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Slide 10.1 Advanced Programming 2004, based on LY Stefanus’s Slides Object Serialization Object serialization: the process of converting an object into.

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Presentation on theme: "Slide 10.1 Advanced Programming 2004, based on LY Stefanus’s Slides Object Serialization Object serialization: the process of converting an object into."— Presentation transcript:

1 slide 10.1 Advanced Programming 2004, based on LY Stefanus’s Slides Object Serialization Object serialization: the process of converting an object into a stream of bytes.

2 slide 10.2 Advanced Programming 2004, based on LY Stefanus’s Slides Where to use as the basis for transfering objects via cut-and-paste. (Alternative: MIME type string, see page. 162 JiaN) to transfer objects between a client and a server for remote method invocation (RMI). Pre-initialized java beans, persistent java beans Pre-initialized applets Save user preferences and application state – a serialized object is an instant file format that works for any application. GUI Builder Tools, saving complete hierarchy Component hierarchy. OS, hibernate (e.g. Windows XP)

3 slide 10.3 Advanced Programming 2004, based on LY Stefanus’s Slides Serialization Object serialization: the process of converting an object into a stream of bytes. writeObject() Object deserialization: the process of converting a stream of bytes back into a copy of the original object. readObject() An object of any class that implements the java.io.Serializable interface can be saved and restored from a stream of bytes. Special stream classes from the package java.io, ObjectInputStream and ObjectOutputStream, are used to serialize primitive types and objects. Subclasses of Serializable classes are also serializable.

4 slide 10.4 Advanced Programming 2004, based on LY Stefanus’s Slides java.io.Serializable is an example of marker interface. Marker interface is an interface that is entirely empty. A class can implement this interface simply by naming it in its implements clause without having to implement any methods. We can check whether an object is an instance of the interface using the instanceof operator, so this technique is a useful way to provide additional information about an object. Another example of marker interface is java.lang.Cloneable.

5 slide 10.5 Advanced Programming 2004, based on LY Stefanus’s Slides The default serialization mechanism saves the value of an object's nonstatic and nontransient member variables. When an object is serialized, any object references that it contains are also serialized. The implication is that any object we serialize must contain only references to other Serializable objects. We can take control by marking nonserializable members as transient or overriding the default serialization mechanisms. The transient modifier can be applied to any instance variable to indicate that its contents are not useful outside of the current context and should never be saved or serialized. See the examples: Save.java, Load.java

6 slide 10.6 Advanced Programming 2004, based on LY Stefanus’s Slides Save.java //Create a hashtable and write (serialize) it to a file called h.ser //The Hashtable object is serializable because it implements //the Serializable interface. import java.io.*; import java.util.*; public class Save { public static void main(String[] args) { Hashtable h = new Hashtable(); h.put("harry", "Harry Potter"); h.put("satu", new Integer(1)); h.put("pi", new Double(Math.PI)); try { FileOutputStream fos = new FileOutputStream("h.ser"); ObjectOutputStream oos = new ObjectOutputStream(fos); oos.writeObject(h); } catch (Exception e) { System.err.println(e); }

7 slide 10.7 Advanced Programming 2004, based on LY Stefanus’s Slides Load.java //Read (deserialize) the hashtable from the h.ser file. import java.io.*; import java.util.*; public class Load { public static void main(String[] args) { try { FileInputStream fis = new FileInputStream("h.ser"); ObjectInputStream ois = new ObjectInputStream(fis); Hashtable h = (Hashtable) ois.readObject(); System.out.println(h.get("harry"));//print: Harry Potter System.out.println(h.get("satu"));//print: 1 } catch (Exception e) { System.err.println(e); }

8 slide 10.8 Advanced Programming 2004, based on LY Stefanus’s Slides Class Versioning When an object is serialized, some information about its class must obviously be serialized with it, so that the correct class file cab be loaded when the object is deserialized. The information about the class is represented by the : Java.io.ObjectStreamClass. It contains the fully qualified name of the class and a version number. Static final long serialVersionUID = L; To compute an initial value use JDK: serialver

9 slide 10.9 Advanced Programming 2004, based on LY Stefanus’s Slides Custom Serialization Example 9-2 (page 175 JiaN2) // This example is from the book "Java in a Nutshell, Second Edition". // Written by David Flanagan. Copyright (c) 1997 O'Reilly & Associates. // You may distribute this source code for non-commercial purposes only. // You may study, modify, and use this example for any purpose, as long as // this notice is retained. Note that this example is provided "as is", // WITHOUT WARRANTY of any kind either expressed or implied. import java.io.*; /** A simple class that implements a growable array or ints, and knows * how to serialize itself as efficiently as a non-growable array. */ public class IntList implements Serializable {

