Make MACs thread-local to avoid repeated reinstantiation/garbage collection.

Author: jchambers

src/main/java/com/eatthepath/otp/HmacOneTimePasswordGenerator.java

@@ -42,6 +42,8 @@ public class HmacOneTimePasswordGenerator {
 
     private final int modDivisor;
 
+    private final ThreadLocal<Mac> macThreadLocal;
+
     /**
      * The default length, in decimal digits, for one-time passwords.
      */
@@ -118,6 +120,16 @@ protected HmacOneTimePasswordGenerator(final int passwordLength, final String al
         // Our purpose here is just to throw an exception immediately if the algorithm is bogus.
         Mac.getInstance(algorithm);
         this.algorithm = algorithm;
+
+        this.macThreadLocal = ThreadLocal.withInitial(() -> {
+            try {
+                return Mac.getInstance(algorithm);
+            } catch (final NoSuchAlgorithmException e) {
+                // This should never happen; we just checked to make sure we could instantiate a MAC with this
+                // algorithm, and if we made it this far, we know it worked.
+                throw new RuntimeException(e);
+            }
+        });
     }
 
     /**
@@ -132,15 +144,8 @@ protected HmacOneTimePasswordGenerator(final int passwordLength, final String al
      * @throws InvalidKeyException if the given key is inappropriate for initializing the {@link Mac} for this generator
      */
     public int generateOneTimePassword(final Key key, final long counter) throws InvalidKeyException {
-        final Mac mac;
-
-        try {
-            mac = Mac.getInstance(this.algorithm);
-            mac.init(key);
-        } catch (final NoSuchAlgorithmException e) {
-            // This should never happen since we verify that the algorithm is legit in the constructor.
-            throw new RuntimeException(e);
-        }
+        final Mac mac = this.macThreadLocal.get();
+        mac.init(key);
 
         final ByteBuffer buffer = ByteBuffer.allocate(8);
         buffer.putLong(0, counter);