Wednesday, 31 October 2012

Breaking String encapsulation using Unsafe

As we all know, String is immutable, which is great and it is also defensive about it's internals to maintain that immutability which is also great, but... sometimes you want to be able to get/set that damn internal char[] without copying it. While arguably this is not very important to most people, it is quite desirable on other occasions when you are trying to get the best performance from a given piece of code. Here's how to break the encapsulation using Unsafe:
1. Acquire Unsafe:
2. Get the field offsets for String fields:
3. Use the offsets to get/set the field values:
Now this seems a bit excessive doesn't it? Isn't encapsulation important? Why would you do that? The bottom line is that this is to scrape some extra performance juice out of your system, and if you are willing to get your hands dirty the above can give you a nice boost.
Getting the data out of String is far less questionable then altering it's internal final fields, just so we are clear. So it is not really recommended that you use the set functionality as illustrated above, unless you are sure it's going to work. Here is why it's generally a bad idea to write into final fields.
Using other techniques it should be possible to hack your way into the package private String constructor that would spare us that bit of hackiness to the same effect.
Measurements and a real world use case to follow...

As per the source code used for the next post, the above source code would break for JDK7 as String no longer has the fields offset and count. The sentiment however stays the same and the final result is on GitHub.

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