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Friday, October 25, 2013

The difference between JSON streaming builder and the default JSON builder in WSO2 ESB

When a request/response is received by ESB, the message will initially be converted into XML infoset. This process is known as message building and the builders are responsible to do this transformation based on the HTTP Content-Type of the incoming message. The default JSONBuilder (org.apache.axis2.json.JSONBuilder) converts the JSON into corresponding XML representation. If I explain this through an example, suppose your incoming JSON response/request is similar to the following.

  "getQuote": {
    "request": { "symbol": "charitha" }

If you log this message inside inSequence of your message flow (using log mediator), you will observe an output similar to the following.

LogMediator To: http://www.w3.org/2005/08/addressing/anonymous, 
WSAction: , SOAPAction: , MessageID: urn:uuid:f37b8466-7cb3-41d6-9187-b5a0a7648f16, 
Direction: response, Envelope: <?xml version='1.0' encoding='utf-8'?><soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope"><soapenv:Body><getQuote><request><symbol>charitha</symbol></request></getQuote></soapenv:Body></soapenv:Envelope> 

Look at the Body of the SOAP envelope. You may notice the JSON to XML mapping clearly in there.

The message formatters do the complete opposite of builders. They are responsible for formatting the message into the relevant wire format. So, the default JSONMessageFormatter converts XML back into JSON.
Obviously this JSON <--> XML conversion may lead to a certain degree of information loss.

In order to avoid that, we have introduced the JSON streaming builder (org.apache.axis2.json.JSONStreamBuilder) which does not covert JSON message to intermediate XML format. If you conduct the same test which I explained above with streaming JSON builder, you will see a message similar to the following when you use log mediator within the message flow.

LogMediator To: http://www.w3.org/2005/08/addressing/anonymous, WSAction: , SOAPAction: , MessageID: urn:uuid:33d9ad3f-83bc-46ee-adb9-1cafd7e5a9cc, Direction: response, Envelope:

In the above message, there is no XML representation of the JSON message. The Body element is empty.

Therefore, if you use streaming JSON builder/formatter, you will not be able to access the message payload. If you want to access/modify the payload, that can only be achieved using script mediator. To access/modify JSON paylaod, you can use mc.getPayloadJSON() or mc.setPayloadJSON() methods as explained in http://docs.wso2.org/display/ESB470/Sample+441:+Exposing+a+SOAP+Service+Over+JSON