8283681: Improve ZonedDateTime offset handling

Reviewed-by: scolebourne, naoto, rriggs

src/java.base/share/classes/java/time/ZoneId.java

@@ -584,6 +584,11 @@ public abstract sealed class ZoneId implements Serializable permits ZoneOffset,
         return this;
     }
 
+    /**
+     * Get the effective offset for an instant at the given epochSecond.
+     */
+    /* package-private */ abstract ZoneOffset getOffset(long epochSecond);
+
     //-----------------------------------------------------------------------
     /**
      * Checks if this time-zone ID is equal to another time-zone ID.

src/java.base/share/classes/java/time/ZoneOffset.java

@@ -86,6 +86,8 @@ import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 
+import jdk.internal.vm.annotation.Stable;
+
 /**
  * A time-zone offset from Greenwich/UTC, such as {@code +02:00}.
  * <p>
@@ -168,6 +170,11 @@ public final class ZoneOffset
      * The string form of the time-zone offset.
      */
     private final transient String id;
+    /**
+     * The zone rules for an offset will always return this offset. Cache it for efficiency.
+     */
+    @Stable
+    private transient ZoneRules rules;
 
     //-----------------------------------------------------------------------
     /**
@@ -504,7 +511,21 @@ public final class ZoneOffset
      */
     @Override
     public ZoneRules getRules() {
-        return ZoneRules.of(this);
+        ZoneRules rules = this.rules;
+        if (rules == null) {
+            rules = this.rules = ZoneRules.of(this);
+        }
+        return rules;
+    }
+
+    @Override
+    public ZoneId normalized() {
+        return this;
+    }
+
+    @Override
+    /* package-private */ ZoneOffset getOffset(long epochSecond) {
+        return this;
     }
 
     //-----------------------------------------------------------------------

src/java.base/share/classes/java/time/ZoneRegion.java

@@ -178,6 +178,11 @@ final class ZoneRegion extends ZoneId implements Serializable {
         return (rules != null ? rules : ZoneRulesProvider.getRules(id, false));
     }
 
+    @Override
+    /* package-private */ ZoneOffset getOffset(long epochSecond) {
+        return getRules().getOffset(Instant.ofEpochSecond(epochSecond));
+    }
+
     //-----------------------------------------------------------------------
     /**
      * Writes the object using a

src/java.base/share/classes/java/time/ZonedDateTime.java

@@ -452,9 +452,9 @@ public final class ZonedDateTime
      * @throws DateTimeException if the result exceeds the supported range
      */
     private static ZonedDateTime create(long epochSecond, int nanoOfSecond, ZoneId zone) {
-        ZoneRules rules = zone.getRules();
-        Instant instant = Instant.ofEpochSecond(epochSecond, nanoOfSecond);  // TODO: rules should be queryable by epochSeconds
-        ZoneOffset offset = rules.getOffset(instant);
+        // nanoOfSecond is in a range that'll not affect epochSecond, validated
+        // by LocalDateTime.ofEpochSecond
+        ZoneOffset offset = zone.getOffset(epochSecond);
         LocalDateTime ldt = LocalDateTime.ofEpochSecond(epochSecond, nanoOfSecond, offset);
         return new ZonedDateTime(ldt, offset, zone);
     }