TDD in Java Servlet Web Service

I already know how to write JUnit test cases (can also use Mockito). Thanks for the simple tutorials in the web for that. The only problem is that I can't find any examples on implementing it to an actual or let's say realistic project.

I have a simple REST web service which uses Servlets. It has an API method that sends a POST request to another web service then arranges the data as its response.

The API request is processed by two layers:

• Servlet (Controller) - validates the request parameters
• Service - The sender of the POST request to another web service

Here's what the service method does:

1. Prepares a parameter data (some attributes are provided in the paramaters of this request, some are retrieved from the database)
2. sends a request to another web service which responds with an image url for a QR Code
3. Decodes the QRCode image and then responds to its request with the decoded text

What are the unit tests cases needed for this?

How about integration tests? Do I have to use ServletUnit, or can I just run the server and write tests with requests to the running server?