Serializable

java version “1.8.0_131”

一、总结

• Serializable 接口是一个标记接口，接口中无方法定义
• 作用：一个 Class 实现了 Seriablizable 接口，表明该类声明的对象可以转为一个字节序列，保存对象的状态
• 保存对象的状态的作用：可以快速恢复原来对象的状态（对象的状态指的是某一时刻的对象的属性赋值，如 Person person = new Person(“张三”,18); // 姓名、年龄），可将字节序列输出到某个文件中，需要时再从文件中将该字节序列读入
• 如果一个类可以被序列化，那么其子类也可以被序列化
• 类中被 static（static修饰的类的变量，不是对象私有的，是公有的） 修饰、transient（transient修饰的变量表示临时变量） 修饰的成员变量，不可序列化
• 反序列化时，static 修饰的成员变量反序列化为当前JVM中对应的static值；而不需要进行序列化的变量，使用 transient 修饰

二、源码分析

/** * 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 */public interface Serializable {}

类注释

• 一个类的序列化是通过实现 java.io.Serializable 接口实现的，如果Class 类没有实现 Serializable 接口，则该类声明的对象没有序列化与反序列化的状态，所有类型的序列化类本身是可序列化的。序列化接口没有任何方法或字段，只能识别可序列化的语义，即当前类的对象是可以序列化的。

总结

• 类实现 Serializable 接口，IDE ： Eclispe 默认在该CLASS类名称下方黄色 WARNING 提示 ，生成 serialVersionUID ；
• serialVersionUID 两个生成方式
• 一个是默认的1L，比如：private static final long serialVersionUID = 1L;
• 一个是根据类名、接口名、成员方法及属性等来生成一个64位的哈希字段，比如：private static final long serialVersionUID = 8565360594299563028L;
• serialVersionUID 作用：序列化时为了保持版本的兼容性，即在版本升级时反序列化仍保持对象的唯一性。

注意

• 当 Class 实现 Serializable 后，若类中有成员变量的增减，不需要重新生成 serialVersionUID ； 原因：在系统运行过程中会存在以原有的 serialVersionUID 为标识的字节序列存在，如果重新生成了 serialVersionUID 那么这些字节序列在反序列化时就会抛出异常InvalidClassException；而且当 serialVersionUID 相同时，反序列化时新增的 field 成员变量会按照其相应类型的默认值进行反序列，而不会抛出异常

博文参考：
java对象实现Serializable接口