java.io.InvalidClassException 異常解決, 實現Serializable接口的注意事項

解決方案:　在類中顯式指定

ｐｒｉｖａｔｅ　static final long serialVersionUID = 42L;

 

類實現序列化接口, 進行序列化反序列化的時候, 拋出 java.io.InvalidClassException 異常

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

 

這個異常是因爲反序列化時, 當前類的serialVersionUID 與 bytes中的類反序列化後的類的serialVersionUID 不一樣所致, 這個serialVersionUID 若是不在類中顯式聲明, 則是經過類名，方法名等諸多因素通過計算而得，理論上是一一映射的關係，也就是惟一的

JDK中Serializable接口的聲明

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

重要的幾點:

1. 全部實現序列化的類，　都推薦顯式聲明序列化ID

2. 序列化ID的訪問類型 推薦爲 private, 由於只在本身內部被使用, 不會由於繼承而流到子類

3. 數組是沒法顯示聲明序列化ID的(好比String[], 你沒法在其中聲明serialVersionUID), 可是java的序列化也不會對數組對象進行serialVersionUID 的比較