One thread adding/updating value and other thread getting values....issue

Question

am using a spring bean (singleton) and this is how it looks like
public class MySingletonService {
private Map<String, List<TaskVO>> alerts = new HashMap<String, List<TaskVO>>();
//spring will call this method every 15 minutes
public void pollDatabase() {
//initialize the alerts instance variable and create/fill the alerts instance variable with new alerts for the employee(from database) .The key for the hashmap is the employee id
//some client code (e.g. GUI) will call this method to get the alert for the operator id.This can be called anytime to get the latest info from the alert Map
public List<TaskAlertVO> getOutstandingAlerts(String operatorId) {
return alerts.get(operatorId);
The issue here is that when getOutstandingAlerts is invoked by a thread,some other thread(Spring) calling the pollDatabase method might be updating the value.Please give me some idea and solution to get around this issue.
I was thinking of creating a copy of alerts instance variable in the getOutstandingAlerts method and then use the copy to find the key and return the alert for that operator id.This way we dont have to worry about data conflict.
Please advice

Answer 1

Manjit wrote:
jtahlborn wrote:
if the pollDatabase() method is getting a complete new copy of the alerts every time, and the alerts map isn't being modified at all after load, then you can just:
- make the alerts member variable volatible
- create a new HashMap each time pollDatabase() is called, fill it in, and then assign it to the alerts member variable. Jtalhorn,I think I got your point.Basically you are suggesting that by creating a brand new map in each pollDatabase operation, then I dont have to worry about get operation ,simply because the client code is still looking at old memory location(old copy) and hence should not worry about values getting changes by pollDatabase method.
I got it.
But the small doubt is that why even I should bother about making the instance variable volatile ? Your suggestion should work without making it volatilebecause volatile ensures correct memory visibility between threads. if you didn't use volatile, then a caller to getOutstandingAlerts() could see some partially initialized/corrupt version of the map.