Does Java have (or is there a library available) that allows me to have a disk based HashMap? It doesn't need to be atomic or anything, but it will be accessed via multiple threads and shouldn't crash if two are accessing the same element at the same time.
Anyone know of anything?
Sounds like you need something close to a lightweight db. Have you looked at/considered Java DB? A light db with a single, indexed table would basically be a disk-based, thread-safe hash map.
Isn't NOSQL supposed to be just some kind of HashMap? Propably that's not what you want because there is still overhead of a database and not just calls to a map. I havn't tried any of them with Java but some API might look quite simple, almost feel like map?
Either properties files or Berkeley DB might be what you're looking for. The java.util.Properties itself implements java.util.Map and provides methods to load from and store to a file. The Berkeley DB is often been recommended as a lightweight key-value pair datastore.
Project Voldemort is also a really fast/scalable/replication "Hashmap". It is used at LinkedIn an performance is also pretty good:
A quote from their site:
Here is the throughput we see from a single multithreaded client talking to a single server where the "hot" data set is in memory under artificially heavy load in our performance lab:
Reads: 19,384 req/sec
Writes: 16,559 req/sec