Transcript Rangkaian Komputer TK6383
Socket-based Client Server Application
Client-Server application architecture Choosing services –
Connectionless
atau
Connection-oriented
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Evolution of Application Architecture
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3-tier Client-server application architecture
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Internet Protocol and Network Application
Internet protocol supports: General-purpose service for reliable data transmission Mechanism for connecting hosts Network application programs: Use Internet Protocol to connect to other application Provide user-level services 4
Client-Server Model
Server Application acts as a “listener” Waits for incoming messages Executes services Returns the results/outcomes Client Application makes connection Sends messages to the server Waits for response 5
Features of a Client
An application program Becomes a client when a network service is needed It can also executes other calculation Is directly created by the users Is executed locally by at the users’ computer Initiates connection to a server Able to access various services (at a given time) Does not require a special device or operating system 6
Features of a Server
Special-purpose application for providing a network service Is executed on a remote computer (usually is centralised and shared) Waits and receives request for services from the clients Requires high-performance device and operating system 7
Transport Protocol and Client Server
Client and server exchange messages via transport protocol such as TCP or UDP Both client and server must have the same protocol stack and both interact with the transport layer 8
Transport Protocol and Client Server
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Several Services on a Server
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Identifying a Service
Every service has a unique identifier which is used by the client and server Example - TCP uses port number protocol (port_num) as identifier Server is registered under a specific port number for a service Client asks for a session with the port number for the service Every transport session contains 2 unique indentifier (IP address, port number) at the server (IP address, port number) at the client 11
Connection-oriented and Connectionless Transport Protocol
Which one should be chosen? UDP - connectionless Client builds a message Client sends the message to a server The server responds Message must be loaded into a UDP datagram TCP - connection-oriented Client makes a connection to a server Client and server exchange messages Client terminates the connection 12
UDP
UDP is a connectionless transport protocol, i.e. it doesn't guarantee either packet delivery or that packets arrive in sequential order.
With UDP, bytes of data are grouped together in discrete packets which are sent over the network.
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Packets may travel along different paths depending on the state of the network.
No two packets are guaranteed the same route. Each packet has a time-to-live (TTL) counter, which is updated when it is routed along to the next point in the network. When the timer expires, it will be discarded, and the recipient will not be notified.
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If a packet does arrive, it will always arrive intact. Packets that are corrupt or only partially delivered are discarded.
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Advantages of UDP
UDP communication can be more efficient than guaranteed-delivery data streams.
Unlike TCP streams, which establish a connection, UDP causes fewer overheads.
Real-time applications that demand up-to the-second or better performance may be candidates for UDP, as there are fewer delays due to error checking and flow control of TCP.
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UDP sockets can receive data from more than one host machine.
Some network protocols specify UDP as the transport mechanism.
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Java Support for UDP
Two classes are provided: DatagramPacket class (java.net) DatagramSocket class (java.net) 18
DatagramPacket Class
A DatagramPacket object represents a data packet intended for transmission using UDP.
It contains addressing information such as an IP address and a port.
When a DatagramPacket is read from a UDP socket, the IP address/port number of the packet represents the address/port number of the sender.
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When a DatagramPacket is used to send a UDP packet, the IP address/port represents the address/port of the recipient.
DatagramPacket IP address (java.net.InetAddr) Port address (int) Packet data (byte[]) 20
Creating a DatagramPacket
Constructor to use for creating a DatagramPacket for receiving incoming UDP packets: DatagramPacket(byte[] buffer, int length) Example: DatagramPacket packet; packet = new DatagramPacket(new byte[256], 256); 21
Constructor to use for sending a DatagramPacket to a remote machine
DatagramPacket(byte[] buffer, int length, InetAddress dest_addr, int dest_port)
Example:
DatagramPacket packet; InetAddress addr; addr = InetAddress.getByName("192.168.0.1"); packet = new DatagramPacket(new byte[128], 128,addr, 2000);
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DatagramPacket Methods
InetAddress getAddress() byte[] getData() int getLength() int getPort() void setAddress(InetAddress addr) void setData(byte[] buffer) void setLength(int length) void setPort(int port) 23
DatagramSocket Class
The DatagramSocket class provides access to a UDP socket, which allows UDP packets to be sent and received.
