Migrating the JSON Module

A new JSON module in Mule 4 replaces the one that was bundled in with Mule 3 for schema validation.

To migrate transformations involving JSON, MuleSoft recommends that you use DataWeave.

Migrating Transformers

In Mule 3, many components required the payload to be a POJO so that its fields could be accessed. In Mule 4, DataWeave can be used as a query language directly on the JSON payload, which completely avoids the need to convert the payload to a POJO as an intermediate step.

JSON to Object Transformer

For mapping rules that are configured for the json:json-to-object-transformer, you can express the rules in DataWeave.

In Mule 3:

Example: Mixin used in the Mapper Configuration

public abstract class AppleMixin
{
    AppleMixin(@JsonProperty("bitten") boolean wasBitten)
    {
        super();
    }

}

Example: Transformer Referencing the Mapper

<json:mapper name="myMapper">
    <json:mixin mixinClass="org.mule.module.json.transformers.AppleMixin"
                    targetClass="org.mule.tck.testmodels.fruit.Apple"/>
</json:mapper>

<flow name="flow">
    <json:json-to-object-transformer returnClass="org.mule.tck.testmodels.fruit.Apple"
                                     mapper-ref="myMapper"/>
</flow>

In Mule 4:

Example: Using DataWeave as the Mapper

<flow name="flow">
    <ee:transform>
        <ee:message>
            <ee:set-payload>
                <![CDATA[%dw 2.0
                    type apple = Object { class: "org.mule.tck.testmodels.fruit.Apple"}
                    output application/java
                    ---
                    {
                        wasBitten = bitten
                    } as apple
                ]]>
            </ee:set-payload>
        </ee:message>
    </ee:transform>
</flow>

You don’t have to define a mapper or a mixin class in Mule 4. The mapping rules are defined using DataWeave code.

Object to JSON Transformer

The mapping rules for the json:object-to-json-transformer are also expressed using DataWeave:

In Mule 3:

Example: Transformer with Mixins

<flow name="flow">
    <json:object-to-json-transformer returnClass="org.mule.tck.testmodels.fruit.Apple">
        <json:serialization-mixin mixinClass="org.mule.module.json.transformers.AppleMixin"
                                  targetClass="org.mule.tck.testmodels.fruit.Apple"/>

    </json:object-to-json-transformer>
</flow>

In Mule 4:

Example: Using DataWeave as the Mapper

<flow name="flow">
    <ee:transform>
        <ee:message>
            <ee:set-payload>
                <![CDATA[%dw 2.0
                    output application/json
                    ---
                    {
                        bitten = wasBitten
                    }
                ]]>
            </ee:set-payload>
        </ee:message>
    </ee:transform>
</flow>

In Mule 4 you can set the output format in the transformation. In this case, the expression defines the output as application/json, so the transformation function returns a JSON payload.

Migrating XSLT Operations

In Mule 4, the JSON module does not provide a component that transforms the JSON based on an XSLT mapping. Possible ways to handle this case are:

Migrate the XSLT to a DataWeave transformation (recommended)
Use XSLT support of the XML module, first converting the payload to XML.

Mule 4 Example: Using XSLT to Transform JSON

<flow name="flow">
    ...
	<xml-module:xslt-transform>
	    <xml-module:content>#[output application/xml --- payload]</xml-module:content>
	    <xml-module:xslt>
	        (the XSLT transformation) ...
	    </xml-module:xslt>
	</xml-module:xslt-transform>
	...
</flow>