Java 日看一类（54）之IO包中的Serializable接口

该接口无引入包

无继承类

该接口的类头注释如下：

/**
 * Serializability of a class is enabled by the class implementing the
 * java.io.Serializable interface. Classes that do not implement this
 * interface will not have any of their state serialized or
 * deserialized.  All subtypes of a serializable class are themselves
 * serializable.  The serialization interface has no methods or fields
 * and serves only to identify the semantics of being serializable. <p>
 *
 * To allow subtypes of non-serializable classes to be serialized, the
 * subtype may assume responsibility for saving and restoring the
 * state of the supertype's public, protected, and (if accessible)
 * package fields.  The subtype may assume this responsibility only if
 * the class it extends has an accessible no-arg constructor to
 * initialize the class's state.  It is an error to declare a class
 * Serializable if this is not the case.  The error will be detected at
 * runtime. <p>
 *
 * During deserialization, the fields of non-serializable classes will
 * be initialized using the public or protected no-arg constructor of
 * the class.  A no-arg constructor must be accessible to the subclass
 * that is serializable.  The fields of serializable subclasses will
 * be restored from the stream. <p>
 *
 * When traversing a graph, an object may be encountered that does not
 * support the Serializable interface. In this case the
 * NotSerializableException will be thrown and will identify the class
 * of the non-serializable object. <p>
 *
 * Classes that require special handling during the serialization and
 * deserialization process must implement special methods with these exact
 * signatures:
 *
 * <PRE>
 * private void writeObject(java.io.ObjectOutputStream out)
 *     throws IOException
 * private void readObject(java.io.ObjectInputStream in)
 *     throws IOException, ClassNotFoundException;
 * private void readObjectNoData()
 *     throws ObjectStreamException;
 * </PRE>
 *
 * <p>The writeObject method is responsible for writing the state of the
 * object for its particular class so that the corresponding
 * readObject method can restore it.  The default mechanism for saving
 * the Object's fields can be invoked by calling
 * out.defaultWriteObject. The method does not need to concern
 * itself with the state belonging to its superclasses or subclasses.
 * State is saved by writing the individual fields to the
 * ObjectOutputStream using the writeObject method or by using the
 * methods for primitive data types supported by DataOutput.
 *
 * <p>The readObject method is responsible for reading from the stream and
 * restoring the classes fields. It may call in.defaultReadObject to invoke
 * the default mechanism for restoring the object's non-static and
 * non-transient fields.  The defaultReadObject method uses information in
 * the stream to assign the fields of the object saved in the stream with the
 * correspondingly named fields in the current object.  This handles the case
 * when the class has evolved to add new fields. The method does not need to
 * concern itself with the state belonging to its superclasses or subclasses.
 * State is saved by writing the individual fields to the
 * ObjectOutputStream using the writeObject method or by using the
 * methods for primitive data types supported by DataOutput.
 *
 * <p>The readObjectNoData method is responsible for initializing the state of
 * the object for its particular class in the event that the serialization
 * stream does not list the given class as a superclass of the object being
 * deserialized.  This may occur in cases where the receiving party uses a
 * different version of the deserialized instance's class than the sending
 * party, and the receiver's version extends classes that are not extended by
 * the sender's version.  This may also occur if the serialization stream has
 * been tampered; hence, readObjectNoData is useful for initializing
 * deserialized objects properly despite a "hostile" or incomplete source
 * stream.
 *
 * <p>Serializable classes that need to designate an alternative object to be
 * used when writing an object to the stream should implement this
 * special method with the exact signature:
 *
 * <PRE>
 * ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException;
 * </PRE><p>
 *
 * This writeReplace method is invoked by serialization if the method
 * exists and it would be accessible from a method defined within the
 * class of the object being serialized. Thus, the method can have private,
 * protected and package-private access. Subclass access to this method
 * follows java accessibility rules. <p>
 *
 * Classes that need to designate a replacement when an instance of it
 * is read from the stream should implement this special method with the
 * exact signature.
 *
 * <PRE>
 * ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;
 * </PRE><p>
 *
 * This readResolve method follows the same invocation rules and
 * accessibility rules as writeReplace.<p>
 *
 * The serialization runtime associates with each serializable class a version
 * number, called a serialVersionUID, which is used during deserialization to
 * verify that the sender and receiver of a serialized object have loaded
 * classes for that object that are compatible with respect to serialization.
 * If the receiver has loaded a class for the object that has a different
 * serialVersionUID than that of the corresponding sender's class, then
 * deserialization will result in an {@link InvalidClassException}.  A
 * serializable class can declare its own serialVersionUID explicitly by
 * declaring a field named <code>"serialVersionUID"</code> that must be static,
 * final, and of type <code>long</code>:
 *
 * <PRE>
 * ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
 * </PRE>
 *
 * If a serializable class does not explicitly declare a serialVersionUID, then
 * the serialization runtime will calculate a default serialVersionUID value
 * for that class based on various aspects of the class, as described in the
 * Java(TM) Object Serialization Specification.  However, it is <em>strongly
 * recommended</em> that all serializable classes explicitly declare
 * serialVersionUID values, since the default serialVersionUID computation is
 * highly sensitive to class details that may vary depending on compiler
 * implementations, and can thus result in unexpected
 * <code>InvalidClassException</code>s during deserialization.  Therefore, to
 * guarantee a consistent serialVersionUID value across different java compiler
 * implementations, a serializable class must declare an explicit
 * serialVersionUID value.  It is also strongly advised that explicit
 * serialVersionUID declarations use the <code>private</code> modifier where
 * possible, since such declarations apply only to the immediately declaring
 * class--serialVersionUID fields are not useful as inherited members. Array
 * classes cannot declare an explicit serialVersionUID, so they always have
 * the default computed value, but the requirement for matching
 * serialVersionUID values is waived for array classes.
 *
 * @author  unascribed
 * @see java.io.ObjectOutputStream
 * @see java.io.ObjectInputStream
 * @see java.io.ObjectOutput
 * @see java.io.ObjectInput
 * @see java.io.Externalizable
 * @since   JDK1.1
 */

