Before anyone questions the fact of using string.intern() at all, let me say that I need it in my particular application for memory and performance reasons. [1]

So, until now I used String.intern() and assumed it was the most efficient way to do it. However, I noticed since ages it is a bottleneck in the software. [2]

Then, just recently, I tried to replace the String.intern() by a huge map where I put/get the strings in order to obtain each time a unique instance. I expected this would be slower... but it was exactly the opposite! It was tremendously faster! Replacing the intern() by pushing/polling a map (which achieves exactly the same) resulted in more than one order of magnitude faster.

The question is: why is intern() so slow?!? Why isn't it then simply backed up by a map (or actually, just a customized set) and would be tremendously faster? I'm puzzled.

[1]: For the unconvinced ones: It is in natural language processing and has to process gigabytes of text, therefore needs to avoid many instances of a same string to avoid blowing up the memory and referential string comparison to be fast enough.

[2]: without it (normal strings) it is impossible, with it, this particular step remains the most computation intensive one

EDIT:

Due to the surprising interest in this post, here is some code to test it out:

http://pastebin.com/4CD8ac69

And the results of interning a bit more than 1 million strings:

  • HashMap: 4 seconds
  • String.intern(): 54 seconds

Due to avoid some warm-up / OS IO caching and stuff like this, the experiment was repeated by inverting the order of both benchmarks:

  • String.intern(): 69 seconds
  • HashMap: 3 second
java string performance
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