Working with JSON Message Payloads - WSO2 Enterprise Integrator 6.5.0 (2023)

The ESB Profile of WSO2 Enterprise Integrator (WSO2 EI) provides support for JavaScript Object Notation (JSON) payloads in messages. The following sections describe how to work with JSON via the ESB Profile of WSO2 EI:

Handling JSON to XML conversion

When building the XML tree, JSON builders attach the converted XML infoset to a special XML element that acts as the root element of the final XML tree. If the original JSON payload is of type object, the special element is <jsonObject/>. If it is an array, the special element is <jsonArray/>. Following are examples of JSON and XML representations of various objects and arrays.

Empty objects




<jsonObject> <object></object></jsonObject>
Empty strings




<jsonObject> <object></object></jsonObject>
Empty array



XML (JsonStreamBuilder):


XML (JsonBuilder):

<jsonArray> <?xml-multiple jsonElement?></jsonArray>
Named arrays



XML (JsonStreamBuilder):

<jsonObject> <array>1</array> <array>2</array></jsonObject>

XML (JsonBuilder):

<jsonObject> <?xml-multiple array?> <array>1</array> <array>2</array></jsonObject>



XML (JsonStreamBuilder):


XML (JsonBuilder):

<jsonObject> <?xml-multiple array?></jsonObject>
Anonymous arrays



XML (JsonStreamBuilder):

<jsonArray> <jsonElement>1</jsonElement> <jsonElement>2</jsonElement></jsonArray>

XML (JsonBuilder):

<jsonArray> <?xml-multiple jsonElement?> <jsonElement>1</jsonElement> <jsonElement>2</jsonElement></jsonArray>


[1, []]

XML (JsonStreamBuilder):

<jsonArray> <jsonElement>1</jsonElement> <jsonElement> <jsonArray></jsonArray> </jsonElement></jsonArray>

XML (JsonBuilder):

<jsonArray> <?xml-multiple jsonElement?> <jsonElement>1</jsonElement> <jsonElement> <jsonArray> <?xml-multiple jsonElement?> </jsonArray> </jsonElement></jsonArray>
XML processing instructions (PIs)

Note the addition of xml-multiple processing instructions to the XML payloads whose JSON representations contain arrays. JsonBuilder (via StAXON) adds these instructions to the XML payload that it builds during the JSON to XML conversion so that during the XML to JSON conversion, JsonFormatter can reconstruct the arrays that are present in the original JSON payload. JsonFormatter interprets the elements immediately following a processing instruction to construct an array.

Special characters

When building XML elements, the EI handles the ‘$’ character and digits in a special manner when they appear as the first character of a JSON key. Following are examples of two such occurrences. Note the addition of the _JsonReader_PS_ and _JsonReader_PD_ prefixes in place of the ‘$’ and digit characters, respectively.




<jsonObject> <_JsonReader_PS_key>1234</_JsonReader_PS_key></jsonObject>




<jsonObject> <_JsonReader_PD_32X32>image_32x32.png</_JsonReader_PD_32X32></jsonObject>
Converting spaces

Although you can have spaces in JSON elements, you cannot have them when converted to XML. Therefore, you can handle spaces when converting JSON message payloads to XML, by adding the following property to the<EI_HOME>/conf/synapse.propertiesfile:synapse.commons.json.buildValidNCNames

For example, consider the following JSON message:

{ "abc def" : "this is a sample value"}

The output converted to XML is as follows:

<abc_jsonreader_32_def>this is a sample value</abc_jsonreader_32_def> 

The value 32 represents the standard char value of the space.

This works other way around as well. When you need to convert XML to JSON, with a JSON element that needs to have a space within it. Then, you use "_JsonReader_32_" in the XML element, to get the space in the JSON output. For example, if you consider the following XML payload;

<abc_jsonreader_32_def>this is a sample value</abc_jsonreader_32_def>

The JSON output will be as follows:


"abc def" : "this is a sample value"


Handling XML to JSON conversion

When an XML element is converted to JSON, the following rules apply:

Empty XML elements

Consider the following empty XML elements:

  • Example 1
  • Example 2
<jsonObject> <object></object></jsonObject>
<jsonObject> <object/></jsonObject>

By default, empty XML elements convert to JSON as null objects as shown below.


If you set the 'synapse.commons.enableXmlNullForEmptyElement=false'property in thesynapse.propertiesfile (stored in the<EI_HOME>/conf/directory), the JSON representaion of empty XML elements will change as shown below.

Empty XML elements with the 'nil' attribute

Consider the following XML element that has the 'nil' attribute set to true.

