If you want to implement Alexa's custom skill, you need to implement it with AWS Lambda, but since you can simply use Https as an endpoint, I investigated whether it can be implemented with a normal web service.
For the time being, it is listed on the following site, but it is written as AWS is essential.
Host custom skills as web services
Web service requirement that the service supports HTTP over SSL/TLS leveraging Amazon trusted certificates. However, it seems that it is useless if you do not use the CA listed below, so you need to check the server you are using.
The implementation in Java is done by servlet, but the library is published on Alexa's GitHub site.
https://github.com/alexa/alexa-skills-kit-sdk-for-java
I thought I'd implement it based on this, but it's sad. Alexa's library is prepared, but other samples are prepared, but only Servlet does not have samples. .. .. But I found it on another git!
For the time being, I have all the materials to make, so let's start here!
*Notes used in development
How to get free text with Alexa