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:
And the results of interning a bit more than 1 million strings:
HashMap: 4 secondsString.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 secondsHashMap: 3 seconds
As you see, the difference is very noticeable, more than tenfolds. (Using OpenJDK 1.6.0_22 64bits …but using the sun one resulted in similar results I think)
Most likely reason for the performance difference:
String.intern()is a native method, and calling a native method incurs massive overhead.So why is it a native method? Probably because it uses the constant pool, which is a low-level VM construct.