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

@@ -138,10 +138,11 @@
      */
     public static final Period ZERO = new Period(0, 0, 0);
     /**
      * Serialization version.
      */
+    @java.io.Serial
     private static final long serialVersionUID = -3587258372562876L;
     /**
      * The pattern for parsing.
      */
     private static final Pattern PATTERN =

@@ -1050,20 +1051,22 @@
      *  out.writeInt(days);
      * </pre>
      *
      * @return the instance of {@code Ser}, not null
      */
+    @java.io.Serial
     private Object writeReplace() {
         return new Ser(Ser.PERIOD_TYPE, this);
     }
 
     /**
      * Defend against malicious streams.
      *
      * @param s the stream to read
      * @throws java.io.InvalidObjectException always
      */
+    @java.io.Serial
     private void readObject(ObjectInputStream s) throws InvalidObjectException {
         throw new InvalidObjectException("Deserialization via serialization delegate");
     }
 
     void writeExternal(DataOutput out) throws IOException {