Need thread safe MessageDigest in Java

Question

I need to hash multiple keys from multiple threads using MessageDigest in a performance critical environment. I came to know that MessageDigest is not thread safe as it stores its state in it's object. What can be the best possible way to achieve thread safe hashing of keys?

Use case:

MessageDigest messageDigest = MessageDigest.getInstance("SHA-1");

//somewhere later, just need to hash a key, nothing else
messageDigest.update(key);
byte[] bytes = messageDigest.digest(); 

Specifically:

1. Will ThreadLocal guaranteed to work? Will it have performance penalty?
2. Are the objects returned by getInstance different and they do not interfere with each other? The documentation says 'new' object, but I am not sure whether it is just a wrapper on (shared) shared concrete class?
3. If getInstance() returns 'real' new objects, is it advisable to create a new instance each time I need to calculate the hash? In terms of performance penalty - how costly is it?

My use case is very simple - just hash a simple key. I cannot afford to use synchronization.

Thanks,

Answer 1

Create a newMessageDigest instance each time you need one.

All of the instances returned from getInstance() are distinct. They need to be, as they maintain separate digests (and if that's not enough for you, here's a link to the source).

ThreadLocal can provide a performance benefit when used with a threadpool, to maintain expensive-to-construct objects. MessageDigest isn't particularly expensive to construct (again, look at the source).

Answer 2

As an alternative, use DigestUtils, Apache Commons' thread-safe wrapper for MessageDigest.

sha1() does what you need:

byte[] bytes = sha1(key)

Answer 3

You could use ImmutableMessageDigest from Caesar, an open source library I wrote.

It essentially wraps a MessageDigest instance and clones it before every digest() or update() call.