Java Network Programming

We will learn how Java handles  Internet Addresses  URLs  Sockets  Server Sockets  UDP

Some Background  Hosts  Internet Addresses  Ports  Protocols

Network Layers

Hosts  Devices connected to the Internet are called hosts  Most hosts are computers, but hosts also include routers, printers, fax machines, soda machines, bat houses, etc.

Internet addresses  Every host on the Internet is identified by a unique, four-byte Internet Protocol (IP) address.  This is written in dotted quad format like where each byte is an unsigned integer between 0 and 255.  There are about four billion unique IP addresses, but they aren’t very efficiently allocated

Sockets and ports 7 message agreed port any port socket Internet address = Internet address = other ports client server

TCP  TCP stream communication  A dedicated point-to-point channel  Use TCP (Transmission Control Protocol) for data transmission. –For integrity, checksum is used to detect and reject corrupt packets an sequence numbers to detect and reject duplicate packets. For validity, timeouts and retransmission to deal with lost packets. –So messages are guaranteed to be delivered. Lossless and reliable. Sent and received in the same order. Use of TCP: many frequently used services run over TCP connections HTTP, FTP, Telnet, SMTP 8

9CS551 - Lecture 11 Application Presentation Session Transport Application Presentation Session Transport Input Stream Output Stream Requests Results ClientServer TCP for Request Implementation

10CS551 - Lecture 11 Connection-oriented Communication

UDP  UDP datagram communication  No dedicated point-to-point channel  Use UDP (User Datagram Protocol) for data transmission. No acknowledge and retries. –Omission failure as messages may be dropped occasionally either because of checksum error or no buffer space at source or destination –Messages can be sometimes be delivered out of sender order –So applications using UDP are left to provide their own checks to achieve quality of reliable communication, which can be constructed by using acknowledgements. –Applications that are acceptable to have occasional omission failure and out of order. For example, DNS and Voice Over IP (VOIP) 11

12CS551 - Lecture 11 Result Datagrams Application Presentation Session Transport Application Presentation Session Transport Request Datagrams ClientServer UDP for Request Implementation

13CS551 - Lecture 11 Datagram Communication

TCP Protocol

UDP Protocol

The InetAddress Class 16 Occasionally, you would like to know who is connecting to the server. You can use the InetAddress class to find the client's host name and IP address. The InetAddress class models an IP address. You can use the statement shown below to create an instance of InetAddress for the client on a socket. InetAddress inetAddress = socket.getInetAddress(); Next, you can display the client's host name and IP address, as follows: System.out.println("Client's host name is " + inetAddress.getHostName()); System.out.println("Client's IP Address is " + inetAddress.getHostAddress());

The InetAddress Class  The java.net.InetAddress class represents an IP address.  It converts numeric addresses to host names and host names to numeric addresses.  It is used by other network classes like Socket and ServerSocket to identify hosts

Creating InetAddresses  There are no public InetAddress() constructors. Arbitrary addresses may not be created.  All addresses that are created must be checked with DNS

The getByName() factory method public static InetAddress getByName(String host) throws UnknownHostException InetAddress uit, duke; try { uit = InetAddress.getByName(“uit.edu.vn"); duke = InetAddress.getByName(" "); } catch (UnknownHostException e) { System.err.println(e); }

Other ways to create InetAddress objects public static InetAddress[] getAllByName(String host) throws UnknownHostException public static InetAddress getLocalHost() throws UnknownHostException

Getter Methods  public boolean isMulticastAddress()  public String getHostName()  public byte[] getAddress()  public String getHostAddress()

Datagrams  Before data is sent across the Internet from one host to another using TCP/IP, it is split into packets of varying but finite size called datagrams.  Datagrams range in size from a few dozen bytes to about 60,000 bytes.  Packets larger than this, and often smaller than this, must be split into smaller pieces before they can be transmitted.

Packets Allow Error Correction  If one packet is lost, it can be retransmitted without requiring redelivery of all other packets.  If packets arrive out of order they can be reordered at the receiving end of the connection.

Abstraction  Datagrams are mostly hidden from the Java programmer.  The host's native networking software transparently splits data into packets on the sending end of a connection, and then reassembles packets on the receiving end.  Instead, the Java programmer is presented with a higher level abstraction called a socket.

Sockets  A socket is a reliable connection for the transmission of data between two hosts.  Sockets isolate programmers from the details of packet encodings, lost and retransmitted packets, and packets that arrive out of order.  There are limits. Sockets are more likely to throw IOExceptions than files, for example.

Socket Operations  There are four fundamental operations a socket performs. These are: 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.  A socket may not reconnect after it's closed.

A Simple Example

The java.net.Socket class  The java.net.Socket class allows you to create socket objects that perform all four fundamental socket operations.  You can connect to remote machines; you can send data; you can receive data; you can close the connection.  Each Socket object is associated with exactly one remote host. To connect to a different host, you must create a new Socket object.

Constructing a Socket  Connection is accomplished through the constructors. public Socket(String host, int port) throws UnknownHostException, IOException public Socket(InetAddress address, int port) throws IOException public Socket(String host, int port, InetAddress localAddr, int localPort) throws IOException public Socket(InetAddress address, int port, InetAddress localAddr, int localPort) throws IOException

Opening Sockets  The Socket() constructors do not just create a Socket object. They 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.

