properties Are Now Attributes

Outbound Attachments

message.outboundAttachments

Multipart data formats

See:

Outbound Properties

message.outboundProperties

Removed:

See Outbound Properties.

message.dataType

dataType

message.id, message.rootId, message.replyTo

Removed

Record Variables

recordVars

  • Example: #[recordVars.myVar]

Mule Event Variables

vars

  • Example: #[vars.myVar]

Session Variables

sessionVars

  • Example: #[sessionVars.myVar]

Removed:

Transport barriers do not exist in Mule 4. See:

server.dateTime

Removed:

Use the DataWeave now function.

Extracting Data

You can use DataWeave Selectors to query data as you did with MEL. Only now, you won’t need extra transformations when dealing with types like JSON.

The next examples log the ID from this JSON payload:

JSON Payload
{
  "name" : "Rachel Duncan",
  "id": "779H41"
}
Mule 3 Example
<json:json-to-object-transformer returnClass="java.util.Map" />
<logger message="Updating health check record for subject '#[payload.id]'" level="INFO" />
Mule 4 Example
<logger message="Updating health check record for subject '#[payload.id]'" />

Assigning Values

As mentioned earlier, DataWeave does not support modifying data in the way the expression-component allowed with MEL. To achieve those use cases, you must use the Scripting module.

Consider payload described above as a Java Map. The examples modify the name attribute.

Mule 3 Example
<expression-component>
  <![CDATA[
    payload.name = 'R. Duncan'
  ]]>
</expression-component>
Mule 4 Example
<script:execute engine="groovy">
  <script:code>
    payload.put('name', 'R. Duncan')
    return payload
  </script:code>
</script:execute>

To use the Scripting module, simply add it to your app using the Studio palette, or add the following dependency in your pom.xml file:

<dependency>
  <groupId>org.mule.modules</groupId>
  <artifactId>mule-scripting-module</artifactId>
  <version>1.1.0</version> <!-- or newer -->
  <classifier>mule-plugin</classifier>
</dependency>

Invoking Java Methods

DataWeave can be used to invoke static methods. For regular methods, you can use the Java module, which exposes both an operation and a function to invoke Java methods.

Static Methods

The following examples use Java’s Locale getDefault static method in Mule 3 and 4.

Mule 3 example
<validation:is-time time="#[payload]" pattern="h:mm a" locale="#[java.util.Locale.getDefault().getLanguage()]"/>
Mule 4 example
<validation:is-time time="#[payload]" pattern="h:mm a" locale="#[java!java::util::Locale::getDefault().getLanguage()]"/>

Note that DataWeave requires the java! prefix to indicate that a Java static method is to be searched, and it requires the fully qualified Class name (separated by :: instead of .).

Instance Methods

The following examples show the usage of Java’s String equalsIgnoreCase method in Mule 3 and 4.

Mule 3 example
<choice>
  <when expression="#[payload.equalsIgnoreCase('error')]">
    <logger message="An error message has been received." level="ERROR"/>
  </when>
  <otherwise>
    <logger message="Message received: #[payload]" level="INFO"/>
  </otherwise>
</choice>
Mule 4 example
<choice doc:name="Choice">
  <when expression="#[Java::invoke('java.lang.String', 'equalsIgnoreCase(String)', payload, {arg0: 'error'})]">
		<logger message="An error message has been received." level="ERROR"/>
	</when>
	<otherwise >
		<logger message="Message received: #[payload]" level="INFO"/>
	</otherwise>
</choice>

To use the Java module, simply add it to your app using the Studio palette, or add the following dependency to your pom.xml file:

<dependency>
  <groupId>org.mule.modules</groupId>
  <artifactId>mule-java-module</artifactId>
  <version>1.0.0</version> <!-- or newer -->
  <classifier>mule-plugin</classifier>
</dependency>

Target Definitions

The enricher has been removed and replaced by target variables, which are now supported by every operation. The example below shows how to send the result of an operation to a variable in Mule 3 and 4.

Mule 3 example
<enricher target="#[flowVars.response]">
    <http:request config-ref="HTTP_Request_Configuration" path="/get" method="GET"/>
</enricher>
Mule 4 example
<http:request config-ref="HTTP_Request_Configuration" path="/get" method="GET" target="response"/>

XPath Function

DataWeave can be used to query XML content using its selectors, but you can also use the XML module, which exposes both an operation and a function to execute XPath queries.

The following examples show how to take lines containing a specific word taken from a variable in Mule 3 and 4.

Mule 3 example
<set‐variable variableName="word" value="handkerchief"/>
<expression‐transformer>
   xpath3('//LINE[contains(.,$word)]',payload,'NODESET')
</expression‐transformer>
Mule 4 example
<set‐variable variableName="word" value="handkerchief"/>
<set-payload value="#[XmlModule::xpath('//LINE[contains(., \$word)]', payload, {'word': vars.word})]" />

To use the XML module, simply add it to your app using the Studio palette, or add the following dependency to your pom.xml file:

<dependency>
  <groupId>org.mule.modules</groupId>
  <artifactId>mule-xml-module</artifactId>
  <version>1.1.0</version> <!-- or newer -->
  <classifier>mule-plugin</classifier>
</dependency>

Wildcard and Regex functions

DataWeave matching functions match and matches (see Core DataWeave Functions) can be used instead. The next example shows how a regex is used in DataWeave to replace the use of the wildcard MEL function.

Mule 3 example
<choice>
  <when expression="#[wildcard('Hello *')]">
    <set-payload value="Hello, how can I help?"/>
  </when>
  <otherwise>
    <set-payload value="Courtesy requires a greeting."/>
  </otherwise>
</choice>
Mule 4 example
<choice doc:name="Choice">
  <when expression="#[payload matches /Hello\s[A-z]+/]">
    <set-payload value="Hello, how can I help?"/>
  </when>
  <otherwise >
    <set-payload value="Courtesy requires a greeting."/>
  </otherwise>
</choice>

See Also

DataWeave Language

Why DataWeave is the Main Expression Language in Mule 4 Beta

Mule Expression Language (MEL) (3.9)