Object Serialization

1
Object Serialization

• CIS 413 - Spring 2006

2
Object Serialization

• ObjectInputStream and ObjectOutputStream classes
  read and write objects.
• Thus reading and writing objects is a process
  called object serialization.
• Object serialization is used for
• Remote Method Invocation (RMI)
• communication between objects via sockets
• Lightweight persistence
• The archival of an object for use in a later
  invocation of the same program.

3
How to Write an Object

• FileOutputStream out
• out new FileOutputStream("theTime")
• ObjectOutputStream s
• s new ObjectOutputStream(out)
• s.writeObject("Today")
• s.writeObject(new Date())
• s.flush()
• Serializes the String Today and the current Date.
• ObjectOutputStreams have to be wrapped on another
  stream.
• This code is serializing the objects to a file
  named theTime

4
The writeObject method

• Serializes the specified object, traverses its
  references to other objects recursively, and
  writes them all.
• Preserves relationships between objects
• ObjectOutputStream implements the DataOutput
  interface
• Has methods for writing primitive data types,
  such as writeInt, writeFloat, or writeUTF.
• You can use these methods to write primitive data
  types to an ObjectOutputStream.
• The writeObject method throws a
  NotSerializableException if it's given an object
  that is not serializable.
• An object is serializable only if its class
  implements the Serializable interface.

5
Serializable Interface

• Known as a marker interface
• Has no methods or fields and serves only to
  identify the semantics of being serializable.
• Classes that do not implement this interface will
  not have any of their state serialized or
  deserialized.
• All subtypes of a serializable class are
  themselves serializable.

6
How to Read an Object

• FileInputStream in
• in new FileInputStream("theTime")
• ObjectInputStream s
• s new ObjectInputStream(in)
• String today (String)s.readObject()
• Date date (Date)s.readObject()
• Deserializes the String Today and the Date.
• ObjectInputStreams also have to be wrapped on
  another stream
• We are reading in from the same file to which we
  wrote the objects

7
Improving Serialization

• Obvious, the more attributes in a class, the more
  time it takes to write all the values
• You may or may not want to read in all values
  again (especially the case with shared memory)
• remove some attributes
• only write the minimal set of attributes
• To say that an attribute will NOT be written, use
  the keyword transient

8
Special Handling

• Classes that require special handling during the
  serialization and deserialization process must
  implement special methods with these exact
  signatures
• private void writeObject(java.io.ObjectOutputStrea
  m out) throws IOException
• private void readObject(java.io.ObjectInputStream
  in) throws IOException, ClassNotFoundException

9
Special Handling

• readObject(ObjectInputStream in)
• Reads from the stream
• Restores the classes fields.
• May call in.defaultReadObject to invoke the
  default mechanism for restoring the object's
  non-static and non-transient fields.
• The defaultReadObject method uses information in
  the stream to assign the fields of the object
  saved in the stream with the correspondingly
  named fields in the current object.
• This handles the case when the class has evolved
  to add new fields.
• writeObject
• Save state
• Write the individual fields to the
  ObjectOutputStream using the writeObject method
  or by using the methods for primitive data types
  supported by DataOutput.