JAVA: Serialization

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Web Design & Development CS506

Lesson 22

Serialization

What?

. You want to send an object to a stream.

Motivation

A lot of code involves boring conversion from a file to memory

As you might recall that AddressBook program reads data from file and then parses it

This is a common problem

Revisiting AddressBook

We read record from a text file named persons.txt. The person record was present in the file in the

following format.

persons.txt

The code that was used to construct Person objects after reading information from the file is given below.

Here only the part of code is shown, for complete listing, see AddressBook code in your earlier handout.

FileReader fr = new FileReader("persons.txt");BufferedReader br = new

BufferedReader(fr);

String line = br.readLine();

while ( line != null ) {

tokens = line.split(",");

name = tokens[0];

add = tokens[1];

ph = tokens[2];

PersonInfo p = new PersonInfo(name, add, ph); // you can add p into arraylist, if

needed line = br.readLine();

}

As you have seen a lot of parsing code is required for converting a line into PersonInfo objects.

Serialization mechanism eases developer's life by achieving all above in a very simple way.

Serialization in Java

Java provides an extensive support for serialization

Object knows how to read or write themselves to streams

Problem:

As you know, objects get created on heap and have some values therefore Objects have some state

in memory

You need to save and restore that state.

The good news is that java serialization takes care of it automatically

Serializable Interface

By implementing this interface a class declares that it is willing to be read/written by automatic

serialization machinery

Found in java.iopackage

Tagging interface has no methods and serves only to identify the semantics of being serializable

Automatic Writing

System knows how to recursively write out the state of an object to stream

If an object has the reference of another object, the java serialization mechanism takes care of it

and writes it too.

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Automatic Reading

System knows how to read the data from Stream and re-create object in memory

The recreated object is of type "Object" therefore Down-casting is required to convert it into actual

type.

Serialization: How it works?

To write an object of PersonInfo, ObejctOutputStream and its method writeObject( ) will be used

PersonInfo p = new PersonInfo( );

ObejctOutputStream out;

// writing PersonInfo's object p

out.writeObject(p);

To read that object back, ObejctInputStream and its method readObject()will be used

ObejctInputStream in;

// reading PersonInfo's object. Remember type casting// is required

PersonInfo obj = (PersonInfo)in.readObject( );

Example Code 22.1: Reading / Writing PersonInfo objects

We want to send PersonInfo object to stream. You have already seen this class number of times before.

Here it will also implement serializable interface.

PersonInfo.java

import javax.swing.*;

import java.io.*

class PersonInfo implements Serializable{

String name;

String address;

String phoneNum;

//parameterized constructorpublic PresonInfo(String n, String a, String p) {

name = n;

address = a;

phoneNm = p;

}

//method for displaying person record on GUIpublic void printPersonInfo( ) {

JOptionPane.showMessageDialog(null ,

"name: " + name + "address:" +address +

"phone no:" + phoneNum);

}

} // end class

WriteEx.java

The following class will serialize PersonInfo object to a file

import java.io*;

public class WriteEx{

public static void main(String args[ ]){

PersonInfo pWrite =new PersonInfo("ali", "defence", "9201211");

try {

// attaching FileOutput stream with "ali.dat"

FileOutputStream fos =new FileOutputStream("ali.dat");

// attaching ObjectOutput stream over FileOutput// stream

ObjectOutputStream out =new ObjectOutputStream(fos);

//serialization

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// writing object to `ali.dat'

out.writeObject(pWrite);

// closing streams

out.close();

fos.close();

} catch (Exception ex){

System.out.println(ex)

}

} // end class

ReadEx.java

The following class will read serialized object of PersonInfo from file i.e "ali.data"

import java.io*; public class ReadEx{ public

static void main(String args[ ]){ try { //

attaching FileInput stream with "ali.dat"

FileInputStream fin = new FileInputStream("ali.dat");

// attaching FileInput stream over ObjectInput stream

ObjectInputStream in = new ObjectInputStream(fis);

//de-serialization

// reading object from `ali.dat'

PersonInfo pRead = (PersoInfo)in.ReadObject( );

// calling printPersonInfo method to confirm that// object contains same set of values

before// serializatoion

pRead.printPersonInfo();

// closing streams

in.close();

fis.close();

} catch (Exception ex){

System.out.println(ex)

}

} // end class

Compile & Execute

After compilation, first run the WriteEx.java file and visit the "ali.dat" file. Then run ReadEx.java from

different command or same command prompt.

