I have seen many so-called “reverse geocoding” libraries in various languages; all depend on calling an external provider via REST or some similar method. However, you cannot call a REST provider if you must handle thousands of requests per second.
On the other hand, the problem should be simple to solve – CSV-based databases are available for free with this information. The issue is the time and cost of writing an efficient and well-tested in-memory search implementation, versus downloading or buying an existing one.
I can’t find any after a lot of looking, but I can’t believe there can’t be one. Is there any pre-written library that does this?
This question:
Fastest way to find the location(zip, city, state) given latitude/longitude
came the closest, but essentially indicates how to write the solution, not that there is anything available off the shelf. But there must be some library everyone uses for this. A dozen people a day must have this problem.
Spatial databases (e.g. Postgresql with PostGis) use algorithms which are fast in looking up data for given latitude/longitude information. As you want to use a Java library and have it in memory you could look at the H2 Spatial database. I have never used it, so I can’t comment on its performance.
Edit: Hm, looking closer at the link I’ve provided shows that this is a planned feature… Personally I’d simply use Postgresql/PostGis (with or without Java as server frontend) and be done with it. If your server has enough memory it will still fit the “in-memory” requirement. Naturally it does not fit the Java library requirement. There is however JSI, which could be used in memory and with Java.