All large scale software should have limits on in memory data growth. Almost every web framework limits the number of concurrent sessions per server, usually by limiting size of thread pool accepting inbound connections.

If you have an in memory map used as a cache where you can discard some of the data if it gets too large then check out Ehcache and you might want to get distributed with memcached since the network is very IO cheap (all depends on your required latencies and cost of computing new values).
Creating a java new object by itself is cheap and keeping many long term objects is a burden on the system’s memory management. Given that, I would not throw cache (and string intern is a type of cache) without getting some numbers first.

From what Eishay is saying, it sounds like that even though the garbage collector has a solid foothold in the perm area, it’s still not completely trivial.
If I would to make extensive intern usage, I’ll make sure I have time to tune the GC perm area during stress testing plus a back of the envelope calculation on how many String I’m expecting to place in the perm area, making sure enough space is left for everything except interened Strings.

BTW, in my current project we employ hard limits on the number of sessions and users as a mean to protect against out of memory situations. Seen/implemented something like that in any of the projects you’ve worked on?
I also like to put caps and report if any map/list grows beyond a certain limit. especially maps used as a temporal storage to preserve information between two executions. Provided useful to warn about memory leaks. Something like:

if (ht.size()>10000) {
System.out.println(“WARNING: ht size above 10K, purging it.”);
ht.clean();
}
ht.put(key, value);

CMSPermGenSweepingEnabled

So the final answer is “it depends”
I would still recommend not to use intern in the common application though there are few edge cases where it would make sense.

I’ve checked on both IBM Java5 and IBM Java6.
Both loop and multi-lines concatenations turn into a StringBuilder.
Seems like there are less and less reasons to micro tune your code.
Thanks.

————————————————
concatenating in a loop – Javap will show you StringBuilder
————————————————
public static volatile String s = “”;
public static void main(String[] args) throws InterruptedException {
for (int i=0; i<10; i++) {
String strOfThisCycle = String.valueOf(i);
s += strOfThisCycle;
}
}

————————————————
Concatenating over multiple lines – Javap will show you StringBuilder
————————————————
public static void main(String[] args) throws InterruptedException {
String strOfThisCycle = “0″;
s += strOfThisCycle;
strOfThisCycle = String.valueOf(System.currentTimeMillis());
s += strOfThisCycle;
}

In java 6 I think that if you run javap on the for loop example you will
see that it uses StringBuilder also, But I might be mistaken

Eyal