<jsonObject> <object nil=true></object></jsonObject>

By default, the above XML element converts to JSON as shown below.


If you set thesynapse.commons.enableXmlNilReadWrite=trueproperty in thesynapse.propertiesfile (stored in the<EI_HOME>/conf/directory), XML elements where the 'nil' attribue is set to true will be represented in JSON as null objects as shown below.


Converting a payload between XML and JSON

To convert an XML payload to JSON, set the messageType property to application/json in the axis2 scope before sending message to an endpoint. Similarly, to convert a JSON payload to XML, set themessageType property to application/xml or text/xml. For example:

<proxy xmlns="" name="tojson" transports="https,http" statistics="disable" trace="disable" startOnLoad="true"> <target> <inSequence> <property name="messageType" value="application/json" scope="axis2"/> <respond/> </inSequence> </target> <description/></proxy>

Use the following command to invoke this proxy service:

curl -v -X POST -H "Content-Type:application/xml" [emailprotected] "http://localhost:8280/services/tojson"

If the request payload is as follows:

<coordinates> <location> <name>Bermuda Triangle</name> <n>25.0000</n> <w>71.0000</w> </location> <location> <name>Eiffel Tower</name> <n>48.8582</n> <e>2.2945</e> </location></coordinates>

The response payload will look like this:

{ "coordinates":{ "location":[ { "name":"Bermuda Triangle", "n":25.0000, "w":71.0000 }, { "name":"Eiffel Tower", "n":48.8582, "e":2.2945 } ] }}

Note that we have used the Property mediator to mark the outgoing payload to be formatted as JSON:

<property name="messageType" value="application/json" scope="axis2"/>

JSON requests cannot be converted to XML if it contains invalid XML characters.

If you need to convert complex XML responses (e.g., XML with with xsi:type values), you will need to set the message type using the Property mediator as follows:

<property name="messageType" value="application/json/badgerfish" scope="axis2" type="STRING"/>

You will also need to ensure you register the following message builder and formatter as specified in Message Builders and Formatters.

<messageBuilder contentType="text/javascript" class="org.apache.axis2.json.JSONBadgerfishOMBuilder"/><messageFormatter contentType="text/javascript" class="org.apache.axis2.json.JSONBadgerfishMessageFormatter"/> 

Accessing content from JSON payloads

There are two ways to access the content of a JSON payload within the EI.

  • JSONPath expressions (with json-eval() method)

  • XPath expressions

JSONPath allows you to access fields of JSON payloads with faster results and less processing overhead. Although it is possible to evaluate XPath expressions on JSON payloads by assuming the XML representation of the JSON payload, we recommend that you use JSONPath to query JSON payloads. It is also possible to evaluate both JSONPath and XPath expressions on a payload (XML/JSON) at the same time.

You can use JSON path expressions with following mediators:


As a log property:

<log> <property name="location" expression="json-eval($.coordinates.location[0].name)"/></log>

As a standalone property:

<property name="location" expression="json-eval($.coordinates.location[0].name)"/>

As the payload arguments:

<payloadFactory media-type="json"> <format>{"RESPONSE":"$1"}</format> <args> <arg evaluator="json" expression="$.coordinates.location[0].name"/> </args></payloadFactory>

IMPORTANT: You MUST omit the json-eval() method within the payload arguments to evaluate JSON paths within the PayloadFactory mediator. Instead, you MUST select the correct expression evaluator (xml or json) for a given argument.


As the switch source:

<switch source="json-eval($.coordinates.location[0].name)">

As the filter source:

<filter source="json-eval($.coordinates.location[0].name)" regex="Eiffel.*">

JSON path syntax

Suppose we have the following payload:

{ "id": 12345, "id_str": "12345", "array": [ 1, 2, [ [], [{"inner_id": 6789}] ] ], "name": null, "object": {}, "$schema_location": "unknown", "12X12": "image12x12.png"}

The following table summarizes sample JSONPath expressions and their outputs:

{ "id":12345, "id_str":"12345", "array":[1, 2, [[],[{"inner_id":6789}]]], "name":null, "object":{}, "$schema_location":"unknown", "12X12":"image12x12.png"}
[1, 2, [[],[{"inner_id":6789}]]]

You can learn more about JSONPath syntax here.

Logging JSON payloads

To log JSON payloads as JSON, use the Log mediator as shown below. The json-eval() method returns the java.lang.String representation of the existing JSON payload.

<log> <property name="JSON-Payload" expression="json-eval($.)"/></log>

To log JSON payloads as XML, use the Log mediator as shown below:

<log level="full"/>

For more information on logging, see Troubleshooting, debugging, and logging below.

