java socket programming - skkumonet.skku.edu/.../2019/03/java_socket_programming.pdf ·...

Copyright 2000-2019 Networking Laboratory

Java socket programming

Sungkyunkwan University

Hyunseung Choo

[email protected]

mailto:[email protected]

Computer Networks

Recap

To connect to other machine we need a socket connection. A socket

connection means the two machines have information about each

other’s network location (IP Address) and port

To communicate over a socket connection, streams are used to both

input and output the data.

Networking Laboratory 2/55

Computer Networks

Java Sockets Programming

The package java.net provides support for sockets programming (and

more).

Typically you import everything defined in this package with:

import java.net.*;


Computer Networks

Classes

InetAddress

Socket

ServerSocket

DatagramSocket

DatagramPacket


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InetAddress

Static methods you can use to create new InetAddress objects.

getByName(String host)

getAllByName(String host)

getLocalHost()

InetAddress x = InetAddress.getByName(“skku.edu”);

❖ Throws UnknownHostException


Computer Networks

InetAddress classSample code

Uses InetAddress class to lookup hostnames found on command line.


> java Lookup skku.edu www.yahoo.com

skku.edu:115.145.128.224

www.yahoo.com:106.10.250.11

http://www.yahoo.com/

Computer Networks

InetAddress classSample code

Try the following code:


try {

InetAddress a = InetAddress.getByName(hostname);

System.out.println(hostname + ":" + a.getHostAddress());

} catch (UnknownHostException e) {

System.out.println("No address found for " + hostname);

Computer Networks

Java Exceptions

When executing Java code, different errors can occur: coding errors

made by the programmer, errors due to wrong input, or other

unforeseeable things.

When an error occurs, Java will normally stop and generate an error

message. The technical term for this is: Java will throw

an exception (throw an error).

The throw statement allows you to create a custom error.


Computer Networks

Java Exception


try {// Block of code to try

}catch(Exception e) {

// Block of code to handle errors}

static void checkAge(int age) { if (age < 18) {

throw new ArithmeticException("Access denied - You must be at least 18 years old.");

}else {

System.out.println("Access granted - You are old enough!"); }

}

Computer Networks

Java socket programming

Standard procedure

Open a socket

Open an input stream and output stream to the socket.

Read from and write to the stream according to the server's protocol.

Close the streams.

Close the socket.


Computer Networks

Socket class

Corresponds to active TCP sockets only

client sockets

socket returned by accept();

Passive sockets are supported by a different class:

ServerSocket

UDP sockets are supported by

DatagramSocket


Computer Networks

JAVA TCP Sockets

java.net.Socket

Implements client sockets (also called just “sockets”).

An endpoint for communication between two machines.

Constructor and Methods

Socket(String host, int port): Creates a stream socket and connects it to the

specified port number on the named host.

InputStream getInputStream()

OutputStream getOutputStream()

close()


Computer Networks

JAVA TCP Sockets

java.net.ServerSocket

Implements server sockets.

Waits for requests to come in over the network.

Performs some operation based on the request.


ServerSocket(int port)

Socket Accept(): Listens for a connection to be made to this socket and accepts

it. This method blocks until a connection is made.


Computer Networks

Sockets


Figure taken from Java socket programming, Dave Hollinger and Joonbok Lee

Client socket, welcoming socket (passive) and connection socket (active)

Computer Networks

Socket Constructors

Constructor creates a TCP connection to a named TCP server

Several constructors:


Socket(InetAddress server, int port);

Socket(InetAddress server, int port,InetAddress local, int localport);

Socket(String hostname, int port);

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Socket Methods

There are a lot of methods, for example:


void close();InetAddress getInetAddress();InetAddress getLocalAddress();InputStream getInputStream();OutputStream getOutputStream();

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Socket I/O

Socket I/O is based on the Java I/O support

in the package java.io

InputStream and OutputStream are abstract classes

common operations defined for all kinds of InputStreams,

OutputStreams…


Computer Networks

InputStream Basics

Both of the followings can throw IOException and return -1 on EOF


// reads some number of bytes and// puts in buffer array bint read(byte[] b);

