You can send API messages through the API Manager without any transformation configurations, if the back-end accepts messages of the same format. For example, the API Manager handles JSON to JSON transformations out of the box. In cases where the back-end does not accept the same format, the transformations are done as described below:
JSON message builders and formatters
There are two types of message builders and formatters for JSON. The default builder and formatter keep the JSON representation intact without converting it to XML. You can access the payload content using JSON Path or XPath and convert the payload to XML at any point in the mediation flow.
org.apache.synapse.commons.json.JsonStreamBuilderorg.apache.synapse.commons.json.JsonStreamFormatter
If you want to convert the JSON representation to XML before the mediation flow begins, use the following builder and formatter instead. Note that some data loss can occur during the JSON to XML to JSON conversion process.
org.apache.synapse.commons.json.JsonBuilderorg.apache.synapse.commons.json.JsonFormatter
The builders and formatters are configured in the messageBuilders and messageFormatters sections, respectively, of the Axis2 configuration files located in the <PRODUCT_HOME>/repository/conf/axis2 directory. Both types of JSON builders use StAXON as the underlying JSON processor.
The following builders and formatters are also included for compatibility with older API Manager versions:
org.apache.axis2.json.JSONBuilder/JSONMessageFormatterorg.apache.axis2.json.JSONStreamBuilder/JSONStreamFormatterorg.apache.axis2.json.JSONBadgerfishOMBuilder/JSONBadgerfishMessageFormatter
Always use the same type of builder and formatter combination. Mixing different builders and formatters will cause errors at runtime.
If you want to handle JSON payloads that are sent using a media type other than application/json, you must register the JSON builder and formatter for that media type in the following two files at minimum (for best results, register them in all Axis2 configuration files found in the <PRODUCT_HOME>/repository/conf/axis2 directory):
<PRODUCT_HOME>/repository/conf/axis2/axis2.xml<PRODUCT_HOME>/repository/conf/axis2/axis2_blocking_client.xml
For example, if the media type is text/javascript, register the message builder and formatter as follows:
<messageBuilder contentType="text/javascript"
class="org.apache.synapse.commons.json.JsonStreamBuilder"/>
<messageFormatter contentType="text/javascript"
class="org.apache.synapse.commons.json.JsonStreamFormatter"/>
When you modify the builders/formatters in Axis2 configuration, make sure that you have enabled only one correct message builder/formatter pair for a given media type.
XML representation of JSON payloads
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.
Null objects
JSON:
{"object":null}
XML:
<jsonObject>
<object></object>
</jsonObject>
Empty objects
JSON:
{"object":{}}
XML:
<jsonObject> <object></object> </jsonObject>
Empty strings
JSON:
{"object":""}
XML:
<jsonObject> <object></object> </jsonObject>
Empty array
JSON:
[]
XML (JsonStreamBuilder):
<jsonArray></jsonArray>
XML (JsonBuilder):
<jsonArray> <?xml-multiple jsonElement?> </jsonArray>
Named arrays
JSON:
{"array":[1,2]}
XML (JsonStreamBuilder):
<jsonObject> <array>1</array> <array>2</array> </jsonObject>
XML (JsonBuilder):
<jsonObject> <?xml-multiple array?> <array>1</array> <array>2</array> </jsonObject>
JSON:
{"array":[]}
XML (JsonStreamBuilder):
<jsonObject></jsonObject>
XML (JsonBuilder):
<jsonObject> <?xml-multiple array?> </jsonObject>
Anonymous arrays
JSON:
[1,2]
XML (JsonStreamBuilder):
<jsonArray> <jsonElement>1</jsonElement> <jsonElement>2</jsonElement> </jsonArray>
XML (JsonBuilder):
<jsonArray> <?xml-multiple jsonElement?> <jsonElement>1</jsonElement> <jsonElement>2</jsonElement> </jsonArray>
JSON:
[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 that 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 ‘$’ character and digits are handled in a special manner when they appear as the first character of a JSON key. Following are examples of two such occurrences. Note the addition of the _JsonReader_PS_ and _JsonReader_PD_ prefixes in place of the ‘$’ and digit characters, respectively.
JSON:
{"$key":1234}
XML:
<jsonObject> <_JsonReader_PS_key>1234</_JsonReader_PS_key> </jsonObject>
JSON:
{"32X32":"image_32x32.png"}
XML:
<jsonObject> <_JsonReader_PD_32X32>image_32x32.png</_JsonReader_PD_32X32> </jsonObject>
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 the messageType property to application/xml or text/xml. For example:
<proxy xmlns="http://ws.apache.org/ns/synapse"
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" -d@request1.xml "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. For more information about the Property Mediator, see the Property Mediator page on WSO2 ESB documentation.