The same DatagramSocket can be used to receive as well as to send packets.
Read operations are blocking - i.e. the application will continue to wait until a packet arrives.
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Each DatagramSocket binds to a port on the local machine. The port number need not match the port number of the remote machine.
If the application is a UDP server, it will usually bind to a specific port number.
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Creating a DatagramSocket
Constructor to use for creating a client DatagramSocket :
DatagramSocket() throws java.net.SocketException
Example:
DatagramSocket socket; try { socket = new DatagramSocket(); } catch (SocketException exception) { … }
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Constructor to use for creating a server DatagramSocket :
DatagramSocket(int port) throws java.net.SocketException
Example:
DatagramSocket socket; try { socket = new DatagramSocket(2000); } catch (SocketException exception) { … }
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DatagramSocket Methods
void close() void connect(InetAddress r_addr, int r_port) void disconnect() InetAddress getInetAddress() int getPort() InetAddress getLocalAddress() int getLocalPort() int getReceiveBufferSize() throws java.net.SocketException
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int getSendBufferSize() throws java.net.SocketException
int getSoTimeout() throws java.net.SocketException
void receive(DatagramPacket packet) throws java.io.IOException
void send(DatagramPacket packet) throws java.io.IOException
int setReceiveBufferSize(int length) throws java.net.SocketException
int setSendBufferSize(int length) throws java.net.SocketException
void setSoTimeout(int duration) throws java.net.SocketException
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Listening for UDP Packets
Before an application can read UDP packets, it must bind a socket to a local UDP port using DatagramSocket create a DatagramPacket that will contain the data. 30
Packet Reads packets DatagramSocket DatagramPacket Translates packets Into a DatagramPacket UDP application 31
The following code illustrates the process for reading UDP packets.
DatagramPacket packet; DatagramSocket socket; packet = new DatagramPacket(new byte[256], 256); socket = new DatagramSocket(2000); boolean finished = false; while (!finished) { socket.receive(packet);
// process the packet
} socket.close(); 32
Java I/O streams are usually used to access the contents of the byte array in a DatagramPacket . See example later.
DatagramPacket IP address (java.net.InetAddr) Port address (int) Packet data (byte[]) ByteArrayInputStream DataInputStream UDP application 33
Sending UDP Packets
When sending a packet, the application must create a DatagramPacket that will contain the data. The address and port information must also be set.
When the packet is ready for transmission, the send method of DatagramSocket should be invoked.
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Binds to a UDP port DatagramSocket UDP application Send DatagramPacket using DatagramSocket Packet Constructs packet DatagramPacket 35
The following code illustrates the process for sending UDP packets.
DatagramPacket packet; DatagramSocket socket; packet = new DatagramPacket(new byte[256], 256); socket = new DatagramSocket(2000); packet.setAddress(…); packet.setPort(2000); boolean finished = false; while (!finished) {
// write data to packet buffer
socket.send(packet); … } socket.close(); 36
User Datagram Protocol Example
Run receiving application
java PacketReceiveDemo
Run sending application
java PacketSendDemo
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PacketSendDemo print(str) PrintStream DatagramPacket ByteArrayOutputStream toByteArray() send(packet) DatagramSocket 39
DatagramSocket receive(packet) DatagramPacket getData() ByteArrayInputStream PacketReceiveDemo read() 40
Building a UDP Client/Server
Run echo server
java EchoServer
Run echo client
java EchoClient
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Algorithm for Echo Server
1.
2.
3.
Create socket Create an empty packet 1.
2.
Repeat the following forever Wait for a packet Send the packet back to sender 42
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2.
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Set timeout value for socket 1.
Repeat the following ten times Create the message to be sent 2.
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Create packet containing the message as well as the destination IP and the port Send the packet through socket Wait for packet from receiver through socket or timeout 1.
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if packet received Create an input stream to access data in the packet Use the input stream to read the data and then display it on the screen.
Sleep for a second 43
Overcoming UDP Limitations
UDP limitations: Lack of Guaranteed Delivery Lack of Guaranteed Packet Sequencing Lack of Flow Control 44
Lack of Guaranteed Delivery
Packets sent via UDP may become lost in transit.
UDP packets can also become damaged or lost.
For some applications, the loss of individual packets may not have a noticeable effect (e.g. video streams).
