src/java.base/share/classes/java/util/TimeZone.java

*** 291,300 ****
--- 291,301 ----
      {
          if (ID == null) {
              throw new NullPointerException();
          }
          this.ID = ID;
+         this.zoneId = null;   // invalidate cache
      }
  
      /**
       * Returns a long standard time name of this {@code TimeZone} suitable for
       * presentation to the user in the default locale.
*** 542,552 ****
--- 543,569 ----
       * @return a {@code ZoneId} representing the same time zone as this
       *         {@code TimeZone}
       * @since 1.8
       */
      public ZoneId toZoneId() {
+         ZoneId zId = zoneId;
+         if (zId == null) {
+             zoneId = zId = toZoneId0();
+         }
+         return zId;
+     }
+ 
+     private ZoneId toZoneId0() {
          String id = getID();
+         TimeZone defaultZone = defaultTimeZone;
+         // are we not defaultTimeZone but our id is equal to default's?
+         if (defaultZone != this &&
+             defaultZone != null && id.equals(defaultZone.getID())) {
+             // delegate to default TZ which is effectively immutable
+             return defaultZone.toZoneId();
+         }
+         // derive it ourselves
          if (ZoneInfoFile.useOldMapping() && id.length() == 3) {
              if ("EST".equals(id))
                  return ZoneId.of("America/New_York");
              if ("MST".equals(id))
                  return ZoneId.of("America/Denver");
*** 708,718 ****
          SecurityManager sm = System.getSecurityManager();
          if (sm != null) {
              sm.checkPermission(new PropertyPermission
                                 ("user.timezone", "write"));
          }
!         defaultTimeZone = zone;
      }
  
      /**
       * Returns true if this zone has the same rule and offset as another zone.
       * That is, if this zone differs only in ID, if at all.  Returns false
--- 725,740 ----
          SecurityManager sm = System.getSecurityManager();
          if (sm != null) {
              sm.checkPermission(new PropertyPermission
                                 ("user.timezone", "write"));
          }
!         // by saving a defensive clone and returning a clone in getDefault() too,
!         // the defaultTimeZone instance is isolated from user code which makes it
!         // effectively immutable. This is important to avoid races when the
!         // following is evaluated in ZoneId.systemDefault():
!         // TimeZone.getDefault().toZoneId().
!         defaultTimeZone = (zone == null) ? null : (TimeZone) zone.clone();
      }
  
      /**
       * Returns true if this zone has the same rule and offset as another zone.
       * That is, if this zone differs only in ID, if at all.  Returns false
*** 733,745 ****
       * @return a clone of this <code>TimeZone</code>
       */
      public Object clone()
      {
          try {
!             TimeZone other = (TimeZone) super.clone();
!             other.ID = ID;
!             return other;
          } catch (CloneNotSupportedException e) {
              throw new InternalError(e);
          }
      }
  
--- 755,765 ----
       * @return a clone of this <code>TimeZone</code>
       */
      public Object clone()
      {
          try {
!             return super.clone();
          } catch (CloneNotSupportedException e) {
              throw new InternalError(e);
          }
      }
  
*** 757,766 ****
--- 777,792 ----
       * display names.  <code>ID</code> values are unique in the system
       * table but may not be for dynamically created zones.
       * @serial
       */
      private String           ID;
+ 
+     /**
+      * Cached {@link ZoneId} for this TimeZone
+      */
+     private transient ZoneId zoneId;
+ 
      private static volatile TimeZone defaultTimeZone;
  
      static final String         GMT_ID        = "GMT";
      private static final int    GMT_ID_LENGTH = 3;