Java Serialization

Java Serialization

• 1.
  Serialization in JavaCritical for Java network abilities Jeslie Chermak ( [email_address] )
• 2.
  What is “serialization”?Serialization is the (reversible) encoding of objects, and the objects reachable in them, into a stream of bytes. It is by no means unique to Java: http://en.wikipedia.org/wiki/Serialization This talk is based on information from http://download.oracle.com/javase/tutorial/essential/io/objectstreams.html
• 3.
  Why Should IUse It? BASIS FOR ALL PERSISTENCE IN JAVA ! Simple to add to most classes. Allows for easy extension. Handles versioning.
• 4.
  Simple Class Createa new class in package com.jcc.training.generics Class will implement a traditional “stack” containing Integer s Only methods are push , pop , top Allow NULL s in stack
• 5.
  Java 1.4 Implementationpackage com.jcc.training.generics; import java.util.ArrayList; import java.util.List; public class Stack { private final List stack = new ArrayList(); public void push( final Integer value) { this . stack .add(value); // NULL allowed! } public Integer top() { if ( this . stack .isEmpty()) throw new IllegalStateException(); return (Integer) this . stack .get( this . stack .size() - 1); } public Integer pop() { final Integer value = this .top(); this . stack .remove( this . stack .size() - 1); return value; } }
• 6.
  WIBNIF Problem: Howdo we make this class serializable? Solution: Use a special “marker” interface -- Serializable
• 7.
  Java 1.4 Implementationpackage com.jcc.training.generics; import java.util.ArrayList; import java.util.List; public class Stack implements java.io.Serializable { // marker interface private final List stack = new ArrayList(); public void push( final Integer value) { this . stack .add(value); // NULL allowed! } public Integer top() { if ( this . stack .isEmpty()) throw new IllegalStateException(); return (Integer) this . stack .get( this . stack .size() - 1); } public Integer pop() { final Integer value = this .top(); this . stack .remove( this . stack .size() - 1); return value; } }
• 8.
  WIBNIF Problem: Javahas evolved since 1.4 Solution: Serialization has been present since initial public release (1.0.2) -- it is a core feature of the language implementation
• 9.
  Java 5 withGenerics package com.jcc.training.generics; import java.io.Serializable; // missing earlier import java.util.ArrayList; import java.util.List; public class Stack implements Serializable { // marker interface private final List<Integer> stack = new ArrayList<Integer>(); // field typed! public void push( final Integer value) { this . stack .add(value); // NULL allowed! } public Integer top() { if ( this . stack .isEmpty()) throw new IllegalStateException(); return this . stack .get( this . stack .size() - 1); // no cast! } public Integer pop() { final int value = this .top(); // OOPS! this . stack .remove( this . stack .size() - 1); return value; } }
• 10.
  WIBNIF Problem: class es evolve over time Solution: introduce a version UID to capture “real” changes
• 11.
  Improved and Versioned package com.jcc.training.generics; import java.util.ArrayList; import java.util.List; public class Stack<E> implements java.io.Serializable { // class typed! private static final long serialVersionUID = 1L; // we specified private final List<E> stack = new ArrayList<E>(); // typed by class public void push( final E value) { // typed by class this . stack .add(value); // NULL allowed! } public E top() { // typed by class if ( this . stack .isEmpty()) throw new IllegalStateException(); return this . stack .get( this . stack .size() - 1); } public E pop() { // typed by class final E value = this .top(); // typed by class this . stack .remove( this . stack .size() - 1); return value; } }
• 12.
  WIBNIF Problem: somecode lacks the UID Solution: define default UID to match that normally built in a class -- serialver program
• 13.
  Good News Most(all?) IDEs provide a serialver hook Writing simple objects is easy Reading objects is comparable
• 14.
  Object I/O Writing:public void writeIt( OutputStream os, Foo foo) { ObjectOutputStream oos = new ObjectOutputStream (os); oos.writeObject(foo); oos.close(); } Reading: public Foo readIt( InputStream is) { ObjectInputStream ois = new ObjectInputStream (is); Foo foo = ( Foo ) ois.readObject(); ois.close(); return foo; }
• 15.
  WIBNIF Problem: notevery field should be serialized Solution: transient fields Problem: static fields are the same for each object instance Solution: they are not written by default
• 16.
  Bad News Default readObject() and writeObject() methods may not properly handle objects with transient and/or static fields
• 17.
  Good News Object.readObject() and Object.writeObject() methods (used in default handling) are overloadable: private void readObject( ObjectInputStream ) throws IOException, ClassNotFoundException; private void writeObject( ObjectOutputStream ) throws IOException;
• 18.
  Better News Whendefault handling is insufficient, use the Externalizable interface instead of Serializable and implement readExternal() and writeExternal() -- a lot more work for total control of serialization
• 19.
  Going Further …Java tutorials: http://download.oracle.com/javase/tutorial/index.html Serialization in depth w/examples: http://download.oracle.com/javase/7/docs/technotes/guides/serialization/index.html Object streams: http://download.oracle.com/javase/tutorial/essential/io/objectstreams.html Google “java serialization”