Java Serialization

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Java Serialization
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What is Serialization?

• Allows the persistent storage of objects
• Uses the java.io.Serializable interface
• ObjectInputStream and ObjectOutputStream
• Allows you to save objects to file and load them
  at a later date
• Really should only be used for temporary storage
  of objects

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What is actually saved?

• Only class name and objects data is saved
• If that data is an object, it is also saved
• Each object is given a serial number
• If an object has already been saved (e.g. within
  a graph) then only the serial number is saved
• Methods are not saved
• Static information not saved

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An example
Network structure
Nan
granddaughter
Stephanie
Family
daughter
daughter
daughter
Mum
Dad
Serialized structure
1
2
3
4
5
family
dad
mum
nan
stephanie
daughter 3
daughter 5
members 2 3 4 5
daughter 5
granddaughter 5
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To save and load structure

• To save the objects to file
• FileOutputStream file new FileOutputStream("data
  .ser")
• ObjectOutputStream output new
  ObjectOutputStream(file)
• output.writeObject(family)
• output.close()
• To load the objects from file
• FileInputStream file new FileInputStream("data.s
  er")
• ObjectInputStream input new ObjectInputStream(fi
  le)
• Family family (Family) input.readObject()
• input.close()
• Note .ser file naming convention

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Indicate data to ignore

• In some situations you may not require the
  serialized object to store all its data
• Some classes cannot be serialized
• To indicate that some data should not be stored
  use the keyword transient
• public class Test implements Serializable
• public transient PrintWriter pw
• public int total
• ...

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Making an object serializable

• Implement the Serializable interface
• Does not require any methods to be implemented
• Implement the Externalizable interface
• default behaviour only saves class name
• must implement readExternal and writeExternal

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Serialized Object V. Class Compatibility

• Handled via the serialVersionUID value
• private static final long serialVersionUID
• -2767605614048989439L
• View using serialver -show
• Stored with serialized object
• If not explicitly defined then a value is
  inserted at compile time
• If the values are not the same -
  InvalidClassException