Java Essential 2

7/27/2019 Java Essential 2

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Java API Essentials(Part II)

Web Languages Course 2009

University of Trento


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Lab Objective More exercises on

Object serialization

Client-Server application through sockets Refresh basic Java concepts on

Threads

Exercise on threads The TCPPortScannerapplication


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The java.net package

The package java.net provides support for

sockets programming

The package java.net contains: Socket, URL,InetAddress, ServerSocket, etc.

Sockets + Object Serialization: combine java.netand java.io.Serializable to transport Javaobjects through the network


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Sockets

Endpointof a two-way communication link between two

programs running on the network.

An endpoint is a combination of an IP address and a

port number.

Every TCP connection can be uniquely identified by its

two endpoints.


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TCP Sockets

Connection-oriented sockets, almost always based on

TCP

java.net.ServerSocket

Implements the server side of the connection Used to listen for connection requests from clients; Should be bound to a known port to listen on, Its accept method blocks until a client requests a connection

java.net.Socket Implements the client side of the connection Used to connect to a specific port in a server machine


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Socket TCP

Constructor Create

Socket(String host, int

port)

a stream socket and connects it to thespecifiedport number on the named

host.

Socket(InetAddress address,

int port)

a stream socket and connects it to thespecifiedport number at the

specified IP address

ServerSocket(int port)

a server socket, bound to the specified

port .

ServerSocket(int port, int

backlog )

a server socket and binds it to thespecified localport number, with the

specifiedbacklog (maximum queue

length for incoming connection)


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Socket Timeout

If data are not available (e.g., the host is not reachable),

the reading methods remain blocked

To solve the problem: use the setSoTimeout()

method

Socket s = new Socket();

s.setSoTimeout(1000); //time out in millisecond

When a timeout is set, all the following operations throwthe exception InterruptedIOException


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try {

//Create connection to the server "localhost" on port 8189

s = new Socket("localhost",8189);

//Set streams to read from and write to a socket

s_in = new BufferedReader(new

InputStreamReader(s.getInputStream()));

s_out = new PrintWriter(s.getOutputStream(),true);

System.out.println(s_in.readLine());

//Set input stream for reading from console

console = new BufferedReader(new InputStreamReader(System.in));

String user_input; String received;

while(true){user_input = console.readLine();

s_out.println(user_input);

received = s_in.readLine();

System.out.println(received);

}

EchoClient


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try {

ServerSocket s = new ServerSocket (8189);

//Wait for the client

Socket client = s.accept();

System.out.println(Connection requested from host: " +incoming.getInetAddress().getHostName() +

"\nto port: " + incoming.getLocalPort() +

"\nfrom port: " + incoming.getPort() +

"\nwith IP: " +

incoming.getInetAddress().getHostAddress());

BufferedReader in = new BufferedReader (new InputStreamReader

(incoming.getInputStream ()));

PrintWriter out = new PrintWriter (incoming.getOutputStream

(), true);

out.println ("Hello! Enter BYE to exit.");

EchoServer


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boolean go = true;

String line;

while (go){

line = in.readLine ();

out.println ("Echo: " + line);if (line.trim().equals("BYE")){

go = false;

}

}

incoming.close ()

} catch (IOException e) {System.out.println ("Error. " + e);

}

EchoServer


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Exercises (15 min.)

1. Test EchoServer.java and EchoClient.java:

use the client EchoClient.java to invoke the service

provided by EchoServer.java

2. Write a client-server application so that when the client

connect to the server, the server replies by sending a

Date object to the client (Object Serialization throughsockets)


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TCP client examplemakes connection to server, sends request and receives reply

import java.net.*;

import java.io.*;

public class TCPClient {

public static void main (String args[]) {

// arguments supply message and hostname of destination

Socket s = null;

try{

int serverPort = 7896;

s = new Socket(args[1], serverPort);ObjectInputStream in = new ObjectInputStream( s.getInputStream());

