java - Performance of ThreadLocal variable

Question

How much is read from ThreadLocal variable slower than from regular field?

More concretely is simple object creation faster or slower than access to ThreadLocal variable?

I assume that it is fast enough so that having ThreadLocal<MessageDigest> instance is much faster then creating instance of MessageDigest every time. But does that also apply for byte[10] or byte[1000] for example?

Edit: Question is what is really going on when calling ThreadLocal's get? If that is just a field, like any other, then answer would be "it's always fastest", right?

Answer 1

In 2009, some JVMs implemented ThreadLocal using an unsynchronised HashMap in the Thread.currentThread() object. This made it extremely fast (though not nearly as fast as using a regular field access, of course), as well as ensuring that the ThreadLocal object got tidied up when the Thread died. Updating this answer in 2016, it seems most (all?) newer JVMs use a ThreadLocalMap with linear probing. I am uncertain about the performance of those – but I cannot imagine it is significantly worse than the earlier implementation.

Of course, new Object() is also very fast these days, and the garbage collectors are also very good at reclaiming short-lived objects.

Unless you are certain that object creation is going to be expensive, or you need to persist some state on a thread by thread basis, you are better off going for the simpler allocate when needed solution, and only switching over to a ThreadLocal implementation when a profiler tells you that you need to.