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 * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package java.io;

import java.lang.annotation.*;


Indicates that an annotated field or method is part of the serialization mechanism defined by the Java Object Serialization Specification. This annotation type is intended to allow compile-time checking of serialization-related declarations, analogous to the checking enabled by the Override annotation type to validate method overriding. Serializable classes are encouraged to use @Serial annotations to help a compiler catch
mis-declared serialization-related fields and methods,
mis-declarations that may otherwise be difficult to detect.

Specifically, annotations of this type should be applied to serialization-related methods and fields in classes declared to be Serializable. The five serialization-related methods are: 
    

  • private void writeObject(java.io.ObjectOutputStream stream) throws IOException 
  • private void readObject(java.io.ObjectInputStream stream) throws IOException, ClassNotFoundException 
  • private void readObjectNoData() throws ObjectStreamException 
  • ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException 
  • ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException 

The two serialization-related fields are:

    

  • private static final ObjectStreamField[] serialPersistentFields 
  • private static final long serialVersionUID 

Compilers are encouraged to validate that a method or field marked with a
@Serial 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.

It is a semantic error to apply this annotation to other fields or methods, including:

    

  • fields or methods in a class that is not Serializable 
  • fields or methods of the proper structural declaration, but in a type where they are ineffectual. For example, enum types are defined to have a serialVersionUID of 0L so a serialVersionUID field declared in an enum type is ignored. The five serialization-related methods identified above are likewise ignored for an enum type. 
  • in a class that is Externalizable: 
      
  
    •  method declarations of writeObject, 
  readObject, and readObjectNoData 
    • a field declaration for serialPersistentFields 
     While the Externalizable interface extends 
Serializable, the three methods and one field above are not used for externalizable classes.

 Note that serialization mechanism accesses its designated fields and methods reflectively and those fields and methods may appear otherwise unused in a Serializable class. 
See Also:
Since:14
/**
 * 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
 * @since 14
 */
@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.SOURCE)
public @interface Serial {}