di-example | REST API to manage a list of car stored in mongodb | REST library

For https://bpm.zciok.blog/2017/08/17/hibernate-cdi-example/ do the following changes in pom.xml, please

add persistence-api dependency

Because your are using compile time dependency injection, you control the number of instances created, and in your case HomeController is created only once. As requests come in, this single instance will be shared between threads so indeed you have to make sure it is thread-safe. All the dependencies of HomeController will also need to be thread-safe, thus object Workflow has to be thread-safe. Currently, Workflow is not publicly exposing any shared state, so it is thread-safe. In general, val definitions within object are thread-safe.

In effect HomeController is behaving like a singleton and avoiding singletons could be safer. For example, by default Play Framework uses Guice dependency injection which creates a new controller instance per request as long as it is not a @Singleton. One motivation is there is less state to worry about regarding concurrency protection as suggested by Nio's answer:

In general, it is probably best to not use @Singleton unless you have a fair understanding of immutability and thread-safety. If you think you have a use case for Singleton though just make sure you are protecting any shared state.