1. Strategic Consulting Group IO-1 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. File Input and Output

2. Strategic Consulting Group IO-2 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Objectives l Java File I/O l Stream classes l Readers and Writers l Random Access Files l Object Serialization

3. Strategic Consulting Group IO-3 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Introduction l Reading and writing is accomplished using streams and readers and writers l A stream can be either a source or destination of bytes and are used to implement byte oriented I/O l readers and writers implement Unicode oriented I/O l Input Output classes are defined in java.io package l We start our discussion with the examination of several stream classes. l This is followed by examples of readers and writers, object streams and random access files

4. Strategic Consulting Group IO-4 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Streams l Streams can be compared to a hose that carries data l Just as different sized of pipes can be interconnected, different types of streams can be interconnected l In general, stream classes can be thought of as node streams and filter streams l node streams connect with devices from which a byte stream is read or to which a byte stream is written to l filter streams connect with existing node streams and reorganizes the data bits for use by a program l InputStream class is responsible for reading l OutputStream class is responsible for writing l Both these streams implement byte oriented I/O

5. Strategic Consulting Group IO-5 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. InputStream methods l The input stream has an abstract method to read one byte, and 2 non-abstract methods to read many bytes m int read()-read next byte m int read(byte[] b)-read some bytes into b m int read(byte[] b, int off, int len) - reads up to len bytes and store in b starting with index = off l All of the read methods throw IOException l All of the read methods return the number of bytes read and -1 at End of Stream

6. Strategic Consulting Group IO-6 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. InputStream methods l Other available methods of input stream include m int available() - returns the number of bytes that can be read without interruption by another m void close() - closes the stream and releases resources m void mark(int readlimit) - specifies a logical mark; readlimit defines the boundary of the mark, mark is lost if readlimit is exceeded m boolean markSupported() - tests if the input stream supports mark m void reset() - reposition the stream to the mark m long skip(long n) - skip n bytes

7. Strategic Consulting Group IO-7 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. OutputStream methods l Writing to the stream is conceptually a mirror of reading l OutputStream contains an abstract write method that writes one byte to the output stream, and two non-abstract methods to write many bytes m void write(int b) - write the byte to the stream m void write(byte[] b) - write b.length bytes m void write(byte[] b, int off, int len) - write len bytes from starting from index = off l void close() - close the stream l void flush() - flushes the stream

8. Strategic Consulting Group IO-8 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Implementing a Stream I/O l Both InputStream and OutputStream are abstract classes l Therefore, implementation of these streams are done through subclasses l Example of subclasses of InputStream include m ByteArrayInputStream - Uses an internal byte array as a buffer m FileInputStream - Obtains input from a file m FilterInputStream- Obtains input from some other input stream m ObjectInputStream - Used to read previously serialized objects m PipedInputStream - Sets up a circular buffer in conjunction with a PipedOutputStream; best used with a thread m SequenceInputStream - Multiple input streams can be read in sequence

9. Strategic Consulting Group IO-9 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Implementing a Stream I/O l Examples of subclasses of OutputStream include m ByteArrayOutputStream - data is written to a byte array m FileOutputStream- data is written to a file m FilterOutputStream - Interacts with other streams to transform data from a program to a device specific stream m ObjectOutputStream - serializes an object’s data m PipedOutputStream - Couples with the PipedInputStream l Notice that most input streams have their corresponding output stream classes l Example using FileInputStream and FileOutputStreamwith and without buffer is shown next

10. Strategic Consulting Group IO-10 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. CopyFileUsingByteStream.java import java.io.*; class CopyFileUsingByteStream { public static void main(String [] args) { FileInputStream fis = null; FileOutputStream fos = null; byte inByte; try { fis = new FileInputStream(new File(args[0])); File outFile = new File(args[1]); if (outFile.exists()) { System.out.println("File "+args[1]+" already exists"); return; } else fos = new FileOutputStream(args[1]); Name of the input file Name of the output file

11. Strategic Consulting Group IO-11 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. CopyFileUsingByteStream.java inByte = (byte)fis.read(); while (inByte != -1) { fos.write(inByte); inByte = (byte) fis.read(); } } catch (FileNotFoundException e) { System.out.println ("File not found "+args[0]); } catch (IOException e) { System.out.println(e.getMessage()); } finally { try { if (fis != null) fis.close(); if (fos != null) fos.close(); } catch (IOException e) {} } A source file is copied one byte at a time

