To Pool or not to Pool java crypto service providers

Question

Solution

• MessageDigest => create new instances as often as needed
• KeyFactory => use a single shared instance
• SecureRandom => use a StackObject

Answer 1

If you give 100 threads access to a shared MessageDigest and get them to calculate 1,000,000 hashes each then on my machine the first thread finishes in 70,160ms and the last finishes in 98,748ms.

If the threads create a new instance of MessageDigest each time, then the first thread finishes in 43,392ms and the last 58,691ms.

Edit:
In fact with this example, with only two threads the example creating new instances runs quicker.

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Share {

  final byte[] bytes = new byte[100];
  final MessageDigest sharedDigest;
  final ExecutorService pool;
  int threads = 100;

  Share() throws NoSuchAlgorithmException {
    sharedDigest = MessageDigest.getInstance("MD5");
    pool = Executors.newFixedThreadPool(threads);
  }

  void go() {

    for (int i=0; i<threads; i++) {
      pool.execute(new Runnable() {
        public void run() {
          long start = System.currentTimeMillis();
          for (int i=0; i<1000000; i++) {
            /*
            synchronized (sharedDigest) {
              sharedDigest.reset();
              sharedDigest.update(bytes);
              sharedDigest.digest();
            }*/
            try {
              MessageDigest digest = MessageDigest.getInstance("MD5");
              digest.reset();
              digest.update(bytes);
              digest.digest();
            } catch (Exception ex) {
              ex.printStackTrace();
            }
          }
          long end = System.currentTimeMillis();
          System.out.println(end-start);
          pool.shutdown();
        }
      });
    }

  }

  public static void main(String[] args) throws Exception {
    Share share = new Share();
    share.go();
  }

}

Answer 2

This test seems to be in favour of caching

long t0 = System.currentTimeMillis();
byte[] bytes = new byte[100];
MessageDigest md = MessageDigest.getInstance("MD5");
for(int i = 0; i < 1000000; i++) {
    //MessageDigest md = MessageDigest.getInstance("MD5");
    md.reset();
    md.update(bytes);
    md.digest();
}
System.out.println(System.currentTimeMillis() - t0);

When md is outside the loop it prints 579, when inside - 953.