<property name="messageType" value="application/json" scope="axis2"/>
Accessing content from JSON payloads
There are two ways to access the content of a JSON payload:
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:
| Mediator | Usage | References |
|---|---|---|
| Log | As a log property: <log>
<property name="location"
expression="json-eval($.coordinates.location[0].name)"/>
</log>
| https://docs.wso2.org/display/ESB481/Log+Mediator |
| Property | As a standalone property: <property name="location"
expression="json-eval($.coordinates.location[0].name)"/>
| https://docs.wso2.org/display/ESB481/Property+Mediator |
| PayloadFactory | 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 | https://docs.wso2.org/display/ESB481/PayloadFactory+Mediator |
| Switch | As the switch source: <switch source="json-eval($.coordinates.location[0].name)"> | https://docs.wso2.org/display/ESB481/Switch+Mediator |
| Filter | As the filter source: <filter source="json-eval($.coordinates.location[0].name)"
regex="Eiffel.*">
| https://docs.wso2.org/display/ESB481/Filter+Mediator |
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:
| Expression | Result |
|---|---|
$. | { "id":12345, "id_str":"12345", "array":[1, 2, [[],[{"inner_id":6789}]]], "name":null, "object":{}, "$schema_location":"unknown", "12X12":"image12x12.png"} |
$.id | 12345 |
$.name | null |
$.object | {} |
$.['$schema_location'] | unknown |
$.12X12 | image12x12.png |
$.array | [1, 2, [[],[{"inner_id":6789}]]] |
$.array[2][1][0].inner_id | 6789 |
You can learn more about JSONPath syntax here.
Logging JSON payloads
To log JSON payloads as JSON, use the Log mediator as shown below. The json-eval() method returns the java.lang.String representation of the existing JSON payload.
<log>
<property name="JSON-Payload" expression="json-eval($.)"/>
</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. For information on the mediator, see WSO2 ESB documentation: https://docs.wso2.org/display/ESB480/PayloadFactory+Mediator. 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="http://ws.apache.org/ns/synapse"
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:82801/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
payloadFactorymediator to construct the new JSON payload. - The
media-typeattribute is set tojson. - Because JSONPath expressions are used in arguments, the
jsonevaluators 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/esb/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/esb/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 (https://docs.wso2.org/display/ESB480/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:
getPayloadJSONsetPayloadJSONgetPayloadXMLsetPayloadXML
By combining any of the setters with a getter, we can handle almost any type of content transformation. 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="http://ws.apache.org/ns/synapse"
name="locations"
transports="https,http"
statistics="disable"
trace="disable"
startOnLoad="true">
<target>
<outSequence>
<script language="js"
key="conf:/repository/esb/transform.js"
function="transform"/>
<send/>
</outSequence>
<endpoint>
<address uri="http://localhost:82801/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.name = location_object.name;
l.tags = location_object.types;
l.id = "ID:" + (location_object.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/esb/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>{elem.id}</id>
<name>{elem.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 the locations proxy service as the request for the following proxy service, transformjson:
<proxy xmlns="http://ws.apache.org/ns/synapse"
name="transformjson"
transports="https,http"
statistics="disable"
trace="disable"
startOnLoad="true">
<target>
<inSequence>
<script language="js"><![CDATA[
payload = mc.getPayloadJSON();
for (i = 0; i < 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.
Troubleshooting, debugging, and logging
To assist with troubleshooting, you can enable debug logging at several stages of the mediation of a JSON payload by adding one or more of the following loggers to the <PRODUCT_HOME>/repository/conf/log4j.properties file and restarting the server.
Be sure to turn off these loggers when running the server in a production environment, as logging every message will significantly reduce performance.
Following are the available loggers:
Message builders and formatters
log4j.logger.org.apache.synapse.commons.json.JsonStreamBuilder=DEBUGlog4j.logger.org.apache.synapse.commons.json.JsonStreamFormatter=DEBUGlog4j.logger.org.apache.synapse.commons.json.JsonBuilder=DEBUGlog4j.logger.org.apache.synapse.commons.json.JsonFormatter=DEBUG
JSON utility class
log4j.logger.org.apache.synapse.commons.json.JsonUtil=DEBUG
PayloadFactory mediator
log4j.logger.org.apache.synapse.mediators.transform.PayloadFactoryMediator=DEBUG
JSONPath evaluator
log4j.logger.org.apache.synapse.util.xpath.SynapseJsonPath=DEBUG