Download Java Essential II

Java API Essentials
(Part II)
Web Languages Course 2009
University of Trento
Lab Objective

More exercises on

Refresh basic Java concepts on

Exercise on threads
• Object serialization
• Client-Server application through sockets
• Threads
• The TCPPortScanner application
2
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.net
and java.io.Serializable to transport Java
objects through the network
3
Sockets

Endpoint of 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.
4
TCP Sockets

Connection-oriented sockets, almost always based on
TCP
java.net.ServerSocket

java.net.Socket

• 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
• Implements the client side of the connection
• Used to connect to a specific port in a server machine
5
Socket TCP
Constructor
Create
Socket(String host, int
port)
a stream socket and connects it to the
specified port number on the named
host.
Socket(InetAddress address,
int port)
a stream socket and connects it to the
specified port 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 the
specified local port number, with the
specified backlog (maximum queue
length for incoming connection)
6
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 throw
the exception InterruptedIOException
7
…
EchoClient
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);
}
…
8
EchoServer
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.");
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EchoServer
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);
}
10
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 through
sockets)
11
TCP client example
makes connection to server, sends request and receives reply
Or with timeout:
s = new Socket();
s.connect(new InetSocketAddress(host, port),
timeout);
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();
blocking
(timeout)
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());}}
Error
Handling
} }
12
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
13
Java Threads
The package java.lang.*
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
15
How to create Java Threads

There are two ways to create a Java thread:
1. Extend the java.lang.Thread class
2. Implement the java.lang.Runnable
interface
16
Extending the Thread class

In order to create a new thread we may subclass
java.lang.Thread and 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 the
start method.
17
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");
}
}
18
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 in parallel
}
}
19
Example I

3
Typical output of the previous example:
thread1
thread1
thread2
thread2
thread1
thread2
thread1
thread2
thread1
thread2
thread1
thread2
thread1
thread2
starts its execution
says: ping
starts its execution
says: pong
says: ping
says: pong
says: ping
says: pong
says: ping
says: pong
says: ping
says: pong
finished execution
finished execution
20
Implementing the Runnable
interface

In order to create a new thread we may also provide
a class that implements the java.lang.Runnable
interface

Preferred way in case our class has to subclass
some other class

The thread’s logic is included inside the run method
of the runnable object
21
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");
}
}
22
Example II
2
public class A {
private String value;
public A(String value) { this.value = value; }
public String getValue() {
return value;
}
Shared variable
}
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();
}
}
23
Example II

3
Typical output of the previous example:
thread1
thread1
thread2
thread2
thread1
thread2
thread1
thread2
thread1
thread2
thread1
thread2
thread1
thread2
starts execution
says: some value
starts execution
says: some value
says: some value
says: some value
says: some value
says: some value
says: some value
says: some value
says: some value
says: some value
finished execution
finished execution
24
Supporting Multiple Connection
1
makes a connection for each client and then echoes the client’s
request
Or:
Connection c = new Connection(clientSocket);
import java.net.*;
c.start();
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());
}
}
}
25
Supporting Multiple Connection
2
makes a connection for each client and then echoes the client’s
request
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 server
String data = in.readObject();
out.writeObject(data);
}catch(IOException e){
System.out.println("IO:"+e.getMessage());
}
26
Exercises (15 min.)
4.
Modify the server code of the previous exercises
(EchoServer.java, DateServer,
RevertStringServer) so that it can accept multiple
connections (a thread is started once a connection is
opened)
27
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 modifies
the state of an object then the method should be
declared 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.
28
Synchronization of Threads
2
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 or
wait() is called inside the synchronized method.
29
Synchronization of Threads

3
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...
}
30
Summary Exercise
TCP PortScanner
Summary Exercise:
TCPPortScanner
Realize an application which :

Scans all ports (from number X to number Y) of a specific host, with
only 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 and
parallel threads)
Assumptions:
 “Scanning a port” means checking if the given port is open or not to a
socket connection;
 Y is greater than X
 N is minor or equal to Y-X;
 N, X e Y can be specified as input parameters
32
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
1
2
3
4
5
6
7
8
9
10
State
OPEN
CLOSED
CLOSED
CLOSED
OPEN
CLOSED
CLOSED
OPEN
CLOSED
CLOSED
33
TCPPortScanner :
Log Info
…
Thread-0 SCANNING PORT:
Thread-0 SCANNING PORT:
…
Thread-0 SCANNING PORT:
port per Threads: 3
left Ports: 1
Thread: 0 from 1 to 3
Thread: 1 from 4 to 6
Thread: 2 from 7 to 10
….
Thread-3 SCANNING PORT:
Thread-3 SCANNING PORT:
Thread-1 SCANNING PORT:
Thread-2 SCANNING PORT:
Thread-3 SCANNING PORT:
Thread-1 SCANNING PORT:
Thread-2 SCANNING PORT:
Thread-3 SCANNING PORT:
2
3
10
7
8
2
5
9
3
6
10
34
References

http://java.sun.com/docs/books/tutorial/essential/con
currency/
35