I am going to access a WebService which is written in C# .net, and my application is a J2EE application and it will act as the Web service client.
the webservice is designed as Document style. Please suggest me as to which technology/API I need to use in the J2EE side, so that I can create the request XML (for document style) that can be formed and need to be maintained easily.
This depends a little on the complexity of the documents used to interact with the web service.
If the complexity is low to moderate, I would try JAXB. JAXB is, by far, the easiest choice since JAXB bean classes can be generated automatically from XML schema. Performance is also good.
If you want to build more complex documents in memory, take a look at DOM or JDOM.
If your documents are extremely large, you may want to consider StAX to write generated XML directly to disc.