12. Strategic Consulting Group IO-12 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. FileCopy.java l Extracts from FileCopy.java is shown l This program is an exact duplicate of the CopyFileUsingByteStream.java, except for the buffered read and the use of available() method byte[] inByte; int size; // code to open file as in the last example size = fis.available(); if (size > 0) { inByte = new byte[size]; fis.read(inByte); fos.write(inByte); }

13. Strategic Consulting Group IO-13 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Data Streams l FileOutputStream and FileInputStream operate using byte oriented output and input - not very useful in many cases l Programs often use ‘binary’ data other than bytes i.e int, float l Data stream classes provide this capability l Data streams derive from Filter streams and do not interact with devices l Data stream classes are normally chained to other streams that do interact with a device l Data Streams provide methods to read and write all basic java data types

14. Strategic Consulting Group IO-14 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Data Streams l There are two data stream classes m DataOutputStream m DataInputStream l These classes are derived from FilterOutputStream and FilterInputStream classes respectively l Examples of write/read methods include writeCharreadChar writeIntreadInt writeDoublereadDouble writeBooleanreadBoolean

15. Strategic Consulting Group IO-15 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Data Streams l Other methods void flush()- flush the output stream l The general chaining process of a data stream (filter) to a node stream (FileInput) can be shown as follows Node device Node streamFilter stream Program Node device Node streamFilter stream Program

16. Strategic Consulting Group IO-16 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Data Input and Output Streams l To set up a data output stream DataOutputStream dos = new DataOutputStream( new FileOutputStream(tempFile)); m tempFile can be a File object or a String l To set up a data input stream DataInputStream dis = new DataInputStream( new FileInputStream(tempFile)); l An example is shown next; this example generates a set of random integers, writes them to a file and then reads them back

17. Strategic Consulting Group IO-17 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestDataStream.java import java.io.*; class TestDataStream { public static void main(String [] args) { DataInputStream dis = null; DataOutputStream dos = null; long writeTime=0, readTime=0; long swTime, ewTime, srTime, erTime; File tempFile = new File("temp.dat"); if (tempFile.exists()) { System.out.println("File temp.dat already exists"); System.out.println("Delete the file and rerun the program"); System.exit(0); } try { dos = new DataOutputStream(new FileOutputStream(tempFile)); swTime = System.currentTimeMillis(); Notice chaining of streams Variables used to measure read write efficiency

18. Strategic Consulting Group IO-18 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestDataStream.java for (int i =0; i < 10000; i++) { dos.writeInt((int)(Math.random()*1000)); } ewTime = System.currentTimeMillis(); writeTime = ewTime - swTime; } catch (IOException e) { System.out.println(e.getMessage()); } finally { try { if (dos != null) dos.close(); } catch (IOException e) {} } // read data try { dis = new DataInputStream(new FileInputStream(tempFile)); srTime = System.currentTimeMillis(); Chaining of streams on the input side

19. Strategic Consulting Group IO-19 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestDataStream.java for (int i = 0; i < 10000; i++) System.out.print(" "+dis.readInt()); erTime= System.currentTimeMillis(); readTime = erTime - srTime; } catch (FileNotFoundException e) { System.out.println("File not found"); } catch (IOException e) { System.out.println(e.getMessage()); } finally { System.out.println("\nWrite = "+writeTime+" Read = "+readTime); try { if (dis != null) dis.close(); } catch (IOException e) {} } Timing discussed after the next example

20. Strategic Consulting Group IO-20 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Efficiency of Data Streams l The I/O, as used in the TestDataStream.java, is still byte oriented and therefore, disk access intensive l Efficiency can be improved by buffering I/O l To set up a buffered stream, we can add a buffering layer to the data stream DataOutputStream dos = new DataOutputStream( new BufferedOutputStream( new FileOutputStream(tempFile))); l Buffered Streams are also filter classes and provide internal data buffering l Notice chaining of streams through constructors