// reads up to len bytesint read(byte[] b, int off, int len);

Computer Networks

OutputStream Basics

Both of the followings can throw IOException


// writes b.length bytesvoid write(byte[] b);

// writes len bytes starting // at offset offvoid write(byte[] b, int off, int len);

Computer Networks

ServerSocket Class(TCP Passive Socket)

Constructors:


ServerSocket(int port);

ServerSocket(int port, int backlog);

ServerSocket(int port, int backlog, InetAddress bindAddr);

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ServerSocket Methods

Throw: IOException, SecurityException


Socket accept();

void close();

InetAddress getInetAddress();

int getLocalPort();

Computer Networks

Socket programming with TCPExample client-server application

client reads line from standard

input (inFromUser stream) ,

sends to server via socket

(outToServer stream)

server reads line from socket

server converts line to

uppercase, sends back to

client

client reads, prints modified

line from socket

(inFromServer stream)

Networking Laboratory 22/55o

utT

oS

erv

er

to network from network

inF

rom

Se

rve

r

inF

rom

Use

r

keyboard monitor

Process

clientSocket

input

stream

input

stream

output

stream

TCP

socket

Input stream:

sequence of bytes

into processoutput stream:

sequence of bytes

out of process

Client

process

client TCP socket

Computer Networks

Client/server socket interaction: TCP


wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()

create socket,port=x, forincoming request:welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=xclientSocket =

Socket()

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

Server (running on hostid) Client

Computer Networks

TCPClient.java


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

class TCPClient { public static void main(String argv[]) throws Exception

{ String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new

InputStreamReader(System.in));Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = newDataOutputStream(clientSocket.getOutputStream());BufferedReader inFromServer = new BufferedReader(newInputStreamReader(clientSocket.getInputStream()));sentence = inFromUser.readLine();outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); }

}

Computer Networks

TCPServer.java


import java.io.*; import java.net.*; class TCPServer {

public static void main(String argv[]) throws Exception{ String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) {

Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));

DataOutputStream outToClient = newDataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();capitalizedSentence = clientSentence.toUpperCase() + '\n’;outToClient.writeBytes(capitalizedSentence);}

}}

Computer Networks

Sample Echo Server

Check out the following implementation:

https://gist.github.com/ChrisDillinger/b717a13d367e7b99582a22c0e4bcb3

88


https://gist.github.com/ChrisDillinger/b717a13d367e7b99582a22c0e4bcb388

Computer Networks

UDP Sockets

DatagramSocket class

DatagramPacket class needed to specify the payload

incoming or outgoing

There are no separate classes for client and server like TCP sockets.


Computer Networks

Socket Programming with UDP

UDP

Connectionless and unreliable service.

There isn’t an initial handshaking phase.

Doesn’t have a pipe.

Transmitted data may be received out of order, or lost

Socket Programming with UDP

No need for a welcoming socket.

No streams are attached to the sockets.

the sending hosts creates “packets” by attaching the IP destination

address and port number to each batch of bytes.

The receiving process must unravel to received packet to obtain the

packet’s information bytes.


Computer Networks

JAVA UDP Sockets

In Package java.net java.net.DatagramSocket

A socket for sending and receiving datagram packets.


• DatagramSocket(int port): Constructs a datagram socket and binds it to the specified port on the local host machine.

• void receive( DatagramPacket p)

• void send( DatagramPacket p)

• void close()


Computer Networks

DatagramSocket Constructors

All can throw SocketException or SecurityException


DatagramSocket();

DatagramSocket(int port);

DatagramSocket(int port, InetAddress a);

Computer Networks

Datagram Methods


void connect(InetAddress, int port);

void close();

void receive(DatagramPacket p);

void send(DatagramPacket p);

Computer Networks

Datagram Packet

Contain the payload

(a byte array)

Can also be used to specify the destination address

when not using connected mode UDP


Computer Networks

DatagramPacket Constructors

For receiving:

For sending:


DatagramPacket( byte[] buf, int len);

