java.io.InvalidClassException 异常解决, 实现Serializable接口的注意事项

解决方案:　在类中显式指定

ｐｒｉｖａｔｅ　static final long serialVersionUID = 42L;

类实现序列化接口, 进行序列化反序列化的时候, 抛出 java.io.InvalidClassException 异常

java.io.InvalidClassException: com.xx.Xxx; local class incompatible: stream classdesc serialVersionUID = -783991920331, local class serialVersionUID = -331138183213

这个异常是由于反序列化时, 当前类的serialVersionUID 与 bytes中的类反序列化后的类的serialVersionUID 不同所致, 这个serialVersionUID 如果不在类中显式声明, 则是通过类名，方法名等诸多因素经过计算而得，理论上是一一映射的关系，也就是唯一的

JDK中Serializable接口的声明

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 InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named "serialVersionUID" that must be static, final, and of type long:

   ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
   
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 strongly recommended 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 InvalidClassExceptions 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 private 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

重要的几点:

1. 所有实现序列化的类，　都推荐显式声明序列化ID

2. 序列化ID的访问类型 推荐为 private, 因为只在自己内部被使用, 不会因为继承而流到子类

3. 数组是无法显示声明序列化ID的(比如String[], 你无法在其中声明serialVersionUID), 但是java的序列化也不会对数组对象进行serialVersionUID 的比较