Concurrent Thread accessing the shared variable

Question

Hi All,
I need help on following issue
I am working on java program that will have about ~500 threads reading per minute List<Object> and ONE task thread which will reset the List<Object> every 4 hours.
Let imagine this following class,
public class Cache {
private static  List< Object> m_cacheList = null;
privates static final Cache instance = new Cache(new AraryList<Object>());
public getInstance(return instance );
private  Cache (Listc){
     m_cacheList = List< Object>;
public List<Object>  getCacheList(){
     return m_cacheList;
public List<Object>  refreshCacheList(List<Object>  newData){
m_cacheList = m_cacheList;
500 Read Tread
List<Object> list = Cache. getInstance().getCacheList();
// this will iterate
for(Object obj ; list ){
   //do something
1 Write Tharead  every 4 hr
List<Object> newData = ��..
Cache. getInstance().refreshCacheList (newData);Since the read rate is so high, I am thinking about using work �volatile� rather then full synchronization.

Answer 1

This looks fine to me, volatile should work fine. The assignment of a new cache is atomic so should not interfere with the threads accessing the cache object.
As good practice however I would recommend that you should not let the cache object out of the class (make it private and do not provide a method that returns it) so that you can control the access to it. If you return the cache object then it is possible that another class that accesses it may try to perform an operation that is not thread safe. For example if two threads get a reference to the same list and one is using the iterator while another thread modifies it (i know this is not the intention now, but you should code with future use in mind), then you will get a ConcurrentModificationException. The options here are either let the Cache class perform the function on the list and never return it, or return a copy of the list in the "getCacheData()" function.
This way you are keeping the state of the class safely encapsulated inside the class and can control all modifications to it.
Hope that helps
Paul