10 slide Advanced Programming 2004, based on LY Stefanus’s Slides private int[] nums = new int[8]; // An array to store the numbers. private transient int size = 0; // Index of next unused element of nums[]. /** Return an element of the array */ public int elementAt(int index) throws ArrayIndexOutOfBoundsException { if (index >= size) throw new ArrayIndexOutOfBoundsException(index); else return nums[index]; } /** Add an int to the array, growing the array if necessary */ public void add(int x) { if (nums.length == size) resize(nums.length*2); // Grow array, if needed. nums[size++] = x; // Store the int in it. }

11 slide Advanced Programming 2004, based on LY Stefanus’s Slides /** An internal method to change the allocated size of the array */ protected void resize(int newsize) { int[] oldnums = nums; nums = new int[newsize]; // Create a new array. System.arraycopy(oldnums, 0, nums, 0, size); // Copy array elements. } /** Get rid of unused array elements before serializing the array */ private void writeObject(ObjectOutputStream out) throws IOException { if (nums.length > size) resize(size); // Compact the array. out.defaultWriteObject(); // Then write it out normally. } /** Compute the transient size field after deserializing the array */ private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); // Read the array normally. size = nums.length; // Restore the transient field. }

12 slide Advanced Programming 2004, based on LY Stefanus’s Slides Serialized Applets Use Appletviewer.

13 slide Advanced Programming 2004, based on LY Stefanus’s Slides JavaBeans Persistence Java 1.4 introduces a new serialization mechanism intended for use with JavaBeans. Java.io serialization works by saving the state of an object as a sequence of bytes in binary format. Java.beans persistence works by saving the state of an object or a bean as a sequence of calls to the public methods defined by the class, in XML format. Since JavaBeans persistence is based on the public API rather than on the internal state, its mechanism allows interoperability between different implementations of the same API, and is suitable for longer-term storage of serialized objects.

14 slide Advanced Programming 2004, based on LY Stefanus’s Slides JavaBeans Persistence (cont.) A bean or other objects can be serialized with java.beans.XMLEncoder and can be deserialized with java.beans.XMLDecoder. These classes write to and read from specified streams, but they are not stream classes themselves. See the examples: SaveXML.java, LoadXML.java

15 slide Advanced Programming 2004, based on LY Stefanus’s Slides SaveXML.java //Create a hashtable and write (serialize) it to a file called h.xml import java.io.*; import java.util.*; import java.beans.*; public class SaveXML { public static void main(String[] args) { Hashtable h = new Hashtable(); h.put("harry", "Harry Potter"); h.put("satu", new Integer(1)); h.put("pi", new Double(Math.PI)); try { FileOutputStream fos = new FileOutputStream("h.xml"); XMLEncoder enc = new XMLEncoder(fos); enc.writeObject(h); enc.close(); } catch (Exception e) { System.err.println(e); }

16 slide Advanced Programming 2004, based on LY Stefanus’s Slides h.xml harry Harry Potter pi satu 1

17 slide Advanced Programming 2004, based on LY Stefanus’s Slides LoadXML.java //Read (deserialize) the hashtable from the h.XML file. import java.io.*; import java.util.*; import java.beans.*; public class LoadXML { public static void main(String[] args) { try { FileInputStream fis = new FileInputStream("h.xml"); XMLDecoder dec = new XMLDecoder(fis); Hashtable h = (Hashtable) dec.readObject(); System.out.println(h.get("harry"));//print: Harry Potter System.out.println(h.get("satu"));//print: 1 } catch (Exception e) { System.err.println(e); }

18 slide Advanced Programming 2004, based on LY Stefanus’s Slides Advanced Serialization: Externalizable Only the identity of the class of an Externalizable instance is written in the serialization stream. It is the responsibility of the class to save and restore the contents of its instances, and it must coordinate with its superclasses to do so. Each object to be stored is tested for the Externalizable interface. If the object supports Externalizable, the writeExternal method is called. If the object does not support Externalizable and does implement Serializable, the object is saved using ObjectOutputStream.

19 slide Advanced Programming 2004, based on LY Stefanus’s Slides Externalizable Here's the complete definition of the Externalizable interface that extends Serializable: package java.io; public interface Externalizable extends Serializable { public void writeExternal(ObjectOutput out) throws IOException; public void readExternal(ObjectInput in) throws IOException, java.lang.ClassNotFoundException; }

20 slide Advanced Programming 2004, based on LY Stefanus’s Slides What holds for an Externalizable class? It must implement the java.io.Externalizable interface. It must implement a writeExternal method to save the state of the object. Also, it must explicitly coordinate with its supertype to save its state. It must implement a readExternal method to read the data written by the writeExternal method from the stream and restore the state of the object. It must explicitly coordinate with the supertype to restore its state. If externally defined format is being written, the writeExternal and readExternal methods are solely responsible for that format.


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