Bài giảng Công nghệ Java - Chương 14: Java Socket Programming - Trần Quang Diệu

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CÔNG NGHỆ JAVA
CH14. JAVA SOCKET PROGRAMMING
Quang Dieu Tran PhD.
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.*;
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Classes
InetAddress
Socket
ServerSocket
DatagramSocket
DatagramPacket
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InetAddress class
• static methods you can use to create new 
InetAddress objects.
– getByName(String host)
– getAllByName(String host)
– getLocalHost()
InetAddress x = InetAddress.getByName( 
“cse.unr.edu”);
 Throws UnknownHostException
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Sample Code: Lookup.java
• Uses InetAddress class to lookup hostnames 
found on command line.
> java Lookup cse.unr.edu www.yahoo.com
cse.unr.edu:134.197.40.9
www.yahoo.com:209.131.36.158
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try { 
InetAddress a = InetAddress.getByName(hostname);
System.out.println(hostname + ":" + 
a.getHostAddress()); 
} catch (UnknownHostException e) {
System.out.println("No address found for " + 
hostname); 
} 
es
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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
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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()
• java.net.ServerSocket
– Implements server sockets.
– Waits for requests to come in over the network.
– Performs some operation based on the request.
– Constructor and Methods
• 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.
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Sockets
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Client socket, welcoming socket (passive) and connection socket (active)
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Socket Constructors
• Constructor creates a TCP connection to a named 
TCP server.
– There are a number of 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
void close();
InetAddress getInetAddress();
InetAddress getLocalAddress();
InputStream getInputStream();
OutputStream getOutputStream();
• Lots more (setting/getting socket options, 
partial close, etc.)
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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
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InputStream Basics
// reads some number of bytes and
// puts in buffer array b
int read(byte[] b);
// reads up to len bytes
int read(byte[] b, int off, int len);
Both methods can throw IOException.
Both return –1 on EOF.
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OutputStream Basics
// writes b.length bytes
void write(byte[] b);
// writes len bytes starting 
// at offset off
void write(byte[] b, int off, int len);
Both methods can throw IOException.
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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
Socket accept();
void close();
InetAddress getInetAddress();
int getLocalPort();
throw IOException, SecurityException
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Socket programming with TCP
Example client-server app:
• 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)
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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
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Client/server socket interaction: TCP
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wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
create socket,
port=x, for
incoming request:
welcomeSocket = 
ServerSocket()
create socket,
connect to hostid, port=x
clientSocket = 
Socket()
close
connectionSocket
read reply from
clientSocket
close
clientSocket
Server (running on hostid) Client
send request using
clientSocketread request from
connectionSocket
write reply to
connectionSocket
TCP 
connection setup
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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 = 
new DataOutputStream(clientSocket.getOutputStream());
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TCPClient.java
BufferedReader inFromServer = 
new BufferedReader(new 
InputStreamReader(clientSocket.getInputStream())); 
sentence = inFromUser.readLine(); 
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence); 
clientSocket.close();
} 
} 
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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())); 
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TCPServer.java
DataOutputStream outToClient = 
new DataOutputStream(connectionSocket.getOutputStream()); 
clientSentence = inFromClient.readLine(); 
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); 
} 
} 
} 
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Sample Echo Server
TCPEchoServer.java
Simple TCP Echo server.
Based on code from:
TCP/IP Sockets in Java
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UDP Sockets
• DatagramSocket class
• DatagramPacket class needed to specify the 
payload
– incoming or outgoing
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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.
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JAVA UDP Sockets
• In Package java.net
– java.net.DatagramSocket
• A socket for sending and receiving datagram 
packets.
• Constructor and Methods
– 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()
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DatagramSocket Constructors
DatagramSocket();
DatagramSocket(int port);
DatagramSocket(int port, InetAddress a);
All can throw SocketException or 
SecurityException
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Datagram Methods
void connect(InetAddress, int port);
void close();
void receive(DatagramPacket p);
void send(DatagramPacket p);
Lots more!
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Datagram Packet
• Contain the payload
– (a byte array
• Can also be used to specify the destination 
address
– when not using connected mode UDP
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DatagramPacket Constructors
For receiving:
DatagramPacket( byte[] buf, int len);
For sending:
DatagramPacket( byte[] buf, int len
InetAddress a, int port);
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DatagramPacket methods
byte[] getData();
void setData(byte[] buf);
void setAddress(InetAddress a);
void setPort(int port);
InetAddress getAddress();
int getPort();
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Example: Java client (UDP)
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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
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Client/server socket interaction: UDP
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close
clientSocket
Server (running on hostid)
read reply from
clientSocket
create socket,
clientSocket = 
DatagramSocket()
Client
Create, address (hostid, port=x,
send datagram request 
using clientSocket
create socket,
port=x, for
incoming request:
serverSocket = 
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port umber
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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("hostname");
byte[] sendData = new byte[1024]; 
byte[] receiveData = new byte[1024]; 
String sentence = inFromUser.readLine(); 
sendData = sentence.getBytes(); 
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UDPClient.java
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(); 
} 
}
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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()); 
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UDPServer.java
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);
} 
}
} 
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Sample UDP code
UDPEchoServer.java
Simple UDP Echo server.
Test using nc as the client (netcat):
> nc –u hostname port
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Socket functional calls
• socket (): Create a socket
• bind(): bind a socket to a local IP address and port #
• listen(): passively waiting for connections
• connect(): initiating connection to another socket
• accept(): accept a new connection
• Write(): write data to a socket
• Read(): read data from a socket
• sendto(): send a datagram to another UDP socket
• recvfrom(): read a datagram from a UDP socket
• close(): close a socket (tear down the connection)
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Java URL Class
• Represents a Uniform Resource Locator
– scheme (protocol)
– hostname
– port
– path
– query string
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Parsing
• You can use a URL object as a parser:
URL u = new URL(“”);
System.out.println(“Proto:” + u.getProtocol());
System.out.println(“File:” + u.getFile());
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URL construction
• You can also build a URL by setting each part individually:
URL u = new URL(“http”, 
www.cs.unr.edu,80,”/~mgunes/”);
System.out.println(“URL:” + u.toExternalForm());
System.out.println(“URL: “ + u);
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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();
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Getting Header Information
• There are methods that return information extracted from 
response headers:
String getContentType();
String getContentLength();
long getLastModified();
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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
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