> From: Jonathan Mast [mailto:jhmast.develo...@gmail.com]
> Subject: Re: replacement for useBean directive
>
> do I really need synchronization at all?
Short answer: yes.
> Is synchronization really called for here, either
> around the getITEM() methods inside the beans or
> around the methods in BeanBag that return the
> bean?
Yes to both. Your example getApple() shows use of some form of Map, most forms
of which require external synchronization. Even if such Map objects
implemented internal synchronization, the logic that checks for the existence
of the key then retrieves the corresponding value requires that the state of
the Map not change between the containsKey() and get() calls.
BTW, you're creating a ton of throw-away objects by doing String.valueOf(int)
here; you may be better off to keep appleID as a String or Integer throughout.
Also, you don't need the containsKey() call, the get() will return null if the
key isn't present.
synchronized Apple getApple(int appleID) {
purgeIfTimeout(); //calls appleMap.clear() if we've timed out
Apple a = (Apple)appleMap.get(String.valueOf(appleID));
if (a != null) return a;
a = FruitDB.getApple(appleID);
if (a != null) appleMap.put(String.valueOf(appleID), a);
return a;
}
Also, your appleMap should be declared as HashMap<String, Apple> or
HashMap<Integer, Apple> (or whatever Map type you're using) so you can get rid
of the casting.
Your BeanBag class methods also need synchronization to close the window
between testing a field for null and then allocating an object for it. Failure
to do so runs the risk of two threads both seeing the field as null, and both
creating beans for the same item type.
- Chuck
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