如何生成一个随机的字母数字字符串?

I've been looking for a simple Java algorithm to generate a pseudo-random alpha-numeric string. In my situation it would be used as a unique session/key identifier that would "likely" be unique over 500K+ generation (my needs don't really require anything much more sophisticated). Ideally, I would be able to specify a length depending on my uniqueness needs. For example, a generated string of length 12 might look something like Java algorithm t