Programmatically fill the JVM Permanent Generation (PermGen) memory region

Question

I need to test some JMX monitoring scripts I have developed, In particular I would like to verify that my monitoring of the PermGen region is working. So in order to test this I would like to be able to run a bit of code that loads a significant number of classes in order to consume PermGen.

My current plan is to write a script to generate prefix(1..n).java compile them and then on cue run:

for( int i=1 ; i < n ; i ++){
    Class.forName("com.mypackage.prefix"+i);
}

Is there a more elegant solution to achieve this?

I think that if you instantiate a new ClassLoader and load the same class over and over, it will reach your goal without the need to create tons of different classes. Alternatively, I think that String.intern() can also fill your PermGen space — Guillaume Polet, Mar 21 '12 at 10:27
@Tom, yes and no because I am not sure of this, so this is more a suggestion. I don't want to look affirmative on this. ;-) — Guillaume Polet, Mar 21 '12 at 10:36
@Guillaume : Woot! Memory leak successfully created String.intern()..works a tread... one completely broken glassfish instance! (not that it worked to start with). Do you want to add that as an answer — Gareth Davis, Mar 21 '12 at 10:43
`prefix(1..n).java` should be `prefix(1..n).class` in the question, shouldn't it? — user unknown, Mar 21 '12 at 12:25
I didn't really expand on my idea, but I was going to generate the .java files then compile them into .class...I'll update the question to be clear — Gareth Davis, Mar 21 '12 at 12:28

score 10 · Accepted Answer · answered Mar 21 '12 at 10:51

OK, so looks like String.intern() will do the trick. Here is one implementation I found. Credits goes to Gareth as well:

public static void main(String[] args) throws ClassNotFoundException {
    int i = 0;
    StringBuilder sb = new StringBuilder("a");
    for (i = 0; i < 20; i++) {
        sb.append(sb.toString());
    }
    System.err.println(sb.length());
    i = 0;
    Set<String> strings = new HashSet<String>();
    while (true) {
        strings.add(sb.append(i++).toString().intern());
        System.err.println(i);
    }
}

Nice solution. The bit that caught me out the first time around is that unless you retain a reference to the interned string the GC will simply empty the strings from PermGen on the FullGC that you will eventually force. — Gareth Davis, Mar 21 '12 at 11:03

Programmatically fill the JVM Permanent Generation (PermGen) memory region

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