ConcurrentHashMap 使用

Question

Another question today.

I am trying to understand if I should use a ConcurrentHashMap for one of my service or not.

I have a service DataService that fetches data every 10sec from a database and puts it in a map dataMap = Map<Key,Data>
This service is scheduled on a different thread than my main thread.

In my main thread, every now and then i have to get an element from the map dataMap
The main thread only use get and never updates the map.

My question, does the dataMap have to be a ConcurrentHashMap in this case?

Thanks

Answer 1

You have to address the reader and writer accessing the map concurrently. Specifically, a HashMap is not sufficient. From the JavaDoc:

"Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings."

Adding a new map entry from the database is a structural modification and may result in a ConcurrentModificationException.

Note that ConcurrentHashMap is still very efficient. From the Javadoc:

"A hash table supporting full concurrency of retrievals and high expected concurrency for updates. .... Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove)."