Internet Programming

1
Internet Programming

• In Java

2
References

• www.cafeaulait.org
• Java.sun.com
• http//home.att.net/baldwin.rick/Advanced/Java552
  
• Many of the programs shown here come from these 3
  sites

3
InetAddress Class

• Java.net.InetAddress
• Represents an IP address ( xxx.xxx.xxx.xxx)
• Converts
• xxx.xxx.xxx.xxx ? machineName.domainName
• machineName.domainName ? xxx.xxx.xxx.xxx
• Used by other network classes
• Socket
• ServerSocket
• ...

4
InetAddress Class

• No public InetAddress( ) Constructors
• Arbitrary addresses may not be created
• All addresses checked with DNS
• Provides objects that you can use to manipulate
  and deal with IP addresses and domain names.
• Class provides several static methods that return
  an object of type InetAddress.

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InetAddress ClassMethods

• getByName ()
• Public static InetAddress getByName(host)
• Throws UNknownHostException
• Returns an InetAddress object representing host
• Can be used to determine the IP address of a
  host, given the host's name.
• Host
• machine name java.sun.com
• IP address 206.26.48.100

6
InetAddress Class getByName (host)

• InetAddress java1, java2
• try
• java1 InetAddress.getByName(java.sun.com")
• java2 InetAddress.getByName("128.238.2.92")
• catch (UnknownHostException e)
  System.err.println(e)
• System.out.println(java1)
• ...

7
InetAddress Class getAllByName (host)

• Returns an array of InetAddress objects.
• IP addresses of the specified host.

8
InetAddress Class getLocalHost (host)

• Returns an InetAddress object representing the
  local host computer.

9
InetAddress Class

• ? Show Url001.java
• Get and display IP address of URL by name
• wpi.wpi.edu/130.215.24.6
• Do reverse lookup on the IP address
• wpi.WPI.EDU/130.215.24.6
• Get and display current IP address of LocalHost
• grover.WPI.EDU/130.215.25.67
• Do reverse lookup on current IP address of
  LocalHost
• grover.wpi.edu/130.215.25.67
• Get and display current name of LocalHost
• grover.wpi.edu
• Get and display current IP address of LocalHost
• 130 215 25 67

10
Ports

• Many hosts have only one Internet address
• This address is subdivided into 65,536 ports
• Ports are logical abstractions that allow one
  host to communicate simultaneously with many
  other hosts
• Many services run on well-known ports
• (HTTP on port 80)

11
Protocols

• Defines how two hosts talk to each other.

12
URL Class

• A URL object represents a URL.
• contains methods to
• create new URLs
• parse the different parts of a URL
• Get an input stream from a URL so you can read
  data from a server
• Get content from the server as a Java object

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java.net.URL Class

• Content and protocol handlers
• separate the data being downloaded from the the
  protocol used to download it.
• The protocol handler
• negotiates with the server and parses any
  headers.
• Gives the content handler only the actual data of
  the requested resource.
• The content handler
• translates those bytes into a Java object
• InputStream or ImageProducer, ...

14
java.net.URL ClassFinding Protocol Handlers

• When virtual machine creates a URL object
• looks for a protocol handler that understands
  the protocol part of the URL
• "http" or "mailto".
• If no such handler is found
• the constructor throws a MalformedURLException

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java.net.URL ClassSupported Protocols

• Vary http and file are supported pretty much
  everywhere) . Sun's JDK 1
• file
• ftp
• gopher
• http
• mailto
• appletresource
• doc
• netdoc
• systemresource
• verbatim

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java.net.URL ClassFour Constructors

• public URL(String u) throws MalformedURLException
  
• URL u null
• try
• u new URL("http//www.cs.wpi.edu/kal")
• catch (MalformedURLException e)
• ...
• absolute URL.
• Contains all information necessary to reach the
  resource

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java.net.URL ClassFour Constructors

• You can also create URL objects from a relative
  URL address.
• 2. public URL(String protocol, String host,
  String file) throws MalformedURLException
• URL u null
• try
• u new URL("http","www.cs.wpi.edu",
  "/kal/personal.html)
• catch (MalformedURLException e)
• ...
• public URL(URL context, String u) throws
  MalformedURLException

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java.net.URL ClassFour Constructors

• 3. public URL(String protocol, String host, int
  port, String file) throws MalformedURLException
• URL u null
• try
• u new URL("http",penguin.wpi.edu",4546,
  
• "/webrecourse/htdocs/course/087254")
  
• catch (MalformedURLException e)
• ...