DatagramPacket( byte[] buf, int len, InetAddress a, int port);

Computer Networks

DatagramPacket methods


byte[] getData();void setData(byte[] buf);

void setAddress(InetAddress a);void setPort(int port);

InetAddress getAddress();int getPort();

Computer Networks

Example: Java client (UDP)


se

nd

Pa

cke

t

to network from network

rece

ive

Pa

cke

t

inF

rom

Use

r

keyboard monitor

Process

clientSocket

UDP

packet

input

stream

UDP

packet

UDP

socket

Output: sends packet (TCP sent “byte stream”)

Input: receives packet (TCP received “byte stream”)

Client

process

client UDP socket

Computer Networks

Client/server socket interaction: UDP


closeclientSocket

Server (running on hostid)

read reply fromclientSocket

create socket,

clientSocket = DatagramSocket()

Client

Create, address (hostid, port=x,send datagram request using clientSocket

create socket,port=x, forincoming request:serverSocket = DatagramSocket()

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port umber

Computer Networks

UDPClient.java


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

class UDPClient{

public static void main(String args[]) throws Exception{

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

DatagramSocket clientSocket = new DatagramSocket();InetAddress IPAddress = InetAddress.getByName("localhost");byte[] sendData = new byte[1024];byte[] receiveData = new byte[1024];String sentence = inFromUser.readLine();sendData = sentence.getBytes();DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);clientSocket.send(sendPacket);DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);clientSocket.receive(receivePacket);String modifiedSentence = new String(receivePacket.getData());System.out.println("FROM SERVER:" + modifiedSentence);clientSocket.close();

}}

Computer Networks

UDPServer.java


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

class UDPServer{

public static void main(String args[]) throws Exception{

DatagramSocket serverSocket = new DatagramSocket(9876);byte[] receiveData = new byte[1024];byte[] sendData = new byte[1024];while(true)

{DatagramPacket receivePacket = new DatagramPacket(receiveData,

receiveData.length);serverSocket.receive(receivePacket);String sentence = new String( receivePacket.getData());System.out.println("RECEIVED: " + sentence);InetAddress IPAddress = receivePacket.getAddress();int port = receivePacket.getPort();String capitalizedSentence = sentence.toUpperCase();sendData = capitalizedSentence.getBytes();DatagramPacket sendPacket =new DatagramPacket(sendData, sendData.length, IPAddress, port);serverSocket.send(sendPacket);

}}

}

Computer Networks

Java URL Class

Represents a Uniform Resource Locator

scheme (protocol)

hostname

port

path

query string


Computer Networks

URL parsing

You can use a URL object as a parser:


URL u = new URL(“http://www.skku.edu/”);

System.out.println(“Proto:” + u.getProtocol());

System.out.println(“File:” + u.getFile());

Computer Networks

URL construction

You can also build a URL by setting each part individually:


URL u = new URL(“http”, www.skku.edu,80,”/<your path here>/”);

System.out.println(“URL:” + u.toExternalForm());

System.out.println(“URL: “ + u);

Computer Networks

Retrieving URL contents

URL objects can retrieve the documents they refer to!

actually this depends on the protocol part of the URL.

HTTP is supported

File is supported (“file://c:\foo.html”)

You can get “Protocol Handlers” for other protocols.

There are a number of ways to do this:


Object getContent();InputStream openStream();URLConnection openConnection();

Computer Networks

Getting Header Information

There are methods that return information extracted from

response headers:

String getContentType();

String getContentLength();

long getLastModified();


Computer Networks

URLConnection

Represents the connection (not the URL itself).

More control than URL

can write to the connection (send POST data).

can set request headers.

Closely tied to HTTP


Computer Networks

Java socket thread

Why we need socket thread?

To allow many clients to connect to server at a particular time,

simultaneously

Whenever a client request comes, a separate thread can be assigned

for handling each request


https://www.geeksforgeeks.org/introducing-threads-socket-programming-java/

https://www.geeksforgeeks.org/introducing-threads-socket-programming-java/

Computer Networks

Java socket thread

Threading overview


Computer Networks

Java socket threadExample application

A date-time server which returns the date/time per user request

Server asks once connected:

Client enters Date/Time/Exit to get the response


What do you want?[Date | Time]..Type Exit to terminate connection.

