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what are all points to be considered while implementing serializable interface?

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I would like to know the important points that to be considered during implementing serializable interface.
asked Jul 16, 2012 in Java/J2EE by anonymous

1 Answer

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Do not implement Serializable lightly, since it restricts future flexibility, and publicly exposes class implementation details which are usually private. As well, implementing Serializable correctly is not trivial.

The serialVersionUID is a universal version identifier for a Serializable class. Deserialization uses this number to ensure that a loaded class corresponds exactly to a serialized object. If no match is found, then an InvalidClassException is thrown.

Guidelines for serialVersionUID :

  • always include it as a field, for example: "private static final long serialVersionUID = 7526472295622776147L; " include this field even in the first version of the class, as a reminder of its importance
  • do not change the value of this field in future versions, unless you are knowingly making changes to the class which will render it incompatible with old serialized objects
  • new versions of Serializable classes may or may not be able to read old serialized objects;  it depends upon the nature of the change; provide a pointer to Sun's guidelines for what constitutes a compatible change, as a convenience to future maintainers

In Windows, generate serialVersionUID using the JDK's graphical tool like so :

  • use Control Panel | System | Environment to set the classpath to the correct directory
  • run serialver -show from the command line
  • point the tool to the class file including the package, for example, finance.stock.Account - without the .class
  • (here are the serialver docs for both Win and Unix)

readObject and writeObject :

  • readObject implementations always start by calling default methods
  • deserialization must be treated as any constructor : validate the object state at the end of deserializing - this implies that readObject should almost always be implemented in Serializable classes, such that this validation is performed.
  • deserialization must be treated as any constructor :  if constructors make defensive copies for mutable object fields, so must readObject
  • when serializing a Collection, store the number of objects in the Collection as well, and use this number to read them back in upon deserialization; avoid tricks using null

Other points :

  • use javadoc's @serial tag to denote Serializable fields
  • the .ser extension is conventionally used for files representing serialized objects
  • no static or transient fields undergo default serialization
  • extendable classes should not be Serializable, unless necessary
  • inner classes should rarely, if ever, implement Serializable
  • container classes should usually follow the style of Hashtable, which implements Serializable by storing keys and values, as opposed to a large hash table data structure
answered Jul 16, 2012 by sarada Hot Users (2,340 points)

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