该类的类头注释如下：

类通过实现该接口来拥有序列化功能

没有实现该类的无法进行序列化或者反序列化

可序列化类的子类也可序列化

该结构未定义任何的字段以及方法，只是个标识，标志该类可以被序列化

如果允许不可序列化类的子类进行序列化，该子类必须负责父类字段值的储存和恢复

仅在子类型扩展的类有一个可访问的无参数构造方法来初始化该类的状态时，才可以假定子类型有此职责

当不存在无参数构造方法时，声明序列化是错误的，会在运行时报错

在反序列化期间，不可序列化的类的字段值需要通过子类的无参数可访问的（public，protected）构造方法来恢复（子类是可序列化的）。可序列化类的子类的字段会从流中恢复

遍历图形时，可能会遇到不支持Serializable的对象，在这种情况下抛出NotSerializionException异常

在序列化和反序列化过程中需要特殊处理的类必须使用下列准确标识来实现特殊方法：

 private void writeObject(java.io.ObjectOutputStream out)
     throws IOException
 private void readObject(java.io.ObjectInputStream in)
     throws IOException, ClassNotFoundException;
 private void readObjectNoData() 
     throws ObjectStreamException;

writeObject方法负责写出特指的对象状态，相应的、readObject方法用于回复特定的对象状态

通过调用 out.defaultWriteObject 可以调用保存 Object 的字段的默认机制。该方法本身不需要涉及属于其超类或子类的状态。通过使用 writeObject 方法或使用 DataOutput 支持的用于基本数据类型的方法将各个字段写入 ObjectOutputStream，状态可以被保存。

readObject 方法负责从流中读取并恢复类字段。它可以调用 in.defaultReadObject 来调用默认机制，以恢复对象的非静态和非瞬态字段。defaultReadObject 方法使用流中的信息来分配流中通过当前对象中相应指定字段保存的对象的字段。这用于处理类演化后需要添加新字段的情形。该方法本身不需要涉及属于其超类或子类的状态。通过使用 writeObject 方法或使用 DataOutput 支持的用于基本数据类型的方法将各个字段写入 ObjectOutputStream，状态可以被保存。

在序列化流不列出给定类作为将被反序列化对象的超类的情况下，readObjectNoData 方法负责初始化特定类的对象状态。这在接收方使用的反序列化实例类的版本不同于发送方，并且接收者版本扩展的类不是发送者版本扩展的类时发生。在序列化流已经被篡改时也将发生；因此，不管源流是“敌意的”还是不完整的，readObjectNoData 方法都可以用来正确地初始化反序列化的对象。

将对象写入流时需要指定要使用的替代对象的可序列化类，应使用准确的签名来实现此特殊方法：

 ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException;
 

此 writeReplace 方法将由序列化调用，前提是如果此方法存在，而且它可以通过被序列化对象的类中定义的一个方法访问。因此，该方法可以拥有私有 (private)、受保护的 (protected) 和包私有 (package-private) 访问。子类对此方法的访问遵循 java 访问规则。

在从流中读取类的一个实例时需要指定替代的类应使用的准确签名来实现此特殊方法。

 ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;
 

此 readResolve 方法遵循与 writeReplace 相同的调用规则和访问规则。

序列化运行时使用一个称为 serialVersionUID 的版本号与每个可序列化类相关联，该序列号在反序列化过程中用于验证序列化对象的发送者和接收者是否为该对象加载了与序列化兼容的类。如果接收者加载的该对象的类的 serialVersionUID 与对应的发送者的类的版本号不同，则反序列化将会导致 InvalidClassException。可序列化类可以通过声明名为 "serialVersionUID" 的字段（该字段必须是静态 (static)、最终 (final) 的 long 型字段）显式声明其自己的 serialVersionUID：

 ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
 

如果可序列化类未显式声明 serialVersionUID，则序列化运行时将基于该类的各个方面计算该类的默认 serialVersionUID 值，如“Java(TM) 对象序列化规范”中所述。不过， 强烈建议  所有可序列化类都显式声明 serialVersionUID 值，原因是计算默认的 serialVersionUID 对类的详细信息具有较高的敏感性，根据编译器实现的不同可能千差万别，这样在反序列化过程中可能会导致意外的  InvalidClassException 。因此，为保证 serialVersionUID 值跨不同 java 编译器实现的一致性，序列化类必须声明一个明确的 serialVersionUID 值。还强烈建议使用  private  修饰符显示声明 serialVersionUID（如果可能），原因是这种声明仅应用于直接声明类 -- serialVersionUID 字段作为继承成员没有用处。数组类不能声明一个明确的 serialVersionUID，因此它们总是具有默认的计算值，但是数组类没有匹配 serialVersionUID 值的要求。

大部分内容来源于api，该类主要为序列化服务，所以需要使用序列化功能的请仔细学习阅读该注释