ObjectOutputStream out = new ObjectOutputStream( s.getOutputStream());

out.writeUTF(args[0]);

String data = in.readUTF();

System.out.println("Received: + data) ;

}catch (UnknownHostException e){

System.out.println("Sock:"+e.getMessage());

}catch (EOFException e){System.out.println("EOF:"+e.getMessage());

}catch (IOException e){System.out.println("IO:"+e.getMessage());}

}finally {if(s!=null) try {s.close();}catch (IOException e)

{System.out.println("close:"+e.getMessage());}}

} }

ErrorHandling

Or with timeout:

s = new Socket();s.connect(new InetSocketAddress(host, port),

timeout);

blocking

(timeout)


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Exercise (15 min.)

3. Write a client-server application so that: The client process (StringClient.java) :

- Creates an ArrayList object and fill it with 3-5 String object- Displays the content of the object

- Sends the object to the server

- Receives an object from the server

- Displays the content of the object received

The server process (StringServer.java):- Receives an ArrayList object from the client- Reverts the strings contained in the object received

- Creates a new ArrayList object filled with the reversed strings

- Sends the object created to the client


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Java Threads

The package java.lang.*


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Threads

A thread is a lightweight process a single sequential

flow of execution within a program

Threads make possible the implementation of programs

that seem to perform multiple tasks at the same time

(e.g. multi-threaded Web servers)

A new way to think about programming


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How to create Java Threads

There are two ways to create a Java thread:

1. Extend the java.lang.Threadclass

2. Implement the java.lang.Runnable

interface


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Extending the Thread class

In order to create a new thread we may subclassjava.lang.Threadand customize what the thread

does by overriding its empty run method.

The run method is where the action of the thread

takes place.

The execution of a thread starts by calling thestart method.


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Example I 1

public class MyThread extends Thread{

private String name, msg;

public MyThread(String name, String msg) {

this.name = name;

this.msg = msg;

}

public void run() {

System.out.println(name + " starts its execution");

for (int i = 0; i < 5; i++) {

System.out.println(name + " says: " + msg);

try {Thread.sleep(5000);

} catch (InterruptedException ie) {}

}

System.out.println(name + " finished execution");

}

}


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Example I 2

public class Test1 {

public static void main(String[] args) {

MyThread mt1 = new MyThread("thread1", "ping");

MyThread mt2 = new MyThread("thread2", "pong");

mt1.start();

mt2.start();

}

}

The threads run inparallel


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Example I 3

Typical output of the previous example:

thread1 starts its execution

thread1 says: ping

thread2 starts its execution

thread2 says: pong

thread1 says: ping

thread2 says: pong

thread1 says: ping

thread2 says: pong

thread1 says: pingthread2 says: pong

thread1 says: ping

thread2 says: pong

thread1 finished execution



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Implementing the Runnable

interface

In order to create a new thread we may also providea class that implements the java.lang.Runnable

interface

Preferred way in case our class has to subclass

some other class

The threads logic is included inside the run methodof the runnable object


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Example II 1

public class MyClass implements Runnable {

private String name;

private A sharedObj;

public MyClass(String name, A sharedObj) {

this.name = name; this.sharedObj = sharedObj;

}

public void run() {

System.out.println(name + " starts execution");

for (int i = 0; i < 5; i++) {

System.out.println(name + " says: " + sharedObj.getValue());

try{

Thread.sleep(5000);} catch (InterruptedException ie) {}

}

System.out.println(name + " finished execution");

}

}


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Example II 2

public class A {

private String value;

public A(String value) { this.value = value; }

public String getValue() {

return value;

}

}

public class Test2 {

public static void main(String[] args) {

A sharedObj = new A("some value");

Thread mt1 = new Thread(new MyClass("thread1", sharedObj));

Thread mt2 = new Thread(new MyClass("thread2", sharedObj));mt1.start(); mt2.start();