Constructing and transforming JSON payloads

To construct and transform JSON payloads, you can use the PayloadFactory mediator or Script mediator as described in the rest of this section.

PayloadFactory mediator

The PayloadFactory mediator provides the simplest way to work with JSON payloads. Suppose we have a service that returns the following response for a search query:

{ "geometry":{ "location":{ "lat":-33.867260, "lng":151.1958130 } }, "icon":"bar-71.png", "id":"7eaf7", "name":"Biaggio Cafe", "opening_hours":{ "open_now":true }, "photos":[ { "height":600, "html_attributions":[ ], "photo_reference":"CoQBegAAAI", "width":900 } ], "price_level":1, "reference":"CnRqAAAAtz", "types":[ "bar", "restaurant", "food", "establishment" ], "vicinity":"48 Pirrama Road, Pyrmont"}

We can create a proxy service that consumes the above response and creates a new response containing the location name and tags associated with the location based on several fields from the above response.

<proxy xmlns="" name="singleresponse" transports="https,http" statistics="disable" trace="disable" startOnLoad="true"> <target> <outSequence> <payloadFactory media-type="json"> <format>{ "location_response" : { "name" : "$1", "tags" : $2 }} </format> <args> <arg evaluator="json" expression="$.name"/> <arg evaluator="json" expression="$.types"/> </args> </payloadFactory> <send/> </outSequence> <endpoint> <address uri="http://localhost:8280/location"/> </endpoint> </target> <description/></proxy>

Use the following command to invoke this service:

curl -v -X GET "http://localhost:8280/services/singleresponse"

The response payload would look like this:

{ "location_response":{ "name":"Biaggio Cafe", "tags":["bar", "restaurant", "food", "establishment"] }}

Note the following aspects of the proxy service configuration:

  • We use the payloadFactory mediator to construct the new JSON payload.
  • The media-type attribute is set to json.
  • Because JSONPath expressions are used in arguments, the json evaluators are specified.
Configuring the payload format

The <format> section of the proxy service configuration defines the format of the response. Notice that in the example above, the name and tags field values are enclosed by double quotes ("), which creates a string value in the final response. If you do not use quotes, the value that gets assigned uses the real type evaluated by the expression (boolean, number, object, array, or null).

It is also possible to instruct the PayloadFactory mediator to load a payload format definition from the registry. This approach is particularly useful when using large/complex payload formats in the definitions. To load a format from the registry, click Pick From Registry instead of Define inline when defining the PayloadFactory mediator.

For example, suppose we have saved the following text content in the registry under the location conf:/repository/EI/transform. (The resource name is “transform”.)

{ "location_response" : { "name" : "$1", "tags" : $2 }}

We can now modify the definition of the PayloadFactory mediator to use this format text saved as a registry resource as the payload format. The new configuration would look as follows (note that the <format> element now uses the key attribute to point to the registry resource key):

<payloadFactory media-type="json"> <format key="conf:/repository/EI/transform"/> ...</payloadFactory>

When saving format text for the PayloadFactory mediator as a registry resource, be sure to save it as text content with the “text/plain” media type.

Script mediator

The Script mediator in JavaScript is useful when you need to create payloads that have recurring structures such as arrays of objects. The Script mediator defines the following important methods that can be used to manipulate payloads in many different ways:

  • getPayloadJSON
  • setPayloadJSON
  • getPayloadXML
  • setPayloadXML

By combining any of the setters with a getter, we can handle almost any type of content transformation within the EI. For example, by combining getPayloadXML and setPayloadJSON, we can easily implement an XML to JSON transformation scenario.In addition, we can perform various operations (such as deleting individual keys, modifying selected values, and inserting new objects) on JSON payloads to transform from one JSON format to another JSON format by using the getPayloadJSON and setPayloadJSON methods. Following is an example of a JSON to JSON transformation performed by the Script mediator.

Suppose a second service returns the following response:

{ "results" : [ { "geometry" : { "location" : { "lat" : -33.867260, "lng" : 151.1958130 } }, "icon" : "bar-71.png", "id" : "7eaf7", "name" : "Biaggio Cafe", "opening_hours" : { "open_now" : true }, "photos" : [ { "height" : 600, "html_attributions" : [], "photo_reference" : "CoQBegAAAI", "width" : 900 } ], "price_level" : 1, "reference" : "CnRqAAAAtz", "types" : [ "bar", "restaurant", "food", "establishment" ], "vicinity" : "48 Pirrama Road, Pyrmont" }, { "geometry" : { "location" : { "lat" : -33.8668040, "lng" : 151.1955790 } }, "icon" : "generic_business-71.png", "id" : "3ef98", "name" : "Doltone House", "photos" : [ { "height" : 600, "html_attributions" : [], "photo_reference" : "CqQBmgAAAL", "width" : 900 } ], "reference" : "CnRrAAAAV", "types" : [ "food", "establishment" ], "vicinity" : "48 Pirrama Road, Pyrmont" } ], "status" : "OK"}

The following proxy service shows how we can transform the above response using JavaScript with the Script mediator.

