jMonkeyEngine.org

I’m tracking down a memory leak at the moment and just came across this really interesting article on how Strings are implemented in a number of different JVM’s. I haven’t come across this before, but its sure nice to know about.. Scary stuff.

http://eyalsch.wordpress.com/2009/10/27/stringleaks/

TL;DR: The backing char arrays of Strings are held on to and can’t be garbage collected, so…

Do this:

String string = "abcdefghijklmnopqrstuvqxyz";
string = new String(string.substring(2, 5));

Instead of this:

String string = "abcdefghijklmnopqrstuvqxyz";
string = string.substring(2, 5);

Yeah I always think it looks funny seeing new String() but stress testing some server software and seeing your heap usage skyrocket is worse IMO

@pspeed said:In that example, it should make no difference as the strings are literals and will be interned anyway as part of the Class literals.

Yeah for that example it wouldn’t really matter.. I didn’t even think of that, I just consciously decided not to write out all of the code to open and read a file from memory

Interesting indeed, its like doing a substring() on a string causes the returned string to reference the main string and thus it will only be released when the substring object is released. Easy to remember but you really gotta know this one

Also if you have string and want all substrings taht ar possible the additional memory cost is quite low. I agree however, that if the original string is rather large, or only a very small part of it is needed the new String approach should be used. (I think about xml processing and Nodenames for example here , btw how is the ogre loader written as we speak of it?)

Its not a bug, its a feature It saves you from stuffing new objects in the heap all the time.

@shirkit said:
Yeah, but a feature that causes memory leak for me sounds like a bug.

By that logic any language without automatic garbage collection should be killed off… ?

@shirkit said:
I consider it annoying as I spent about 2 hours to discover this thing. So, for me, it’s annoying

Fair enough