For other applications, loss of packets is not acceptable (e.g. file transfers).
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If guaranteed delivery is required, avoid packet-based communication, and use a more suitable transport mechanism (e.g. TCP).
send acknowledgement to sender after receiving packets.
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Lack of Guaranteed Packet Sequencing
Applications that require sequential access to data should include a sequence number in the contents of a datagram packet.
This enables detection of duplicate packets and also missing packets.
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Lack of Flow Control
The technique of flow control is important to avoid flooding a system with more data than it can handle due to limited bandwidth.
One technique of flow control is to limit the number of unacknowledged packets. E.g.: increase control when number of acknowledgement packets received is much less than the number of packets sent.
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Transmission Control Protocol
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Overview
Unlike UDP which is concerned with the transmission of packets of data, TCP establishes a "virtual connection" between two machines through which streams of data may be sent.
TCP guarantees delivery and order, providing a reliable byte communication stream between client and server that supports two-way communication.
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Establish a virtual connection Transmit data back and forth Terminate the connection 51
TCP uses IP (Internet Protocol) to establish the connection between machines.
This connection provides an interface that allows streams of bytes to be sent and received, and transparently converts the data into IP datagram packets.
The virtual connection between two machines is represented by a socket.
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TCP sockets are different from UDP sockets: TCP sockets are connected to a single machine.
UDP sockets only send and receive packets of data. TCP allows tansmission of data through byte streams. They are converted into datagram packets for transmission over the network without programmer intervention.
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Advantages of TCP over UDP
Automatic Error Control Data transmission is more dependable. Delivery of data is guaranteed - lost data packets are retransmitted.
By means of a timer, TCP retransmits a packet if an acknowledgement is not received from the recipient within a specified amount of time.
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Reliability As packets are delivered by IP, they will frequently arrive out of order. However, each packet contains a sequence number. Using this sequence number and queuing out-of-order packets, TCP is able to pass arriving packets to the application in the correct sequence.
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Ease of Use Network programmers will find programming communication via TCP sockets far simpler than via datagram packets. This is because data sent and received can be treated as a continuous stream (like I/O streams). The data need not be packaged into discrete units like UDP.
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Communication Using Ports
Both UDP and TCP uses the concept of a communications port, which distinguishes one application from another.
When a TCP socket establishes a connection to another machine, the following information will be required: the IP address of the remote machine the port number 57
Like ports in UDP, ports in TCP are also represented by a number in the range 1 65535.
Ports below 1024 are restricted to use by well-known services. For example, Telnet (port 23) SMTP (port 25) HTTP (port 80) POP3 (port 110) 58
Socket Operations
TCP sockets can perform a variety of operations: Establish a connection to a remote host Send data to a remote host Receive data from a remote host Close a connection 59
There is a special type of socket that provides a service that will bind to a specific port number. Normally used only in servers, this socket can perform the following operations: Bind to a local port Accept incoming connections from remote hosts Unbind from a local port 60
TCP and the Client/Server Paradigm
In network programming, applications that use sockets are divided into clients and servers.
A client is software that initiates a connection and sends requests.
A server is software that listens for connections and processes requests.
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Note that in the context of UDP programming, no actual connection is established. UDP applications may both initiate and receive requests on the same socket.
In the client/server paradigm, when there is a connection between two applications, one must be a client and the other must be a server. 62
Network Clients
Network clients initiate connections and control network transactions.
The server fulfills the requests of the client but not the other way round.
The network client speaks to the server using a network protocol. E.g an HTTP client communicates with an HTTP server using HTTP.
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Port numbers are used to enable clients to locate servers. E.g. a web server uses port 80. 64
Network Servers
The role of the network server is to bind to a specific port and to listen for new connections.
Unlike the client, the server must run continually in the hope that some client will want its services.
The server runs indefinitely. Normally, it is automatically started when the host computer of the server is started.
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Some servers can handle only one connection at a time, while others can handle many connections concurrently, through the use of threads.
Some protocols (e.g. HTTP/1.0) normally allow only one request per connection. Others, like POP3, support a sequence of requests.
Servers answer the client request by sending either a response or an error message. 66
TCP Sockets and Java
Java provide the following classes for TCP sockets: java.net.Socket
java.net.ServerSocket
The Socket class should be used when writing client software.
