1. Object Serialization Explanation + Example of file + network
Lec 22
3-Dec-18

2. Serialization You want to send an object to a stream Motivation
A lot of code involves boring conversion from a file to memory
AddressBook program reads data from file and then parses it
This is a common problem!
ali, defence,
usman, gulberg,
address.txt

3. Serialization Java’s answer: Serialization
Object know how to read/write themselves to streams
Problem - Objects have state in memory
Serialization is also called
Flattening, Streaming, Dehydrate (rehydrate = read), Archiving

4. Animation Object diagram with stack & heap
After serialized, passed through a stream (pipe) and reconstructed on the other side

5. Java: Automatic Serialization
Serializable Interface
By implementing this interface a class declares that it is willing to be read/written by automatic serialization machinery
Found in java.io package
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
Recursively follows references and writes out those objects too!
Automatic Reading s
System knows how to read the data from Stream and re-create object in memory
Downcasting is required

6. How it works? To write out an object of PersonInfo
PersonInfo p = new PersonInfo();
ObjectOutputStream out;
out.writeObject(p)
To read that object back in
ObjectInputStream in;
PersonInfo obj = (PersonInfo) in.readObject();
Must be of the same type
class and version issue

7. Example Code: Serialization
Reading/Writing PersonInfo objects
3-Dec-18

8. Example Code: Serialization
import javax.swing.*;
import java.io.*;
public class PersonInfo implements Serializable{
String name;
String address;
String phoneNum;
public void printPersonInfo( ){
JOptionPane.showMessageDialog( “name: ” + name + “address:” + address +
“phoneNum: ” + phoneNum); }

9. Example Code: Serialization (cont.)
import java.io*;
public class WriteEx{
public static void main(String args[ ]){
PersonInfo pWrite = new PersonInfo("ali", "defence", " ");
try {
FileOutputStream fos = new FileOutputStream("ali.dat");
ObjectOutputStream out = new ObjectOutputStream(fos);
//serialization
out.writeObject(pWrite);
out.close();
fos.close();
} catch (Exception ex){
System.out.println(ex) }

10. Example Code: Serialization (cont.)
import java.io*;
public class ReadEx{
public static void main(String args[ ]){
try {
FileInputStream fis = new FileInputStream("ali.dat");
ObjectInputStream in = new ObjectInputStream(fis);
//de - serialization
PersonInfo pRead = (PersonInfo) in.readObject( );
pRead.printPersonInfo();
in.close();
fis.close();
} catch (Exception ex){
System.out.println(ex) }

11. Object Serialization and Network
You can read/write objects to network using sockets
The class version should be same on both sides (client & Server) of the network

12. Sending Objects over Network
…………..
PersonInfo p = new PersonInfo (“ali”, “defence”, “ ”);
Socket s = new Socket(“localhost”, 4444);
OutputStream os = s.getOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(os);
oos.write(p);
…………

13. Reading Objects from Network
…………..
Socket s = ss.accept();
InputStream in = s.getInputStream();
ObjectInputStream ois = new ObjectInputStream(is);
PersonInfo p = (PersonInfo) ois.read( );
…………

14. Preventing Serailization
transient keyword is used to mark a field that should not be serialized
Often there is no need to serialize sockets, streams & DB connections etc (they do not represent the state of object, rather connections to external resources)
So, we can mark them as
public transient Socket s;
public transient OutputStream os;
public transient Connection con;
Transient fields are returned as null on reading

15. Example Code: transient
import javax.swing.*;
import java.io.*;
public class PersonInfo implements Serializable{
String name;
String address;
transient String phoneNum;
public void printPersonInfo( ){
JOptionPane.showMessageDialog( “name: ” + name + “address:” + address +
“phoneNum: ” + phoneNum); }

16. Circularity: not an issue
Serialization machinery will take circular references into account and do the right thing!
Animation

17. Multithreading
3-Dec-18

18. Concurrency Hardware VS Software
Hardware Concurrency Headings
Software Concurrency
Processes VS Threads
Three loop code in a single class

19. Hardware Concurrency trends
Multiple CPU’s
Multiple Cores on a Single Chip
Simultaneous Multi-Threading (SMT)

20. Software concurrency Processes Unix-style concurrency
The ability to run multiple applications at once
Example: word, powerpoint , game
Separate address space
Cooperate using read/write streams (pipes)
Synchronization is easy
Since there is no shared address space

21. Threads
The ability to do multiple things at once within the same application
Finer granularity of concurrency
Lightweight
Easy to create and destroy
Shared address-space
Can share memory variables directly
May require more complex synchronization logic because of shared address space

22. Advantages of threads…
Use multiple processors
Code is partitioned in order to be able to use n processors at once
This is not easy to do! But Moore’s Law may force us in this direction
Hide network/disk latency
While one thread is waiting for something, run the others
Dramatic improvements even with a single CPU
Need to efficiently block the connections that are waiting, while doing useful work with the data that has arrived
Writing good network codes relies on concurrency!
Keeping the GUI responsive
Separate worker threads from GUI thread

23. Why Concurrency is a Hard Problem
No language construct to alleviate the problem
Memory management can be solved by a garbage collector, no analog for concurrency
Counter-intuitive
Concurrency bugs are hard to spot in the code
Difficult to get into the concurrency mindset
No fixed programmer recipe either
Client may need to know the internal model to use it correctly
Hard to pass the Clueless-Client test
Concurrency bugs are random
Show up rarely, often not deterministic/reproducible easily
Rule of thumb: if something bizarre happens try and note the current state as well as possible