What is the most popular in-memory data store for Java?

The most popular in-memory data store for Java is Redis. It is a high-performance, memory-resident database that supports various data structures: strings, lists, sets, hashes, and more.

Redis is often used for caching, session management, message queues, and other tasks where fast data access is crucial.

To work with Redis in Java, client libraries such as Jedis or Lettuce are commonly used.

Example of connecting to Redis using Jedis:

Jedis jedis = new Jedis("localhost", 6379);
jedis.set("key", "value");
String value = jedis.get("key");
System.out.println(value); // Will output "value"
jedis.close();