Summary
-------

Add a `java.io.Serial` annotation type to help mark serialization-related fields and methods for better compile-time checking.

Problem
-------

Mis-declaring a serialization-related field or method can silently lead to unexpected and incorrect behavior.

Solution
--------

Add a new annotation to the `java.io` package, where other serialization-related types reside.

Specification
-------------

    package java.io;
    
    import java.lang.annotation.*;
    
    /**
     * Indicates that an annotated field or method is part of the {@linkplain
     * Serializable serialization mechanism} defined by the
     * <cite>Java Object Serialization Specification</cite>. This
     * annotation type is intended to allow compile-time checking of
     * serialization-related declarations, analogous to the checking
     * enabled by the {@link java.lang.Override} annotation type to
     * validate method overriding. {@code Serializable} classes are encouraged to
     * use <code>&#64;Serial</code> annotations to help a compiler catch
     * mis-declared serialization-related fields and methods,
     * mis-declarations that may otherwise be difficult to detect.
     *
     * <p>Specifically, annotations of this type should be
     * applied to serialization-related methods and fields in classes
     * declared to be {@code Serializable}. The five serialization-related
     * methods are:
     *
     * <ul>
     * <li>{@code private void writeObject(java.io.ObjectOutputStream stream) throws IOException}
     * <li>{@code private void readObject(java.io.ObjectInputStream stream) throws IOException, ClassNotFoundException}
     * <li>{@code private void readObjectNoData() throws ObjectStreamException}
     * <li><i>ANY-ACCESS-MODIFIER</i> {@code Object writeReplace() throws ObjectStreamException}
     * <li><i>ANY-ACCESS-MODIFIER</i> {@code Object readResolve() throws ObjectStreamException} 
     * </ul>
     *
     * The two serialization-related fields are:
     *
     * <ul>
     * <li>{@code private static final ObjectStreamField[] serialPersistentFields}
     * <li>{@code private static final long serialVersionUID}
     * </ul>
     * 
     * Compilers are encouraged to validate that a method or field marked with a 
     * <code>&#64;Serial</code> annotation is one of the defined serialization-related
     * methods or fields declared in a meaningful context and issue a warning
     * if that is not the case.
     *
     * <p>It is a semantic error to apply this annotation to other fields or methods, including:
     * <ul>
     * <li>fields or methods in a class that is not {@code Serializable}
     *
     * <li>fields or methods of the proper structural declaration, but in
     * a type where they are ineffectual. For example, {@code enum} types
     * are defined to have a {@code serialVersionUID} of {@code 0L} so a
     * {@code serialVersionUID} field declared in an {@code enum} type is
     * ignored. The five serialization-related methods identified above
     * are likewise ignored for an {@code enum} type.
     *
     * <li>in a class that is {@code Externalizable}:
     * <ul>
     *   <li> method declarations of {@code writeObject}, {@code
     *   readObject}, and {@code readObjectNoData}
     *
     *  <li>a field declaration for {@code serialPersistentFields}
     * </ul>
     *
     * While the {@code Externalizable} interface extends {@code
     * Serializable}, the three methods and one field above are
     * <em>not</em> used for externalizable classes.
     *
     * </ul>
     *
     * Note that serialization mechanism accesses its designated fields
     * and methods reflectively and those fields and methods may appear
     * otherwise unused in a {@code Serializable} class.
     * 
     * @see Serializable
     * @see Externalizable
     */
    @Target({ElementType.METHOD, ElementType.FIELD})
    @Retention(RetentionPolicy.SOURCE) 
    public @interface Serial {}