src/java.base/share/classes/java/lang/Long.java

@@ -32,10 +32,12 @@
  import java.math.*;
  import java.util.Objects;
  import java.util.Optional;
  
  import jdk.internal.misc.CDS;
+ import jdk.internal.misc.PreviewFeatures;
+ import jdk.internal.value.DeserializeConstructor;
  import jdk.internal.util.DecimalDigits;
  import jdk.internal.vm.annotation.ForceInline;
  import jdk.internal.vm.annotation.IntrinsicCandidate;
  import jdk.internal.vm.annotation.Stable;
  

@@ -54,14 +56,23 @@
   * a {@code long} to a {@code String} and a {@code String} to a {@code
   * long}, as well as other constants and methods useful when dealing
   * with a {@code long}.
   *
   * <p>This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
-  * class; programmers should treat instances that are
-  * {@linkplain #equals(Object) equal} as interchangeable and should not
-  * use instances for synchronization, or unpredictable behavior may
-  * occur. For example, in a future release, synchronization may fail.
+  * class; programmers should treat instances that are {@linkplain #equals(Object) equal}
+  * as interchangeable and should not use instances for synchronization, mutexes, or
+  * with {@linkplain java.lang.ref.Reference object references}.
+  *
+  * <div class="preview-block">
+  *      <div class="preview-comment">
+  *          When preview features are enabled, {@code Long} is a {@linkplain Class#isValue value class}.
+  *          Use of value class instances for synchronization, mutexes, or with
+  *          {@linkplain java.lang.ref.Reference object references} result in
+  *          {@link IdentityException}.
+  *      </div>
+  * </div>
+  *
   *
   * <p>Implementation note: The implementations of the "bit twiddling"
   * methods (such as {@link #highestOneBit(long) highestOneBit} and
   * {@link #numberOfTrailingZeros(long) numberOfTrailingZeros}) are
   * based on material from Henry S. Warren, Jr.'s <cite>Hacker's

@@ -72,10 +83,11 @@
   * @author  Arthur van Hoff
   * @author  Josh Bloch
   * @author  Joseph D. Darcy
   * @since   1.0
   */
+ @jdk.internal.MigratedValueClass
  @jdk.internal.ValueBased
  public final class Long extends Number
          implements Comparable<Long>, Constable, ConstantDesc {
      /**
       * A constant holding the minimum value a {@code long} can

@@ -976,28 +988,42 @@
      }
  
      /**
       * Returns a {@code Long} instance representing the specified
       * {@code long} value.
-      * If a new {@code Long} instance is not required, this method
-      * should generally be used in preference to the constructor
-      * {@link #Long(long)}, as this method is likely to yield
-      * significantly better space and time performance by caching
-      * frequently requested values.
-      *
-      * This method will always cache values in the range -128 to 127,
-      * inclusive, and may cache other values outside of this range.
+      * <div class="preview-block">
+      *      <div class="preview-comment">
+      *          <p>
+      *              - When preview features are NOT enabled, {@code Long} is an identity class.
+      *              If a new {@code Long} instance is not required, this method
+      *              should generally be used in preference to the constructor
+      *              {@link #Long(long)}, as this method is likely to yield
+      *              significantly better space and time performance by caching
+      *              frequently requested values.
+      *              This method will always cache values in the range -128 to 127,
+      *              inclusive, and may cache other values outside of this range.
+      *          </p>
+      *          <p>
+      *              - When preview features are enabled, {@code Long} is a {@linkplain Class#isValue value class}.
+      *              The {@code valueOf} behavior is the same as invoking the constructor,
+      *              whether cached or not.
+      *          </p>
+      *      </div>
+      * </div>
       *
       * @param  l a long value.
       * @return a {@code Long} instance representing {@code l}.
       * @since  1.5
       */
      @IntrinsicCandidate
+     @DeserializeConstructor
      public static Long valueOf(long l) {
-         final int offset = 128;
-         if (l >= -128 && l <= 127) { // will cache
-             return LongCache.cache[(int)l + offset];
+         if (!PreviewFeatures.isEnabled()) {
+             if (l >= -128 && l <= 127) { // will cache
+                 final int offset = 128;
+                 return LongCache.cache[(int) l + offset];
+             }
          }
          return new Long(l);
      }
  
      /**