[SOLVED] Need thread safe MessageDigest in Java

Issue

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,

Solution

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).

Answered By – parsifal

Answer Checked By – Katrina (BugsFixing Volunteer)

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