Object Serialization & Network

. You can read / write to a network using sockets

. All you need to do is attach your stream with socket rather than file

. The class version should be same on both sides (client & network) of the network

Example Code 22.2: Sending/Reading Objects to/from Network

We are going to use same PersonInfo class listed in example code 22.1. An object of PersonInfo class will

be sent by client on network using sockets and then be read by server from network.

Sending Objects over Network

The following class ClientWriteNetEx.java will send an object on network

import java.net.*;import java.io.*;import

javax.swing.*;

public class ClientWriteNetEx{ public static void

main(String args[]){ try { PersonInfo p = new

PersonInfo("ali", "defence", "9201211"); // create a

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communication socket

Socket s = new Socket("localhost", 2222);

// Get I/O streams

OutputStream is = s.getOutputStream();

// attaching ObjectOutput stream over Input stream

ObjectOutputStream oos= new ObjectOutputStream(is);

// writing object to network

oos.write(p);

// closing communication socket

s.close();

}catch(Exception ex){

System.out.println(ex);

}

} }// end class

Reading Objects over Network

The following class ServerReadNetEx.java will read an object of PersonInfo sent by client.

import java.net.*;import java.io.*;import

javax.swing.*;

public class ServerReadNetEx{

public static void main(String rgs[])

{

try

{

// create a server socket

ServerSocket ss = new ServerSocket(2222);

System.out.println("Server started...");

/* Loop back to the accept method of the serversocket and wait for a new connection request.

Soserver will continuously listen for requests

while(true) {

// wait for incoming connection

Socket s = ss.accept();

System.out.println("connection request recieved");

// Get I/O streams

InputStream is = s.getInputStream();

// attaching ObjectOutput stream over Input stream

ObjectInputStream ois = new ObjectInputStream(is);

// read PersonInfo object from network

PersonInfo p = (PersonInfo)ois.read( );

p.printPersonInfo();

// closing communication socket

s.close();

} // end while

}catch(Exception ex){ System.out.println(ex); } } } // end class

Compile & Execute

After compiling both files, run ServerReadNetEx.java first, from the command prompt window. Open

another command prompt window and run ClientWriteNetEx.javafrom it.

The ClientWriteNetEx.java will send an Object of PersonInfo to ServerReadNetEx.java that displays that

object values in dialog box after reading it from network.

Preventing Serialization

Often there is no need to serialize sockets, streams & DB connections etc because they do no

represent the state of object, rather connections to external resources

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To do so, transient keyword is used to mark a field that should not be serialized

So we can mark them as,

transient Socket s;

transient OutputStream os;

transient Connecction con;

Transient fields are returned as nullon reading

Example Code 22 . 3: transient

Assume that we do not want to serialize phoneNum attribute of PersonInfo class, this can be done as shown

below

PersonInfo.java

import javax.swing.*;

import java.io.*

class PersonInfo implements Serializable{

String name;

String address;

transient String phoneNum;

public PresonInfo(String n, String a, String p) { name = n;address = a;phoneNm = p;

}

public void printPersonInfo( ) {

JOptionPane.showMessageDialog(null , "name: " + name + "address:" +address +

"phone no:" + phoneNum);

}

} // end class

References

Entire material for this handout is taken from the book JAVA A Lab Course by Umair Javed. This

material is available just for the use of VU students of the course Web Design and Development and not for

any other commercial purpose without the consent of author.

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