The ServerSocket class should be used when writing server software.
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Socket Class
Socket objects represent client sockets, and is a communication channel between two TCP communications ports belonging to one or two machines.
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There are several constructors for the Socket class. The easiest way to create a socket is shown below: Socket mySocket; try { mySocket = new Socket(" www.aol.com
", 80); } catch (Exception e) { … } 69
Some of the other constructors: Socket(InetAddress addr, int port) Throws java.io.IOException, java.lang.SecurityException
Socket(InetAddress rAddr, int rPort, InetAddress lAddr, int lPort) Throws java.io.IOException, java.lang.SecurityException
Socket(String rHost, int rPort, InetAddress lAddr, int lPort) Throws java.net.UnknownHostException, java.io.IOException, java.lang.SecurityException
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Using a Socket
Refer section 6.4.2 (pg 150) for a description of some of the methods of the Socket class.
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Reading from and Writing to TCP Sockets
In Java, once a socket is created, it is connected and ready to read/write by using the socket's input and output streams. Use the methods getInputStream () and getOutputStream () to access those streams.
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Example:
Socket socket; InputStreamReader isr; BufferedReader br; PrintStream ps; try { socket =
new Socket("www.aol.com",80)
; isr = new InputStreamReader(
socket.getInputStream()
); br = new BufferedReader(isr); ps = new PrintStream(
socket.getOutputStream()
); } catch (Exception e) { … } 73
SO_TIMEOUT Socket Option
Socket options are settings that modify how sockets work. They can affect the performance of applications.
SO_TIMEOUT is the most useful socket option. It allows a timer to be started when a read request is made on a socket. When no data arrives in time and the timer expires, a java.io.InterruptedIOException
is thrown which can be caught to check for a timeout.
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Use the
setSoTimeout
() method to set the duration of the timer. Example:
socket.setSoTimeout(5000);
The
getSoTimeout
() method can be used to get the duration of the timer. A value of zero means that timeouts are disabled and read operations will block indefinitely.
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Creating a TCP Server
The given example is a TCP server which returns the current day and time to the client.
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SocketServer Class
The server socket is a special type of socket used to provide TCP services.
Client sockets bind to any free port on the local machine, and connect to a specific port and host.
The difference with server sockets is that they bind to a specific port on the local machine so that remote clients may locate a service.
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Client socket connections will connect to only one machine, whereas server sockets are capable of fulfilling the requests of multiple clients.
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Creating a ServerSocket
Once a server socket is created, it will be bound to a local port and ready to accept incoming connections.
When clients attempt to connect, they are placed into a queue.
When this queue is full, further clients are refused.
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There are several constructors for the ServerSocket class. The easiest way to create a socket is shown below: ServerSocket mySocket; try { mySocket = new ServerSocket(80); } catch (Exception e) { … } 80
Some of the other constructors: ServerSocket(int port) Throws java.io.IOException, java.lang.SecurityException
If port is 0, then any free port will be used.
By default, the queue size is set to 50.
ServerSocket(int port, int maxClients) Throws java.io.IOException, java.lang.SecurityException
Allocates sufficient space to the queue to support the specified number of client sockets.
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Using a ServerSocket
Refer section 6.6.2 (pg 161-Reilly) for a description of some of the methods of the ServerSocket class.
The most important method is the accept () method, which accepts client connection requests.
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Accepting and Processing Requests from TCP Clients
The most important function of a server socket is to accept client sockets. Once a client socket is obtained, the server can perform all the "real work" of server programming, which involves reading from and writing to the socket to implement a network protocol.
Example: a mail server that provides access to stored messages would listen to commands and send back message contents.