<proxy xmlns="" name="locations" transports="https,http" statistics="disable" trace="disable" startOnLoad="true"> <target> <outSequence> <script language="js" key="conf:/repository/EI/transform.js" function="transform"/> <send/> </outSequence> <endpoint> <address uri="http://localhost:8280/locations"/> </endpoint> </target> <description/></proxy>

The registry resource transform.js contains the JavaScript function that performs the transformation:

function transform(mc) { payload = mc.getPayloadJSON(); results = payload.results; var response = new Array(); for (i = 0; i < results.length; ++i) { location_object = results[i]; l = new Object(); =; l.tags = location_object.types; = "ID:" + (; response[i] = l; } mc.setPayloadJSON(response);}

mc.getPayloadJSON() returns the current JSON payload as a JavaScript object. This object can be manipulated as a normal JavaScript variable within a script as shown in the above JavaScript code. The mc.setPayloadJSON() method can be used to replace the existing payload with a new payload. In the above script, we build a new array object by using the fields of the incoming JSON payload and set that array object as the new payload (see the response payload returned by the final proxy service below.)

Use the following command to invoke the proxy service:

curl -v -X GET "http://ggrky:8280/services/locations"

The response payload would look like this:

[ { "id":"ID:7eaf7", "tags":["bar", "restaurant", "food", "establishment"], "name":"Biaggio Cafe" }, { "id":"ID:3ef98", "tags":["food", "establishment"], "name":"Doltone House" }]

If you want to get the response in XML instead of JSON, you would modify the out sequence by adding the Property mediator as follows:

<outSequence> <script language="js" key="conf:/repository/EI/transform.js" function="transform"/> <property name="messageType" value="application/xml" scope="axis2"/> <send/></outSequence>

The response will then look like this:

<jsonArray> <jsonElement> <id>ID:7eaf7</id> <tags>bar</tags> <tags>restaurant</tags> <tags>food</tags> <tags>establishment</tags> <name>Biaggio Cafe</name> </jsonElement> <jsonElement> <id>ID:3ef98</id> <tags>food</tags> <tags>establishment</tags> <name>Doltone House</name> </jsonElement></jsonArray>

If you are not getting the results you want when the Script mediator converts the JSON payload directly into XML, you can build the XML payload iteratively with the Script mediator as shown in the following script.

function transformXML(mc) { payload = mc.getPayloadJSON(); results = payload.results; var response = <locations/>; for (i = 0; i < results.length; ++i) { var elem = results[i]; response.locations += <location> <id>{}</id> <name>{}</name> <tags>{elem.types}</tags> </location> } mc.setPayloadXML(response);}

The response would now look like this:

<locations> <location> <id>7eaf7</id> <name>Biaggio Cafe</name> <tags>bar,restaurant,food,establishment</tags> </location> <location> <id>3ef98</id> <name>Doltone House</name> <tags>food,establishment</tags> </location></locations>

Finally, let's look at how you can perform delete, modify, and add field operations on JSON payloads with the Script mediator in JavaScript. Let's send the JSON message returned by thelocations proxy service as the request for the following proxy service, transformjson:

<proxy xmlns="" name="transformjson" transports="https,http" statistics="disable" trace="disable" startOnLoad="true"> <target> <inSequence> <script language="js"> payload = mc.getPayloadJSON(); for (i = 0; i &lt; payload.length; ++i) { payload[i].id_str = payload[i].id; delete payload[i].id; payload[i].tags[payload[i].tags.length] = "pub"; } mc.setPayloadJSON(payload); </script> <log> <property name="JSON-Payload" expression="json-eval($.)"/> </log> <respond/> </inSequence> </target> <description/></proxy>

The proxy service will convert the request into the following format:

[ { "name":"Biaggio Cafe", "tags":["bar", "restaurant", "food", "establishment", "pub"], "id_str":"ID:7eaf7" }, { "name":"Doltone House", "tags":["food", "establishment", "pub"], "id_str":"ID:3ef98" }]

Note that the transformation (line 9 through 17) has added a new field id_str and removed the old field id from the request, and it has added a new tag pub to the existing tags list of the payload.

