介绍java中serialVersionUID

介绍java中serialVersionUID

serialVersionUID 是java中可序列化类的唯一标识。在对象反序列化时使用，确保加载的类与序列化对象兼容。如果类不匹配，会抛出 InvalidClassException异常。

示例说明

首先创建一个序列化类，并声明serialVersionUID 标识：

public class AppleProduct implements Serializable {

    private static final long serialVersionUID = 1234567L;

    public String headphonePort;
    public String thunderboltPort;
}

接下来，我们需要两个工具类：一个序列化AppleProduct 对象至字符串中，另一个从字符串中反序列化生成对象：

public class SerializationUtility {

   public static void main(String[] args) {
       AppleProduct macBook = new AppleProduct();
       macBook.headphonePort = "headphonePort2020";
       macBook.thunderboltPort = "thunderboltPort2020";

       String serializedObj = serializeObjectToString(macBook);

       System.out.println("Serialized AppleProduct object to string:");
       System.out.println(serializedObj);
   }

   public static String serializeObjectToString(Serializable o) {
       ByteArrayOutputStream baos = new ByteArrayOutputStream();
       ObjectOutputStream oos = new ObjectOutputStream(baos);
       oos.writeObject(o);
       oos.close();

       return Base64.getEncoder().encodeToString(baos.toByteArray());
   }
}

public class DeserializationUtility {

   public static void main(String[] args) {

       String serializedObj = ... // ommited for clarity
       System.out.println(
         "Deserializing AppleProduct...");

       AppleProduct deserializedObj = (AppleProduct) deSerializeObjectFromString(
         serializedObj);

       System.out.println(
         "Headphone port of AppleProduct:"
           + deserializedObj.getHeadphonePort());
       System.out.println(
         "Thunderbolt port of AppleProduct:"
          + deserializedObj.getThunderboltPort());
   }

   public static Object deSerializeObjectFromString(String s)
     throws IOException, ClassNotFoundException {

       byte[] data = Base64.getDecoder().decode(s);
       ObjectInputStream ois = new ObjectInputStream(
         new ByteArrayInputStream(data));
       Object o = ois.readObject();
       ois.close();
       return o;
   }
}

我们开始运行SerializationUtility，其保存（序列化）ApplProduct 为字符串实例，使用Base64编码。

然后运行DeserializationUtility类，使用之前生成的字符串作为deserialization 方法的参数，装配（反序列化）AppleProduct 对象。输出结果大致如下：

Serialized AppleProduct object to string:
rO0ABXNyACljb20uYmFlbGR1bmcuZGVzZXJpYWxpemF0aW9uLkFwcGxlUHJvZHVjdAAAAAAAEta
HAgADTAANaGVhZHBob25lUG9ydHQAEkxqYXZhL2xhbmcvU3RyaW5nO0wADmxpZ2h0ZW5pbmdQb3
J0cQB+AAFMAA90aHVuZGVyYm9sdFBvcnRxAH4AAXhwdAARaGVhZHBob25lUG9ydDIwMjBwdAATd
Gh1bmRlcmJvbHRQb3J0MjAyMA==

Deserializing AppleProduct...
Headphone port of AppleProduct:headphonePort2020
Thunderbolt port of AppleProduct:thunderboltPort2020

现在，让我们修改AppleProduct类中的常量serialVersionUID，并尝试使用之前的字符串反序列化AppleProduct 对象，重新运行会抛出异常：

Deserializing AppleProduct...
Exception in thread "main" java.io.InvalidClassException: com.dataz.deserialization.AppleProduct; local class incompatible: stream classdesc serialVersionUID = 1234567, local class serialVersionUID = 7654321
    at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:616)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1630)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1521)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1781)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
    at com.baeldung.deserialization.DeserializationUtility.deSerializeObjectFromString(DeserializationUtility.java:24)
    at com.dataz.deserialization.DeserializationUtility.main(DeserializationUtility.java:15)

改变类的serialVersionUID ，即修改类的版本/状态，因此在反序列化过程中类不兼容并抛出异常InvalidClassException 。

已存在类中增加新字段

现在我们需要增加lightningPort字段至已存在的类AppleProduct中：

public class AppleProduct implements Serializable {
//...
    public String lightningPort;
}

由于我们只是添加了一个新字段，所以不需要更改serialVersionUID。这是因为在反序列化过程中，lightningPort字段将被赋值null作为默认值。我们修改DeserializationUtility 类，增加打印新字段：

System.out.println("LightningPort port of AppleProduct:"
  + deserializedObj.getLightningPort());

现在，当我们重新运行DeserializationUtility 类，输出结果如下：

Deserializing AppleProduct...
Headphone port of AppleProduct:headphonePort2020
Thunderbolt port of AppleProduct:thunderboltPort2020
Lightning port of AppleProduct:null

总结

本文我们演示了使用serialVersionUID常量来实现序列化数据的版本化。