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Example:
ServerSocket server; BufferedReader reader; PrintWriter writer; server = new ServerSocket(13); while (true) { Socket client = server.accept(); reader = new BufferedReader( new InputStreamReader( client.getInputStream())); writer = new PrintWriter( new OutputStreamWriter( client.getOutputStream())); … } 84
Client Application
In this example, there are 2 java programs The client program is implemented as a class NyietInSengClient 85
import java.io.*; import java.net.*; public class NyietInSengClient { public static void main(String[] args) throws IOException { Socket nisSocket = null; PrintWriter out = null; BufferedReader in = null; try { nisSocket = new Socket("localhost", 8888); out = new PrintWriter( nisSocket.getOutputStream(), true); in = new BufferedReader(new InputStreamReader( nisSocket.getInputStream() )); } catch (UnknownHostException e) { System.err.println("Don't know about host: localhost."); System.exit(1); } catch (IOException e) { System.err.println("Couldn't get I/O for the connection to: localhost."); System.exit(1); } } } BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in)); String fromServer; String fromUser; while ((fromServer = in.readLine()) != null) { System.out.println("Server: " + fromServer); if (fromServer.equals("tata titi tutu")) break; fromUser = stdIn.readLine(); if (fromUser != null) { System.out.println("Client: " + fromUser); out.println(fromUser); } } out.close(); in.close(); stdIn.close(); nisSocket.close(); 86
Server Application
Server application is implemented using 2 classes: NyietInSengServer NyietInSengServer contains method main() for the server program. It listens at a port for incoming connection, accept the connection and reads messages from client/writes responses to the client through a socket 87
import java.net.*; import java.io.*; public class NyietInSengServer { public static void main(String[] args) throws IOException { ServerSocket serverSocket = null; try { serverSocket = new ServerSocket(8888); } catch (IOException e) { System.err.println("Could not listen on port: 8888."); System.exit(1); } } Socket clientSocket = null; try { clientSocket = serverSocket.accept(); } catch (IOException e) { System.err.println("Accept failed."); System.exit(1); PrintWriter out = new PrintWriter( clientSocket.getOutputStream
(), true); BufferedReader in = new BufferedReader( new InputStreamReader( clientSocket.getInputStream
() )); String inputLine, outputLine; NyietInSengProtocol nis = new NyietInSengProtocol(); outputLine = nis.processInput(null); out.println(outputLine); } } while ((inputLine = in.readLine()) != null) { outputLine = nis.processInput(inputLine); out.println(outputLine); if (outputLine.equals("tata titi tutu")) break; } out.close(); in.close(); clientSocket.close(); serverSocket.close(); 88
Protocol class (part of server app)
NyietInSengProtocol NyietInSengProtocol provides the jokes. It tracks the current joke, the current status (SENTTOKTOK, SENTCLUE, etc) and returns joke text based on the current status. It implements a communication protocol (a language) agreed by the client and server. 89
Server App.
Begins by creating a ServerSocket object to listen (wait) at a certain port. When selecting a port no.
, pick one that is not reserved for other services.
NyietInSengServer waits at port 8888 as the port 8888 is not used in my computer environment: try {
serverSocket = new ServerSocket(
8888
);
} catch (IOException e) { System.out.println("Could not listen on port: 8888"); System.exit(-1); } 90
Server App.
The constructor for ServerSocket throws an exception if it fail to listen the specified port (say if it is being used) In this case NyietInSengServer other choice than to exit. has no 91
If the server is able to connect to the specified port, then a ServerSocket object is created and the server app. Will perform the following steps – accept connection from client (in bold): Socket clientSocket = null; try {
clientSocket = serverSocket.accept();
} catch (IOException e) { System.out.println("Accept failed: 8888"); System.exit(-1); } 92
Server App.
Method accept() waits until a client program is executed and requesting for connection at a specified host and port (example, host :
localhost
and port : 8888. When connection between the client and server is succesfully created, method accept() Socket object (in example : will return a new clientSocket ) which is bound to a new port.
NyietInSengServer can communicate with NyietInSengClient through this new socket. It can keep listening and waiting for new incoming connection using the original ServerSocket (in example : serverSocket ) However, in the example, the server application is not able to cater for more than 1 client. 93
Read/Write Process
1.
Gets the socket's input and output stream and opens readers and writers on them PrintWriter out = new PrintWriter( clientSocket.getOutputStream() , true); BufferedReader in = new BufferedReader( new InputStreamReader( clientSocket.getInputStream())); String inputLine, outputLine; 94
Server App.
After the server successfully establishes a connection with a client, it communicates with the client using this code:
// initiate conversation with client NyietInSengProtocol nis = new NyietInSengProtocol(); outputLine = nis .processInput(null); out.println(outputLine);
2.
Initiates communication with the client by writing to the socket (shown in bold). 95