For additional examples that demonstrate different ways to manipulate JSON payloads within the EI mediation flow, see the following samples:

  • Sample 440: Converting JSON to XML Using XSLT
  • Sample 441: Converting JSON to XML Using JavaScript

XML to JSON transformation parameters

You can useXML to JSON transformation parameters when you need totransform XML formatted data into the JSON format.

Following are the XML to JSON transformation parametersand their descriptions:



Default Value


Preserves the namespace declarations in the JSON output in XML to JSON transformations.



Builds valid XML NCNames when building XML element names in XML to JSON transformations.



Allows primitive types in the JSON output in XML to JSON transformations.



The namespace prefix separation character for the JSON output in XML to JSON transformations.

The default separation character is -


Adds XML namespace declarations in the JSON output in XML to JSON transformations.



Disables auto primitive conversion in XML to JSON transformations.



Sets the JSON output to an array element in XML to JSON transformations.



Sets the JSON output to an xml multiple processing instruction in XML to JSON transformations.



Sets the XML output to an array element in XML to JSON transformations.



Sets the XML output to an xml multiple processing instruction in XML to JSON transformations.

synapse.commons.enableXmlNilReadWriteHandles how empty XML elements with the 'nil' attribute are converted to JSON.false
Handles how empty XML elements are converted to JSON.true

Validating JSON messages

You can use theValidate mediatortovalidate JSON messages against a specified JSON schemaas described in the rest of this section.

Validate mediator

The parameters available in this section are as follows.

Parameter NameDescription
Schema keys defined for Validate MediatorThis section is used to specify the key to access the main schema based on which validation is carried out, as well as to specify the JSON, which needs to be validated.
SourceThe JSONPath expression to extract the JSON that needs to be validated.E.g:json-eval($.msg)"

Following example use the below sample schema StockQuoteSchema.jsonfile. Add this sample schema file (i.e.StockQuoteSchema.json)to the following Registry path: conf:/schema/StockQuoteSchema.json. For instructions on adding the schema file to the Registry path, seeAdding a Resource.

When adding this sample schema file to the Registry, specify the Media Type as application/json.

{ "$schema": "", "type": "object", "properties": { "getQuote": { "type": "object", "properties": { "request": { "type": "object", "properties": { "symbol": { "type": "string" } }, "required": [ "symbol" ] } }, "required": [ "request" ] } }, "required": [ "getQuote" ]}

In this example, the required schema for validating messages going through the Validate mediator is given as a registry key (i.e.schema\StockQuoteSchema.json). You do not have any source attributes specified. Therefore, the schema will be used to validate the complete JSON body. The mediation logic to follow if the validation fails is defined within the on-fail element. In this example, thePayloadFactory mediatorcreates a fault to be sent back to the party, which sends the message.

<validate> <schema key="conf:/schema/StockQuoteSchema.json"/> <on-fail> <payloadFactory media-type="json"> <format>{"Error":$1"}</format> <args> <arg evaluator="xml" expression="$ctx:ERROR_MESSAGE"/> </args> </payloadFactory> <property name="HTTP_SC" value="500" scope="axis2"/> <respond/> </on-fail></validate>

An example for a valid JSON payload request is given below.

{ "getQuote": { "request": { "symbol": "WSO2" } }}

Troubleshooting, debugging, and logging

To assist with troubleshooting, you can enable debug logging at several stages of the mediation of a JSON payload within the EI by adding one or more of the following loggers to the <EI_HOME>/conf/ file and restarting the EI.

Be sure to turn off these loggers when running the EI in a production environment, as logging every message will significantly reduce performance.

Following are the available loggers:

Message builders and formatters





JSON utility class

PayloadFactory mediator

JSONPath evaluator

Top Articles
Latest Posts
Article information

Author: Rev. Porsche Oberbrunner

Last Updated: 07/10/2023

Views: 6076

Rating: 4.2 / 5 (53 voted)

Reviews: 92% of readers found this page helpful

Author information

Name: Rev. Porsche Oberbrunner

Birthday: 1994-06-25

Address: Suite 153 582 Lubowitz Walks, Port Alfredoborough, IN 72879-2838

Phone: +128413562823324

Job: IT Strategist

Hobby: Video gaming, Basketball, Web surfing, Book restoration, Jogging, Shooting, Fishing

Introduction: My name is Rev. Porsche Oberbrunner, I am a zany, graceful, talented, witty, determined, shiny, enchanting person who loves writing and wants to share my knowledge and understanding with you.