}

}

Shared variable


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Example II 3

Typical output of the previous example:

thread1 starts execution

thread1 says: some value

thread2 starts execution













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Supporting Multiple Connection 1makes a connection for each client and then echoes the clients

request

import java.net.*;

import java.io.*;

public class TCPServer {

public static void main (String args[]) {

try{

int serverPort = 7896;

ServerSocket listenSocket = new ServerSocket(serverPort);

while(true) {

Socket clientSocket = listenSocket.accept();

(new Thread(new Connection(clientSocket))).start();

}

}catch(IOException e){S

System.out.println("Listen :"+e.getMessage());

}

}

}

Or:

Connection c = new Connection(clientSocket);

c.start();


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public class Connection implements Runnable{

ObjectInputStream in;

ObjectOutputStream out;

Socket clientSocket;

public Connection (Socket s) {

try {

clientSocket = s;in = new ObjectInputStream( s.getInputStream());

out = new ObjectOutputStream( s.getOutputStream());

} catch(IOException e)

{System.out.println("Connection: + e.getMessage());}

}

public void run(){

try {// an echo serverString data = in.readObject();

out.writeObject(data);

}catch(IOException e){

System.out.println("IO:"+e.getMessage());

}

Supporting Multiple Connection 2makes a connection for each client and then echoes the clients

request


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Exercises (15 min.)

4. Modify the server code of the previous exercises(EchoServer.java, DateServer,

RevertStringServer) so that it can accept multipleconnections (a thread is started once a connection isopened)


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Synchronization of Threads 1

In many cases concurrently running threads share data

and must consider the state and activities of other

threads

If two threads can both execute a method that modifiesthe state of an object then the method should bedeclared to be synchronized, allowing only one

thread to execute the method at a time.

If a class has at least one synchronized methods, each

instance of it has a monitor. A monitor is an object that

can block threads and notify them when it is available.


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Example:

public synchronized void updateRecord() {

// critical code goes here

}

Only one thread may be inside the body of this function.

A second call will be blocked until the first call returns orwait() is called inside the synchronized method.



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The Thread class has a method, join(), which

allows an object to wait until the thread terminates

public void myMethod() {

// Do some work here...

//Can't proceed until another thread is done:otherThread.join();

//Continue work...

}



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Summary Exercise

TCP PortScanner


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Summary Exercise:

TCPPortScanner

Realize an application which :

Scans all ports (from numberX to numberY) of a specific host, withonly one thread of execution

Scans the same ports in parallel, by using N threads

Stores log info into a log file (or console)

Stores the scan results into a file

Check and prints the execution time in both cases (single thread andparallel threads)

Assumptions:

Scanning a portmeans checking if the given port is open or not to a

socket connection; Y is greater than X

N is minor or equal toY-X;

N, X e Y can be specified as input parameters


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TCPPortScanner :

Expected Output----- Port Scanning Report -----

Date: Sun Feb 24 21:47:20 CET 2008

Scanned Host: localhost

From port: 1

To port: 10

Execution Time (1 thread): 7 sec

Execution Time (3 threads): 1 sec

Port State

1 OPEN

2 CLOSED

3 CLOSED

4 CLOSED5 OPEN

6 CLOSED

7 CLOSED

8 OPEN

9 CLOSED

10 CLOSED


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TCPPortScanner :

Log InfoThread-0 SCANNING PORT: 2

Thread-0 SCANNING PORT: 3


port per Threads: 3

left Ports: 1

Thread: 0 from 1 to 3



.










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http://java.sun.com/docs/books/tutorial/essential/con

currency/

References
http://java.sun.com/docs/books/tutorial/essential/concurrency/http://java.sun.com/docs/books/tutorial/essential/concurrency/http://java.sun.com/docs/books/tutorial/essential/concurrency/http://java.sun.com/docs/books/tutorial/essential/concurrency/http://www.mindview.net/Books/TIJ/

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