Question

Java Gurus,

Currently we have a HashMap<String,SomeApplicationObject> which is being read frequently and modified occasionally and we are having issues that during the modification/reloading, Read operation returns null which is not acceptable.

To fix this I have following options:

A. Use ConcurrentHashMap

Which looks like the first choice but the operation which we are talking about is reload() – means clear() followed by replaceAll(). So if the Map is read post clear() and pre replaceAll() it returns null which is not desirable. Even if I synchronize this doesn’t resolves the issue.

B. Create another implementation based upon ReentrantReadWriteLock

Where I would create acquire Write Lock before reload() operation. This seems more appropriate but I feel there must be something already available for this and I need not to reinvent the wheel.

What is the best way out?

EDIT Is any Collection already available with such feature?

Answer 1

It seems you are not sure as to how what Peter Lawrey suggests can be implemented. It could look like this:

class YourClass {
    private volatile Map<String, SomeApplicationObject> map;

    //constructors etc.

    public void reload() {
        Map<String,SomeApplicationObject> newMap = getNewValues();
        map = Collections.unmodifiableMap(newMap);
    }
}

There are no concurrency issues because:

• The new map is created via a local variable, which by definition is not shared – getNewValues does not need to be synchronized or atomic
• The assignement to map is atomic
• map is volatile, which guarantees that other threads will see the change