21. Strategic Consulting Group IO-21 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Extracts from TestBufferedDataStream.java import java.io.*; class TestBufferedDataStream { public static void main(String [] args) { DataInputStream dis = null; DataOutputStream dos = null; // set up code try { dos = new DataOutputStream( new BufferedOutputStream( new FileOutputStream(tempFile))); for (int i =0; i < 10000; i++) dos.writeInt((int)(Math.random()*1000)); } // other code try { dis = new DataInputStream(new BufferedInputStream( new FileInputStream(tempFile))); for (int i = 0; i < 10000; i++) System.out.print(" "+dis.readInt()); // rest of the code } See source file for the full code

22. Strategic Consulting Group IO-22 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Efficiency of Data Streams l What kind of efficiency is gained? l You will recall that we had embedded time collection statements in the TestDataStream program l We did the same for the next program TestBufferedDataStream.java which uses a buffered stream; both programs are otherwise comparable Write Time(ms)Read Time (ms) Unbuffered9909280 Buffered507850 l Better timing for read can be computed by removing the println that embeds the readInt call

23. Strategic Consulting Group IO-23 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Text Streams l DataStreams are fine if a data file in binary encoded form is already there l It is quite common to read and write human-readable data file l Text file I/O can be done using descendants of Reader and Writer classes l InputStreamReader can be used to read a text stream l OutputStreamWriter can be used to write a text stream l As in the case of data streams, these streams need to be chained to a node stream class

24. Strategic Consulting Group IO-24 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Text Streams l An InputStreamReader can be setup with the following: InputStreamReader in = new InputStreamReader(System.in); l System.in enables a program to read data from the console InputStreamReader in = new InputStreamReader( new FileInputStream(filename)); l Text data can be read from a file, notice chaining l Text reading can also be buffered BufferedReader br = new BufferedReader( new InputStreamReader(new FileInputStream( new File(filename))));

25. Strategic Consulting Group IO-25 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Encoding Scheme l InputStreamReader and OutputStreamWriter classes can be instantiated with an optional encoding scheme InputStreamReader in = new InputStreamReader( new FileInputStream(filename), encString); m filename can be a File object or a filename string m encString specifies an encoding String m “ 8859_1”ISO Latin-1default for Windows m “Big5”Traditional Chinese m “Cp1253”Windows Greek m “Cp863”PC Canadian French

26. Strategic Consulting Group IO-26 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Reading and writing Text files l It is common to associate text oriented I/O with some file l Special Writer classes exist to write to a file in text format PrintWriter out = new PrintWriter( new FileWriter("testdata.dat")); m PrintWriter class provides a number of overloaded methods for print, println, write for text output l There is no PrintReader class, but only a FileReader class l BufferedReader class has a readLine() method to read an entire line terminated by a new line character

27. Strategic Consulting Group IO-27 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Reading Text files l Text reading from a file can be set up using File input = new File("testdata.dat"); FileReader in = new FileReader(input); BufferedReader br = new BufferedReader(in); l br.readLine(); - returns a String equal to the line terminated by \r or \n, or null at the end of stream l StringTokenizer class can be used to break the line read into tokens provided the string has some delimiter StringTokenizer stk = new StringTokenizer(s,delim); where the delim is the delimiter string

28. Strategic Consulting Group IO-28 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestBufferedReads.java public class TestBufferedReads { public static String getConsoleLine(BufferedReader br) throws IOException { return br.readLine(); } public static void writeDiskFile(PrintWriter out, String s) throws IOException { out.println(s); } public static String[] getColumns(BufferedReader br,String delim) throws NoSuchElementException, IOException { String s = br.readLine(); if ( s == null) return null; StringTokenizer stk = new StringTokenizer(s,delim); int tokenCount = stk.countTokens(); String [] words = new String[tokenCount]; for (int i =0; i < tokenCount; i++) { words[i] = stk.nextToken(); } return words; }

29. Strategic Consulting Group IO-29 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestBufferedReads.java public static void main(String [] args) { InputStreamReader in = null; BufferedReader br = null; FileReader fr = null; PrintWriter out = null; FileWriter fw = null; String inText; String [] fields; String fname = null, lname=null; double salary=0; File input = null; try { in = new InputStreamReader(System.in); br = new BufferedReader(in); fw = new FileWriter("testdata.dat"); out = new PrintWriter(fw); System.out.println("Input for setting up a data file"); System.out.println("Enter three strings: first name, last name, salary"); System.out.println("Separate fields with | or a space");

