src/java.base/share/classes/java/net/PortUnreachableException.java

@@ -31,10 +31,11 @@
  *
  * @since   1.4
  */
 
 public class PortUnreachableException extends SocketException {
+    @java.io.Serial
     private static final long serialVersionUID = 8462541992376507323L;
 
     /**
      * Constructs a new {@code PortUnreachableException} with a
      * detail message.