[jdjlist] Re: AW: problem with serizlization

[Tom Jordan]

Java serialization automatically handles class versioning, within limits. If I understand what you are trying to do, you don't need to write your own writeObject() and readObject() methods. Class versioning is defined as, a class can evolve while still permitting the evolved class to:     1. De-serialize a stream serialized by an older version of the class, or     2. Serialize an object intended to be de-serialized by an older version of the class.   As I said before this works within limits, the limits are you can only change the class as follows: you can rearrange the existing fields, add new fields, or change the types of non-primitive fields in type compatible ways (for instance, change a String to an Object, since a String can be assigned to an Object).   When fields are added this is what happens automatically:     1. An object written by an older version of the class then read by the new version with additional fields, those fields are set to their defaults values according to their type (either false, 0, or null).     2. An object written by the new version of the class then read by an old version of the class without the new fields, the additional fields are silently ignored.   If this is what you want to do, just implement Serializable and follow my previous advice. All of the above is handled automatically without intervention by the programmer.   Tom Jordan  -----Original Message----- From: Zlatko Kostadinov [mailto:[EMAIL PROTECTED]] Sent: Monday, August 26, 2002 10:52 AM To: JDJList Subject: [jdjlist] Re: AW: problem with serizlization Well I have this variable (private static final long serialVersionUID) in the serialized version. But I can't understand how can I use it. Maybe likle the (private static final int version) variable. But then how to get it's value for already serialized instances in my program. Note - the serialized objects are stored into a database and they are many... It will be too slow to run the tool over every instance...   ----- Original Message ----- From: Tom Jordan To: JDJList Sent: Monday, August 26, 2002 5:35 PM Subject: [jdjlist] Re: AW: problem with serizlization If you are Serializing objects you have to learn about the serialVersionUID. It is a 64-bit value that Java's object serialization facility uses to determine whether a serialized object instance is compatiable with the local class of the same name. Every serialized object has one whether you specify it or not. But, you should always specify it. Otherwise practically any change to the class will result in a java.io.InvalidClassException when trying to read (unmarshall) a persisted instantance of the previous version of the class. Furthermore if you don't specify one, the JVM will compute it on the fly; since this is a expensive computation it significantly degrades the serialization performance. You should get in the habit of always including the following class variable to each class that is Serializable.    private static final long serialVersionUID = -7506063353335561975L; Where the above number is found using the java tool "serialver -show" (make sure your classpath is set correctly before running it). It is also printed in the java.io.InvalidClassException if the persisted instance being read is not compatiable with the local class. Note: if serialver is running while you are making changes to a class, exit serialver and restart it after you made your last change to the class and compiled it. Since, serialver loads the class only once, if you don't restart it will compute serialVersionUID on the original class. Tom Jordan -----Original Message----- From: Zlatko Kostadinov [mailto:[EMAIL PROTECTED]] Sent: Monday, August 26, 2002 7:52 AM To: JDJList Subject: [jdjlist] Re: AW: problem with serizlization Yes - this is good desicion. Thanks! But I already have searialized objects - I forgot to put the version field, although I had seen this method already :-(( ----- Original Message ----- From: "Lesden, Kim" <[EMAIL PROTECTED]> To: "JDJList" <[EMAIL PROTECTED]> Sent: Monday, August 26, 2002 2:45 PM Subject: [jdjlist] AW: problem with serizlization > Hello Zlatko, > > a simple way would be to store a version attribute before you store anything else. That way you can also easily migrate old data on the fly. I have seen this work very well in C++. However, if you have lots of data and complex version migrations (v1->v2, v2->v3, ...) this will cost you lots of extra cpu. (see sample below) > > Regards > Kim > > /** >   * Version 1: >   */ > class A { >   private static final int version = 1; >   private ArrayList data = "new" ArrayList(); > >     private void writeObject(ObjectOutputStream ous) throws IOException { >         out.writeObject(version); >         out.writeObject(data); >     } > >     private void readObject(ObjectInputStream ois) throws IOException, > ClassNotFoundException { >         version = out.readObject(); >         if ( 1 == version ) { >           data = "out.readObject();>         } >     } > } > > /** >   * Version 2: >   */ > class A { >     private static final int version = 2; >     private ArrayList data = "new" ArrayList(); >     private String name = "name"; > >     private void writeObject(ObjectOutputStream ous) throws IOException { >         out.writeObject(version); >         out.writeObject(data); >         out.writeObject(name); >     } > >     private void readObject(ObjectInputStream ois) throws IOException, > ClassNotFoundException { >         version = out.readObject(); >         data = "out.readObject();>         if (version > 1) { >           name = out.readObject(); >         } >         else { >            /* convert version 1 data to meaningful version 2 or throw exception */ >            name = "none"; >         } >     } > } > > /** >   * Version 3: >   */ > class A { >     private static final int version = 3; >     private ArrayList data = "new" ArrayList(); >     private String name = "name"; >     private int age = 0; > >     private void writeObject(ObjectOutputStream ous) throws IOException { >         out.writeObject(version); >         out.writeObject(data); >         out.writeObject(name); >         out.writeObject(age); >     } > >     private void readObject(ObjectInputStream ois) throws IOException, > ClassNotFoundException { >         version = out.readObject(); >          data = "out.readObject();>         if (version > 1) { >           name = out.readObject(); >         } >         else { >            /* convert version 1 data to meaningful version 2 or throw exception */ >            name = "name"; >         } >         if (version > 2) { >           age = out.readObject(); >         } >         else { >            /* convert version 2 data to meaningful version 3 or throw exception */ >            age = 0; >         } >     } > } > > > -----Ursprungliche Nachricht----- > Von: Zlatko Kostadinov [mailto:[EMAIL PROTECTED]] > Gesendet: Montag, 26. August 2002 13:14 > An: JDJList > Betreff: [jdjlist] problem with serizlization > > > Hi guys > > I had serizlized objects in this way: > > class A { >     private ArrayList data = "new" ArrayList(); > >     private void writeObject(ObjectOutputStream ous) throws IOException { >         out.writeObject(data); >     } > >     private void readObject(ObjectInputStream ois) throws IOException, > ClassNotFoundException { >         data = "out.readObject();>     } > } > > And my program now works with objects of this type. I want to add new field > in the class so the class will be > > class A { >     private ArrayList data = "new" ArrayList(); >     private name = "name"; > >     private void writeObject(ObjectOutputStream ous) throws IOException { >         out.writeObject(data); >         out.writeObject(name); >     } > >     private void readObject(ObjectInputStream ois) throws IOException, > ClassNotFoundException { >         data = "out.readObject();>         // here is the problem - how to determine is this an instance of the > new version or no. >         // must I read the name or not. >     } > } > > Any ideas are wellcome > Regards > Zlatko > > > To change your JDJList options, please visit: http://www.sys-con.com/java/list.cfm > > To change your JDJList options, please visit: http://www.sys-con.com/java/list.cfm > To change your JDJList options, please visit: http://www.sys-con.com/java/list.cfm To change your JDJList options, please visit: http://www.sys-con.com/java/list.cfm THIS TRANSMISSION, INCLUDING ANY ATTACHMENTS OR FILES, CONTAINS AIRNET COMMUNICATIONS CORPORATION CONFIDENTIAL AND PROPRIETARY INFORMATION WHICH MAY BE OTHERWISE EXEMPT FROM DISCLOSURE. 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