30. Strategic Consulting Group IO-30 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestBufferedReads.java System.out.println("Terminate input with 'end'"); inText = getConsoleLine(br); while (!(inText.equals("end"))) { writeDiskFile(out,inText); inText = getConsoleLine(br); } } catch (IOException e) { System.out.println(e.getClass().getName()+" "+e.getMessage()); } finally { try{ if (br != null) br.close(); if (out != null) { out.flush(); out.close(); }

31. Strategic Consulting Group IO-31 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestBufferedReads.java }catch (IOException f) { f.printStackTrace(); } try { input = new File("testdata.dat"); in = new FileReader(input); br = new BufferedReader(in); fields = getColumns(br," |"); while ( fields != null) { fname = fields[0]; lname = fields[1]; salary = Double.parseDouble(fields[2]); System.out.println("name = "+lname+", "+fname+" salary: "+salary); fields = getColumns(br," |"); }

32. Strategic Consulting Group IO-32 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestBufferedReads.java } catch (IOException e) { System.out.println(e.getClass().getName()+" "+e.getMessage()); } finally { try { if (br != null){ br.close(); } } catch (IOException f) { f.printStackTrace(); }

33. Strategic Consulting Group IO-33 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. File l Examples used in this section have liberally used files; a brief description of file class is, therefore, in order l The stream and reader/writer classes use files to read from and write to l But a program often needs to know, for example, location of the file, readability, file size, length of contents etc l A File object is the answer to many of these questions l A File object is created using one of 3 constructors File(String pathname); File(File parent, String child); File(String parent, String child);

34. Strategic Consulting Group IO-34 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. File l A number of methods exist to manipulate a File object m boolean canRead() - Can the file be read m boolean canWrite() - Can the file be written to m int compareTo(File pathname) - compare two filenames m boolean createNewFile() - Create a new file only if not there m boolean delete() - Delete the file or directory m boolean exists() - Tests if file is there m String getName() - Return the pathname of the file or directory m boolean isDirectory() - Is the object a directory m boolean isAbsolute() - Tests for absolute pathname m long length() - Length of the file in bytes m String[] list() - list of files and directories for the object; object should be a directory

35. Strategic Consulting Group IO-35 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Random Access Files l Java provides a Random Access file class that can be used to access a record directly RandomAccessFile raf = new RandomAccessFile(ra,mode); m ra - A file object or a file name m mode - usage mode: “r” or “rw” l The file pointer can be set to any location with a seek(long) l Program must provide a means of computing the offset where a record ends and the next record begins l Those interested are referred to the TestRandomFile.java available in the examples directory

36. Strategic Consulting Group IO-36 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Object Persistence l Programs sometimes need to save an Object to a permanent storage to be used by another program l Object can be stored to a file by ‘serializing’ the Object l An ObjectOutputStream is used to write Objects ObjectOutputStream os = new ObjectOutputStream( new FileOutputStream(new File(filename))); l An ObjectInputStream is used to read Objects ObjectInputStream is = new ObjectInputStream( new FileInputStream(new File(filename)));

37. Strategic Consulting Group IO-37 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Object Persistence l Basic method to write to an Object stream writeObject(Object o) writeInt(int i) - Other methods exist to write other primitives l Basic method to read from an Object stream readObject() - returns an Object readInt() - returns a primitive int; other such methods are available

38. Strategic Consulting Group IO-38 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Serializing an Object l All Objects cannot be serialized l An Object can be serialized by implementing Serializable interface public class ObjectStack implements Serializable { // } l Serializable interface does not require implementation of any special method and is considered a marker interface

39. Strategic Consulting Group IO-39 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. What is Serialized l Only data is serialized, not any method l Serialization of specific data members of an Object can be prevented by marking the data as transient public class Account implements Serializable { private transient long accountNumber; private transient long accountBalance; private String accountName; } l Seralized objects of type Account would not include accountNumber or accountBalance

