src/share/classes/sun/misc/Hashing.java

rev 5810 : 8006593: Performance and compatibility improvements to hash based Map implementations.
Summary: Use ThreadLocalRandom for hash seed rather than shared Random. Initialize {HashMap|Hashtable}.hashSeed only as needed. Minor optimizations.
Reviewed-by: alanb, bchristi

*** 22,32 ****
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  package sun.misc;
  
! import java.util.Random;
  
  /**
   * Hashing utilities.
   *
   * Little endian implementations of Murmur3 hashing.
--- 22,32 ----
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  package sun.misc;
  
! import java.util.concurrent.ThreadLocalRandom;
  
  /**
   * Hashing utilities.
   *
   * Little endian implementations of Murmur3 hashing.
*** 211,232 ****
       * is fully booted.
       */
      private static class Holder {
  
          /**
-          * Used for generating per-instance hash seeds.
-          *
-          * We try to improve upon the default seeding.
-          */
-         static final Random SEED_MAKER = new Random(
-                 Double.doubleToRawLongBits(Math.random())
-                 ^ System.identityHashCode(Hashing.class)
-                 ^ System.currentTimeMillis()
-                 ^ System.nanoTime()
-                 ^ Runtime.getRuntime().freeMemory());
- 
-         /**
           * Access to {@code String.hash32()}
           */
          static final JavaLangAccess LANG_ACCESS;
  
          static {
--- 211,220 ----
*** 246,259 ****
       */
      public static int stringHash32(String string) {
          return Holder.LANG_ACCESS.getStringHash32(string);
      }
  
      public static int randomHashSeed(Object instance) {
          int seed;
          if (sun.misc.VM.isBooted()) {
!             seed = Holder.SEED_MAKER.nextInt();
          } else {
              // lower quality "random" seed value--still better than zero and not
              // not practically reversible.
              int hashing_seed[] = {
                  System.identityHashCode(Hashing.class),
--- 234,254 ----
       */
      public static int stringHash32(String string) {
          return Holder.LANG_ACCESS.getStringHash32(string);
      }
  
+     /**
+      * Return a non-zero 32-bit pseudo random value. The {@code instance} object
+      * may be used as part of the value.
+      *
+      * @param instance an object to use if desired in choosing value.
+      * @return a non-zero 32-bit pseudo random value.
+      */
      public static int randomHashSeed(Object instance) {
          int seed;
          if (sun.misc.VM.isBooted()) {
!             seed = ThreadLocalRandom.current().nextInt();
          } else {
              // lower quality "random" seed value--still better than zero and not
              // not practically reversible.
              int hashing_seed[] = {
                  System.identityHashCode(Hashing.class),