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

@@ -291,10 +291,11 @@
     {
         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.

@@ -542,11 +543,27 @@
      * @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");

@@ -708,11 +725,16 @@
         SecurityManager sm = System.getSecurityManager();
         if (sm != null) {
             sm.checkPermission(new PropertyPermission
                                ("user.timezone", "write"));
         }
-        defaultTimeZone = zone;
+        // 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

@@ -733,13 +755,11 @@
      * @return a clone of this <code>TimeZone</code>
      */
     public Object clone()
     {
         try {
-            TimeZone other = (TimeZone) super.clone();
-            other.ID = ID;
-            return other;
+            return super.clone();
         } catch (CloneNotSupportedException e) {
             throw new InternalError(e);
         }
     }
 

@@ -757,10 +777,16 @@
      * 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;