40. Strategic Consulting Group IO-40 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestObjectSerialization.java import java.io.*; import java.util.*; public class TestObjectSerialization { public static void serializeObject(Object o, ObjectOutputStream os) throws IOException { os.writeObject(o); } public static Object unSerializeObject(ObjectInputStream is) throws ClassNotFoundException, IOException { return(is.readObject()); } public static void main(String[] args) { if (args.length < 1) { System.out.println("Usage java TestObjectSerialization "+ ” ” + " "); return; }

41. Strategic Consulting Group IO-41 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestObjectSerialization.java ObjectOutputStream os = null; ObjectInputStream is = null; // A number of hard coded object to be serialized int i = 3; float f = 3.4f; String s = "Java String"; Date d = new Date(); char [] c = {'A','B','C','D','E'}; ObjectStack st = new ObjectStack(); st.push(d); st.push(c); st.push(s); try { os = new ObjectOutputStream( new FileOutputStream(new File(args[0]))); serializeObject(new Integer(i),os); serializeObject(new Float(f),os);

42. Strategic Consulting Group IO-42 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestObjectSerialization.java serializeObject(s,os); serializeObject(d,os); serializeObject(c,os); serializeObject(st,os); os.writeInt(i); os.writeFloat(f); } catch (IOException e) { System.out.println("Error: "+e.getClass().getName()+" "+e.getMessage()); } finally { try { if (os != null) os.close(); } catch (IOException e1) {} }

43. Strategic Consulting Group IO-43 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestObjectSerialization.java // Unserialize, we know the order of serialization try { is = new ObjectInputStream(new FileInputStream(new File(args[0]))); System.out.println("Integer: "+unSerializeObject(is)); System.out.println("Float: "+unSerializeObject(is)); System.out.println("String: "+unSerializeObject(is)); System.out.println("Date: "+unSerializeObject(is)); System.out.println("Char array: "+new String((char[])unSerializeObject(is))); System.out.println("Object stack: "+unSerializeObject(is)); System.out.println("Primitive int: "+is.readInt()); System.out.println("Primitive float: "+is.readFloat()); } catch (ClassNotFoundException cne) { System.out.println("Error: "+cne.getClass().getName()+” ” +cne.getMessage()); } catch (IOException e) {

44. Strategic Consulting Group IO-44 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestObjectSerialization.java System.out.println("Error: "+e.getClass().getName()+" "+e.getMessage()); } finally { try { if (is != null) is.close(); } catch (IOException e1) {} } } // end TestObjectSerialization class class ObjectStack implements Serializable { private Vector v = new Vector(100,20); public void push(Object n) { v.add(n); } public String toString() { return ("ObjectStack with size = "+getHeight()); }

45. Strategic Consulting Group IO-45 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. TestObjectSerialization.java public Object pop() { Object i; int ht; while(v.isEmpty()) { try { this.wait(); } catch (InterruptedException ie) { System.out.println(ie.getClass().getName()+" "+ie.getMessage()); } ht = getHeight(); i = v.remove(ht-1); return(i); } public int getHeight() { return v.size(); }

46. Strategic Consulting Group IO-46 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Versioning Problem l Consider the following scenario m Serialization design is done at Time A and code distributed m A program runs and serializes the object at Time B which is after Time A m Between Time A and B, the object design is changed; the code to unserialize is not changed m THUS WHEN AN OBJECT IS READ BACK, THE DESIGN IS A VERSION OLD l If version mismatch occurs, the program throws an exception l How do we handle such version problem?

47. Strategic Consulting Group IO-47 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Versioning Problem l The obvious solution to versioning problem is to redistribute the byte code when versions are updated. l Redistribution does not solve the problem of reading back old serialized data l We can use the concept of version Identification l Every class is associated with a version Id l The version id for a class can be found by executing the serialver command >serialver ByteCodeFile l The command yields a very long integer i.e the serial id

48. Strategic Consulting Group IO-48 © Copyright 1999-2000 by Santosh Misra. All rights reserved. May not be reproduced in whole or in part by any means without prior written permission. Versioning Problem l The serial version id can be embedded in the class definition public class Account implements Serializable { private final static long serialVersionUID = 3745358160550433574L; // old data members // new data members } l The unserialization process uses the serialVersionUID value instead of generating a new serialized id l New data members are ignored