SpaiR/imgui-java is an open source project licensed under Apache License 2.0 which is an OSI approved license.
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If you want a library that has an active community and a lot of examples try out imgui (java port: https://github.com/SpaiR/imgui-java). It is a C based language but seems like the port are pretty close to the original api so you can follow not only the java docs but the c/c++ ones as well. There are a few game engines that utilizes imgui in the editor because it is lightweight, easy to extend, theme and use.