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Network Programming in Java - Part One

A Network Programming Lecture by Steven Choy

Lecture Overview: The InetAddress class, Overview of using socket, the Socket class, General operations of building a TCP client program, Case study: Daytime service.

The InetAddress class

• Provide abstract access to IP addresses
• No constructor, use static methods for creation

• Instance methods

• Program Souce Example 1

• Program Souce Example 2

• API Reference from java.sun.com: http://java.sun.com/javase/6/docs/api/java/net/InetAddress.html

Overview of using socket

• A socket is one end-point of a two-way communication link between two programs running on the network.
• It is through sockets that clients access the network and transmit data.
  On the client-side: The client knows the hostname of the machine on which the server is running and the port number on which the server is listening. To make a connection request, the client attempts to contact the server on the given hostname and port. The client also needs to give the server a way to contact it and so it binds to a local port number that it will use during this connection. This is usually assigned by the system. If everything goes well, the server accepts the connection. Upon acceptance, the server binds a new socket with its remote endpoint set to the address and port of the client and the same local endpoint. It also creates a new socket so that it can continue to listen to the original port for connection requests while tending to the needs of the newly-connected client.

The Socket class

• API Reference from java.sun.com: http://java.sun.com/javase/6/docs/api/java/net/Socket.html
• Represent network connections
  • Support stream-based (TCP) communications
• Specify the remote host name & port
  • Valid port range 1 to 65535
  • 1 to 1023 reserved for system services
• Automatically get a free local port upon connection
• Constructors and main methods

• Socket(String host, int port) – creates a socket and connects it to the specified port on the remote host. The system assigns an available port on the machine as the local port of the socket.
• InputStream getInputStream() – returns an input stream of the socket. Reading from this stream obtains data from the remote host.
• OutputStream getOutputStream() – returns an output stream of the socket. Writing to this stream transmits data to the remote host.
• void close() – closes the socket.
• The Socket class has many other methods for accessing information and setting options of the socket.
• Other useful methods
  getInetAddress, getPort, getLocalAddress, getLocalPort, setSoTimeout, getSoTimeout, setTcpNoDelay, getTcpNoDelay, setSoLinger, getSoLinger, setSendBufferSize, getSendBufferSize, setReceiveBufferSize, getReceiveBufferSize, setSocketImplFactory
• Generally speaking, you don’t need to memorize the details of each constructor and method of the Socket class. Whenever necessary, refer to the API documentation

Build a TCP client program - general operations

• 1) Open a Socket with remote host name and port
• 2) Invoke getInputStream & getOutputStream to obtain I/O streams
• 3) Read/write the streams according to protocol
• 4) Close the streams
• 5) Close the socket
  Client applications make use of the classes Socket, InputStream and OutputStream. The operations of a client are:
  1) Create a socket by using one of the constructors of the Socket class. You must specify the host name (or the IP address) and the port number of the target remote server. Your attempt is successful when the target server accepts your connection request.
  2) Once you create a Socket object, you use that object’s two methods, getInputStream and getOutputStream, to establish input stream and output streams to the socket. These two streams enable you to transmit data between the client and the server.
  3) Read from and write to the streams according to the application-level protocol of the target server. This step will differ from client to client, depending on the server.
  4) Close the socket and the streams.
  These four steps remain largely the same for many different client programs you are going to write, though the implementation of step 3 varies accordingly to the designated protocol.

Case Study: Daytime Service

• Most UNIX servers run the daytime service on TCP port 13.

          C:\>telnet www.ouhk.edu.hk 13

          Mon Oct  5 13:17:39 2009


          遺失與主機的連線。

          C:\>

• It is easy to write a Java daytime client.
• All the program needs to do is to establish a TCP connection on port 13 of a remote host.

Executing socket programs through proxy servers

• Java supports communication via a proxy server in its networking APIs.
• You set the system properties http.proxyHost and http.proxyPort to the proxy server host and port, respectively, to enable the use of the proxy server.
• It can be done when executing the client program, using the –D switch in the Java command line:

    java –Dhttp.proxyHost=<...> –Dhttp.proxyPort=<...> <youClientProgram>

• Alternatively, the system properties can be configured in program code, before creating and using sockets:

    System.setProperty("http.proxyHost", "proxy.ouhk.edu.hk");
    System.setProperty("http.proxyPort", "8080");

Java Programming Lesson for this lecture

• Reading: The Java Tutorial - Lesson: Classes and Objects
• The roles of classes and objects in Java programming
• What are instance variables (also called object variables)?
• What are instance methods (also called object methods)?
• What are static variables (also called class variables)?
• What are static methods (also called class methods)?
• Question: Consider the following class: (Extract from: http://java.sun.com/docs/books/tutorial/java/javaOO/QandE/creating-questions.html)

        public class IdentifyMyParts {
            public static int x = 7; 
            public int y = 3; 
        }

1) What are the class variables?

2) What are the instance variables?

3) What is the output from the following code:

            IdentifyMyParts a = new IdentifyMyParts();
            IdentifyMyParts b = new IdentifyMyParts();
            a.y = 5;
            b.y = 6;
            a.x = 1;
            b.x = 2;
            System.out.println("a.y = " + a.y);
            System.out.println("b.y = " + b.y);
            System.out.println("a.x = " + a.x);
            System.out.println("b.x = " + b.x);
            System.out.println("IdentifyMyParts.x = " + IdentifyMyParts.x);

• What are System.in and System.out?

         System.out.println(data)

• API Reference from java.sun.com: http://java.sun.com/javase/6/docs/api/java/lang/System.html
• What is try and catch blocks?

Thanks for Reading

If you would rather like to have this lecture note in printed format, please click the print action link in the top right corner.

If you find any problem in this lecture note, please feel free to tell Steven via steven@findaway.hk.