Finding serialVersionUID of serialized object

Question

Is there a way to determine the generated serialVersionUID of a serialized Java object?

The problem is that I serialized an object without explicitely specifying the serialVersionUID. Now the deserialization process complains about class incompatibilities. However I didn't change the class in a way which would make it incompatible. So I assume that it is enough to specify the serialVersionUID in the class as it is stored in the object data. In order to do this I need to read the serialVersionUID from the serialized data.

Answer 1

That's exactly what you should do - specify your own static final long serialVersionUID.

There's a section about it in the docs for Serializable.

Unless you've specified a serialVersionUID I don't believe there's an easy way to get it other than deciphering the stream as @WMR suggests.

Answer 2

There is a specified grammar for the serialization of objects:

See chapter 6.4 in http://java.sun.com/javase/6/docs/platform/serialization/spec/serialTOC.html

Using this, you should be able to determine the SerialVersionUID of your serialized object.

Answer 3

You can do this by extending ObjectInputStream:

public class PrintUIDs extends ObjectInputStream {

  public PrintUIDs(InputStream in) throws IOException {
    super(in);
  }

  @Override
  protected ObjectStreamClass readClassDescriptor() throws IOException,
      ClassNotFoundException {
    ObjectStreamClass descriptor = super.readClassDescriptor();
    System.out.println("name=" + descriptor.getName());
    System.out.println("serialVersionUID=" + descriptor.getSerialVersionUID());
    return descriptor;
  }

  public static void main(String[] args) throws IOException,
      ClassNotFoundException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ObjectOutputStream oos = new ObjectOutputStream(baos);
    List<Object> list = Arrays.asList((Object) new Date(), UUID.randomUUID());
    oos.writeObject(list);
    oos.close();
    InputStream in = new ByteArrayInputStream(baos.toByteArray());
    ObjectInputStream ois = new PrintUIDs(in);
    ois.readObject();
  }

}

I believe it would be possible to read all the serialized data by replacing the descriptor returned by the method, but I haven't tried it.

Answer 4

There is metadata associated with the serialized bits (a header if you like). You can read the value from the metadata if you know at which position it is (the SerialVersionUID is written there along with other info such as the class name).

I think this article might help you: The Java serialization algorithm revealed.

Note that the bits are written "in clear" (unless you encrypted the stream explicitly) so a HEX editor might be all you need to see what is the SerialVersionUID.

Answer 5

There's an easy way to find out serialversionUID of a class-

Suppose you have class in which you have forgotten to mention serialversionUID-

import java.io.Serializable;

public class TestSerializable implements Serializable {

}

Just do this-

serialver -classpath . TestSerializable

This prints-

static final long serialVersionUID = 5832063776451490808L;

serialver is a utility that comes along with JDK

Answer 6

What happens in the case when a method is added to the class? Can the older serialized object be read using the modified serialized class?