vrijdag 21 december 2012

A reusable solution for the conversion between JSON and XML

Two popular information exchange notations/languages are Json and XML. In order to provide interoperability between frameworks using different mechanisms of information exchange, a conversion between the two is required. There is however not a strict way to provide a conversion; http://jackson-users.ning.com/forum/topics/xml-to-json-conversion-using.

I've seen several solutions in practice to provide the conversion for example;
- using a service bus to make the translation from XML to JSON and the other way around
- using a Java HttpServlet with JAX-B to provide a mapping

There are also other solutions such as embedding a Java JSON client in the BPEL code; http://technology.amis.nl/2009/12/15/the-oracle-soa-suite-11g-httpbinding-or-another-way-to-call-restful-services-from-soa-composite-applications/

Those solutions however require work per service and are thus not very reusable. When the service landscape consists of a lot of services, this creates an extra mapping layer which needs maintenance.

A reusable solution

JSON is structured (http://www.w3resource.com/JSON/structures.php; objects, arrays and values). The conversion from JSON to XML (and the other way around) can be fixed and a single message definition (XSD) can be provided.

In order to provide a single reusable conversion from JSON to XML and XML to JSON, an XML schema is required which is capable of containing JSON structures. Luckily, the internet is a big place and I could find several examples of such XSD's such as; http://xml.calldei.com/JsonXML. This way, only a single set of JAX-B classes can be used and only two pieces of mapping code to make the transformation work. If you have a different Json message (sending or receiving), it can be mapped to the same XML so no additional programming is required for individual services.

Based on the schema, I used JAX-B to generate Java classes. Then I used Jackson Json processor to create Json (http://jackson.codehaus.org/). I wrote a mapping from the JAX-B objects to JsonNodes and the other way around. Next I exposed this as a webservice so it could for example easily be called from BPEL.


I managed to create a reversible transformation from Json to a fixed Xml schema and the other way around. Also I've provided a test method to check if the conversion works (the following two conversions lead to the original result; JSON -> XML -> JSON) for specific Json messages. Keep in mind that JSON strings need to be enclosed in ". This allows Json to be used in XML based systems for request and response handling. Also, because the schema is fixed, it can easily be used in XML based systems to build JSON messages.

I've used recursive methods to walk the XML and JSON trees respectively and convert them to the other type. The code is not thoroughly tested so should not be used carelessly in production environments.

The example webservice can be downloaded here (JDeveloper project); https://dl.dropbox.com/u/6693935/blog/JsonXml.zip


When combined the above with the SocketAdapter functionality as described in http://javaoraclesoa.blogspot.nl/2012/12/receiving-json-requests-in-oracle-bpel.html the Hello World JSON BPEL process is not farfetched anymore. I didn't manage to complete this yet however. I've spend some time on getting the SocketAdapter to work with the XSL (request/reply) transformations. I came to the conclusion that usage of the socket XSLT functions is hard. One of the issues I encountered is when the server should start sending back a reply. Also getting the body from the HTTP message could be better programmed out in Java (using the Content-Length HTTP header to obtain the correct body).

The other way around; converting an XML to a fixed Json message and back again to obtain the original result, is more difficult. An example of a method of converting XML to Json can be found on; http://javaoraclesoa.blogspot.nl/2012/07/webinterface-restjson-vs-middleware.html. This transformation is however not reversible.