Thanks for your reply. However, the zip you gave here doesn't contain the boilerplate project. 

https://helpx.adobe.com/aem-forms/6-2/prepopulate-adaptive-form-fields.htmlCreate and run a prefill serviceThe prefill service is an OSGi bundle. You create the OSGi bundle, upload, and install it to AEM Forms bundles. Before you get started with creating the bundle:Download the AEM 6.2 Forms Client...

I have the same issue. I have some servlets that read objects from session. But I noticed the session ID keep changing in every request. Every time AEM 6.2 creates a new session. It looks AEM 6.2 doesn't support httpsession at all. Can someone please help?