JAVA SDK
The Java SDK includes two servlets, AccountLinkingServlet and PurchaseServlet that should be extended and served in two different endpoints. The AccountLinkingServlet supports the account linking API call and the PurchaseServlet supports API calls for purchase and subscription fulfillment. Also, both servlets need a credential store with a valid key, which can be found during the profile configuration in the developer portal.
The SDK can be found on GitHub