Computer Networks

DTserver.java

Server class


public class Server {

public static void main(String[] args) throws IOException{

// server is listening on port 5056 ServerSocket ss = new ServerSocket(5056);

// running infinite loop for getting // client request while (true) {

Socket s = null;

try{

// socket object to receive incoming client requests s = ss.accept();

System.out.println("A new client is connected : " + s);

Computer Networks

DTserver.java

Server class


// obtaining input and out streams DataInputStream dis = new DataInputStream(s.getInputStream()); DataOutputStream dos = new DataOutputStream(s.getOutputStream());

System.out.println("Assigning new thread for this client");

// create a new thread object Thread t = new ClientHandler(s, dis, dos);

// Invoking the start() method t.start();

} catch (Exception e){

s.close(); e.printStackTrace();

} }

} }

Tell you what happened and

where in the code it happened

Computer Networks

DTserver.java

Client handler class


class ClientHandler extends Thread {

DateFormat fordate = new SimpleDateFormat("yyyy/MM/dd"); DateFormat fortime = new SimpleDateFormat("hh:mm:ss"); final DataInputStream dis; final DataOutputStream dos; final Socket s;

// Constructor public ClientHandler(Socket s, DataInputStream dis, DataOutputStream dos) {

this.s = s; this.dis = dis; this.dos = dos;

}

Computer Networks

DTserver.java



@Overridepublic void run() {

String received; String toreturn; while (true) {

try {

// Ask user what he wants dos.writeUTF("What do you want?[Date | Time]..\n"+

"Type Exit to terminate connection.");

// receive the answer from client received = dis.readUTF();

if(received.equals("Exit")) {

System.out.println("Client " + this.s + " sends exit..."); System.out.println("Closing this connection."); this.s.close(); System.out.println("Connection closed"); break;

}

Computer Networks

DTserver.java



// creating Date object Date date = new Date();

// write on output stream based on the // answer from the client switch (received) {

case "Date" : toreturn = fordate.format(date); dos.writeUTF(toreturn); break;

case "Time" : toreturn = fortime.format(date); dos.writeUTF(toreturn); break;

default: dos.writeUTF("Invalid input"); break;

} } catch (IOException e) {

e.printStackTrace(); }

}

Computer Networks

DTclient.java


public class Client {

public static void main(String[] args) throws IOException{

try{

Scanner scn = new Scanner(System.in);

// getting localhost ipInetAddress ip = InetAddress.getByName("localhost");

// establish the connection with server port 5056 Socket s = new Socket(ip, 5056);

// obtaining input and out streams DataInputStream dis = new DataInputStream(s.getInputStream()); DataOutputStream dos = new DataOutputStream(s.getOutputStream());

Computer Networks

DTclient.java


// the following loop performs the exchange of // information between client and client handler while (true) {

System.out.println(dis.readUTF()); String tosend = scn.nextLine(); dos.writeUTF(tosend);

// If client sends exit,close this connection // and then break from the while loop if(tosend.equals("Exit")) {

System.out.println("Closing this connection : " + s); s.close(); System.out.println("Connection closed"); break;

}

// printing date or time as requested by client String received = dis.readUTF(); System.out.println(received);

}

// closing resources scn.close(); dis.close(); dos.close();

}catch(Exception e){ e.printStackTrace();

} }

}

Computer Networks

Practical Problems

1. Capitalized Server with TCP Socket or UDP Socket

2. A simple DateTime server

This is perhaps the simplest possible server. It listens on port 59090.

When a client connects, the server sends the current datetime to the client.

The connection socket is created in a try-with-resources block so it is

automatically closed at the end of the block. Only after serving the

datetime and closing the connection will the server go back to waiting for

the next client.


java socket programming - skkumonet.skku.edu/.../2019/03/java_socket_programming.pdf ·...

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