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java.net.URL ClassFour Constructors

• 4. public URL(URL context, String u) throws
  MalformedURLException
• URL u1, u2
• try
• u1 new URL("http//www.cs.wpi.edu")
• u2 new URL(u1, personal.html")
• catch (MalformedURLException e)
• ...

20
java.net.URL ClassParsing URLs

• Five methods to split a URL into its component
  parts
• public String getProtocol()
• public String getHost()
• public int getPort()
• public String getFile()
• public String getRef()
• If a port is not explicitly specified in the URL,
  it's set to -1. ( default port is used)
• If the ref doesn't exist, it's just null, so
  watch out for NullPointerExceptions. Better yet,
  test to see that it's non-null before using it.
• If file is left off completely, e.g.
  http//wpi.cs.wpi.edu, then it's set to "/".

21
java.net.URL ClassParsing URLs

• try
• URL u new URL(http//www.wpi.edu")
• System.out.println("The protocol is "
  u.getProtocol())
• System.out.println("The host is " u.getHost())
  
• System.out.println("The port is " u.getPort())
  
• System.out.println("The file is " u.getFile())
  
• System.out.println("The anchor is "
  u.getRef())
• catch (MalformedURLException e)
• Show Url002.java

22
Sockets

• Socket one end-point of a two-way communication
  link between two programs running on a network.
• Data sent across the Internet from one host to
  another using TCP/IP
• Split into packets of varying but finite size
  called datagrams.
• Range in size from a few dozen bytes to about
  60,000 bytes
• Host transparently handles the splitting of data
  into packets on the sending end of a connection,
  and the reassembly of packets on the receiving
  end.

23
Sockets

• Socket represents a reliable connection for
  transmission of data between two hosts.
• Isolates programmer from details of packet
  encodings, lost and retransmitted packets, and
  packets that arrive out of order.

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Sockets

• Four fundamental operations a socket performs.
• 1. Connect to a remote machine
• 2. Send data
• 3. Receive data
• 4. Close the connection
• A socket may not be connected to more than one
  host at a time.
• Specify the remote host and port to connect to
• Host may be specified as either a string like
  www.nord.is" or as an InetAddress object.
• The port should be an int between 1 and 65535.

25
Sockets

• Socket() constructors do not just create a Socket
  object.
• Also attempt to connect the underlying socket to
  the remote server.
• All the constructors throw an IOException if the
  connection can't be made for any reason.

26
Sockets

• Java programmer is presented with a higher level
  abstraction called a socket.
• Socket classes are used to represent the
  connection between a client program and a server
  program.
• The java.net package provides two classes--Socket
  and ServerSocket--that implement the client side
  of the connection and the server side of the
  connection, respectively.

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java.net.Socket class

• Can connect to remote machines you can send
  data you can receive data you can close the
  connection.
• Connection is accomplished through the
  constructors. Each Socket object is associated
  with exactly one remote host. To connect to a
  different host, you must create a new Socket
  object.

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java.net.Socket classConstructors

• public Socket(String host, int port) throws
  UnknownHostException, IOException
• Socket webSunsite new Socket(wpi.wpi.edu"
  , 80)
• public Socket(InetAddress address, int port)
  throws IOException
• public Socket(String host, int port, InetAddress
  localAddress, int localPort) throws IOException
• Socket metalab new Socket("metalab.unc.edu",
  80, "calzone.oit.unc.edu", 0)
• public Socket(InetAddress address, int port,
  InetAddress localAddress, int localPort) throws
  IOException
• Last two constructors also specify the host and
  port you're connecting from

29
java.net.Socket class

• Show PortScanner.java

30
java.net.Socket classReading Input from a
Socket

• Once a socket has connected
• Send data to the server via an output stream.
• Receive data from the server via an input stream.
  
• Exactly what data you send and receive often
  depends on the protocol.

31
java.net.Socket classReading Input from a
Socket getInputStream()

• getInputStream() method
• returns an InputStream which reads data from the
  socket. You can use all the normal methods of the
  InputStream class to read this data.

32
java.net.Socket classWriting Output to a
Socket getOutputStream()
33
java.net.Socket classReading and Writing to a
Socket

• Some protocols require the reads and the writes
  to be interlaced.
• write read write read write read
• Other protocols, such as HTTP 1.0, have multiple
  writes, followed by multiple reads
• write write write read read read read
• Other protocols don't care and allow client
  requests and server responses to be freely
  intermixed.
• Java places no restrictions on reading and
  writing to sockets. One thread can read from a
  socket while another thread writes to the socket
  at the same time.