问题
The title pretty much sums it up. I have seen people construct an instance of Random globally, and use it in all of their code, and I have also seen people that construct an instance everytime they want to use Random.
My question is: When, if ever, should I construct a new instance of Random for generating random numbers?
Math.random() stores a Random instance in a RandomNumberGeneratorHolder, and calls it every time Math.random() is called.
My view: I should use a global Random() instance, because:
- Save the time of object creation. The no-arg constructor of Random() calls seedUniquifier(), which basically loops forever until it finds a new AtomicLong(), and raises it to the power of System.nanoTime(). Pretty expensive.
- I am currently using random numbers for a custom hashCode(). I overrided equals(), and now I am doing the same for hashCode(). The class that I am doing this on will be mostly used to store data in Collections, which heavily abuses hashCode(). Seeing the no-arg Random() constructor will take more time than a couple of multiplications that I am using to generate my hashcode, it will more than double the execution time. Not good.
I can't think of any more reasons, but if I should use a global Random instance, I can imagine java developers implementing Random with at instance field, in addition to the constructor for special cases. That tells me that I am wrong. Should I use a global Random instance?
回答1:
You should use a global Random instance for performance reasons, instead of initializing one each time - see also API.
Please note that:
- for multi-threaded applications, you should use a ThreadLocalRandom instead, in the form of
ThreadLocalRandom.current().next...for each call (see official recommendation on the matter). - for cryptographically secure random numbers, use a SecureRandom instead.
回答2:
If you use the functionality of the Random class more than once, then IMO yes. Otherwise it would obviously take more memory to instantiate the class more than once instead of instantiating once and using it everywhere.
回答3:
In a single threaded program it's okay to use a single Random object for the duration of the program.
In a multi-threaded program it's required that threads don't share a single Random object. One way to achieve this is to use ThreadLocalRandom or ThreadLocal and Random.
来源：https://stackoverflow.com/questions/25200903/java-should-i-use-a-global-instance-of-java-util-random-or-construct-one-ever