Instead of using the JAR, you can also use https://generator.swagger.io to generate the SDKs (Java, Ruby, PHP, etc) online without installing anything. Here is an example:
curl -X POST -H "content-type:application/json" -d '{"swaggerUrl":"http://petstore.swagger.io/v2/swagger.json"}' https://generator.swagger.io/api/gen/clients/java
and here is a sample response:
{"code":"1445940806041","link":"https://generator.swagger.io/api/gen/download/1445940806041"}
You can then download the zipped SDK from the link.
For more options on customizing the output of https://generator.swagger.io, please refer to https://github.com/swagger-api/swagger-codegen#online-generators
(Swagger Generator is part of the Swagger Codegen project (free, open source) that you can run your local Swagger generator as well)
As of July 2017, the Java API client generator supports the following HTTP libraries: Jersey 1.x & 2.x, Retrofit 1.x & 2.x, okhttp, Feign, RESTEasy, RestTemplate
UPDATE: On May 2018, about 50 top contributors and template creators of Swagger Codegen decided to fork Swagger Codegen to maintain a community-driven version called OpenAPI Generator. Please refer to the Q&A for more information.