How to exclude field from class serialization in runtime?

Question

How to exclude class field from serialization process in runtime ? There is transient modifier for compilation time but what about runtime ? I mean common java serialization with ObjectOutputStream, not gson or something.

Sorry I think I didn't explain right. This is not exactly about serialization , but about de-serialization. I have batch of legacy files and handle them like this:

public class Deserialize {  /**  * @param args  * @throws IOException   * @throws ClassNotFoundException   */ public static void main(String[] args) throws ClassNotFoundException, IOException {     File file = new File("/home/developer/workspace/DDFS/some.ddf");     HackedObjectInputStream in = new HackedObjectInputStream(new GZIPInputStream(new FileInputStream(file)));      System.out.println("Attempt to open " + file.getAbsolutePath());     Object obj = in.readObject();     in.close();   }   static class HackedObjectInputStream extends ObjectInputStream     {          /**          * Migration table. Holds old to new classes representation.          */         private static final Map<String, Class<?>> MIGRATION_MAP = new HashMap<String, Class<?>>();          static         {             MIGRATION_MAP.put("DBOBExit", Exit.class);         }          /**          * Constructor.          * @param stream input stream          * @throws IOException if io error          */         public HackedObjectInputStream(final InputStream stream) throws IOException         {             super(stream);         }          @Override         protected ObjectStreamClass readClassDescriptor() throws IOException, ClassNotFoundException         {             ObjectStreamClass resultClassDescriptor = super.readClassDescriptor();              for (final String oldName : MIGRATION_MAP.keySet())             {                 if (resultClassDescriptor.getName().equals(oldName))                 {                     resultClassDescriptor = ObjectStreamClass.lookup(MIGRATION_MAP.get(oldName));                    }             }              return resultClassDescriptor;         }      } 

}

This code works fine for most of files , but some files throws

Exception in thread "main" java.lang.ClassCastException: cannot assign instance of java.awt.Polygon to field Exit.msgbackPt of type java.awt.Point in instance of Exit at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2053) 

because of different versions of Exit class . New version has new fields. Error disappearing when I add transient to new fields, but another files starts to throwing an exception (latest files).

So can I add transient to these new fileds in runtime if I detect legacy serilized file ? Maybe reflection or something ?

Answer 1

The documentation of ObjectOutputStream says:

The default serialization mechanism for an object writes the class of the object, the class signature, and the values of all non-transient and non-static fields. References to other objects (except in transient or static fields) cause those objects to be written also.

So when you declare a variable as transient, it should be ignored by ObjectOutputStream. Make sure that you use the transient keyword and not a @Transient annotation. Such annotations are used by some ORM frameworks to mark fields which are not supposed to be saved in databases. They are meaningless for the buildin serialization framework.

private transient String foo; // Field gets ignored by ObjectOutputStream @Transient private String bar; // Treated normally by ObjectOutputStream (might mean something for some other framework)