 You must at least specify the remote host and port to connect to.  The host may be specified as either a string like "utopia.poly.edu" or as an InetAddress object.  The port should be an int between 1 and Socket webMetalab = new Socket("metalab.unc.edu", 80);

 You cannot just connect to any port on any host. The remote host must actually be listening for connections on that port.  You can use the constructors to determine which ports on a host are listening for connections.

public static void scan(InetAddress remote) { String hostname = remote.getHostName(); for (int port = 0; port < 65536; port++) { try { Socket s = new Socket(remote, port); System.out.println("There is a server on port " + port + " of " + hostname); s.close(); } catch (IOException e) { // The remote host is not listening on this port }

Picking an IP address  The last two constructors also specify the host and port you're connecting from.  On a system with multiple IP addresses, like many web servers, this allows you to pick your network interface and IP address.

Choosing a Local Port  You can also specify a local port number,  Setting the port to 0 tells the system to randomly choose an available port.  If you need to know the port you're connecting from, you can always get it with getLocalPort(). Socket webMetalab = new Socket("metalab.unc.edu", 80, "calzone.oit.unc.edu", 0);

Create Client

Sending and Receiving Data  Data is sent and received with output and input streams.  There are methods to get an input stream for a socket and an output stream for the socket. public InputStream getInputStream() throws IOException public OutputStream getOutputStream() throws IOException  There's also a method to close a socket. public synchronized void close() throws IOException

Reading Input from a Socket  The 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.  Most of the time you'll chain the input stream to some other input stream or reader object to more easily handle the data.

For example  The following code fragment connects to the daytime server on port 13 of metalab.unc.edu, and displays the data it sends. try { Socket s = new Socket("metalab.unc.edu", 13); InputStream in = s.getInputStream(); InputStreamReader isr = new InputStreamReader(in); BufferedReader br = new BufferedReader(isr); String theTime = br.readLine(); System.out.println(theTime); } catch (IOException e) { return (new Date()).toString(); }

Writing Output to a Socket  The getOutputStream() method returns an output stream which writes data to the socket.  Most of the time you'll chain the raw output stream to some other output stream or writer class to more easily handle the data.

Discard byte[] b = new byte[128]; try { Socket s = new Socket("metalab.unc.edu", 9); OutputStream theOutput = s.getOutputStream(); while (true) { int n = theInput.available(); if (n > b.length) n = b.length; int m = theInput.read(b, 0, n); if (m == -1) break; theOutput.write(b, 0, n); } s.close(); } catch (IOException e) {}

Socket Options  Several methods set various socket options. Most of the time the defaults are fine. public void setTcpNoDelay(boolean on) throws SocketException public boolean getTcpNoDelay() throws SocketException public void setSoLinger(boolean on, int val) throws SocketException public int getSoLinger() throws SocketException public synchronized void setSoTimeout(int timeout) throws SocketException public synchronized int getSoTimeout() throws SocketException

 These methods to return information about the socket: public InetAddress getInetAddress() public InetAddress getLocalAddress() public int getPort() public int getLocalPort()  Finally there's the usual toString() method: public String toString()

Servers  There are two ends to each connection: the client, that is the host that initiates the connection, and the server, that is the host that responds to the connection.  Clients and servers are connected by sockets.  A server, rather than connecting to a remote host, a program waits for other hosts to connect to it.

Create Server

Server Sockets  A server socket binds to a particular port on the local machine.  Once it has successfully bound to a port, it listens for incoming connection attempts.  When a server detects a connection attempt, it accepts the connection. This creates a socket between the client and the server over which the client and the server communicate.

Multiple Clients  Multiple clients can connect to the same port on the server at the same time.  Incoming data is distinguished by the port to which it is addressed and the client host and port from which it came.  The server can tell for which service (like http or ftp) the data is intended by inspecting the port.  It can tell which open socket on that service the data is intended for by looking at the client address and port stored with the data.

Queueing  Incoming connections are stored in a queue until the server can accept them.  On most systems the default queue length is between 5 and 50.  Once the queue fills up further incoming connections are refused until space in the queue opens up.

The java.net.ServerSocket Class  The java.net.ServerSocket class represents a server socket.  A ServerSocket object is constructed on a particular local port. Then it calls accept() to listen for incoming connections.  accept() blocks until a connection is detected. Then accept() returns a java.net.Socket object that performs the actual communication with the client.

Constructors  There are three constructors that let you specify the port to bind to, the queue length for incoming connections, and the IP address to bind to: public ServerSocket(int port) throws IOException public ServerSocket(int port, int backlog) throws IOException public ServerSocket(int port, int backlog, InetAddress networkInterface) throws IOException

Constructing Server Sockets  Normally you only specify the port you want to listen on, like this: try { ServerSocket ss = new ServerSocket(80); } catch (IOException e) { System.err.println(e); }

 When a ServerSocket object is created, it attempts to bind to the port on the local host given by the port argument.  If another server socket is already listening to the port, then a java.net.BindException, a subclass of IOException, is thrown.  No more than one process or thread can listen to a particular port at a time. This includes non-Java processes or threads.  For example, if there's already an HTTP server running on port 80, you won't be able to bind to port 80.

 On Unix systems (but not Windows or the Mac) your program must be running as root to bind to a port between 1 and  0 is a special port number. It tells Java to pick an available port.  The getLocalPort() method tells you what port the server socket is listening on. This is useful if the client and the server have already established a separate channel of communication over which the chosen port number can be communicated.  FTP

Expanding the Queue  If you think you aren't going to be processing connections very quickly you may wish to expand the queue when you construct the server socket. For example, try { ServerSocket httpd = new ServerSocket(80, 50); } catch (IOException e) { System.err.println(e); }

Choosing an IP address  Many hosts have more than one IP address.  By default, a server socket binds to all available IP addresses on a given port.  You can modify that behavior with this constructor: public ServerSocket(int port, int backlog, InetAddress bindAddr)throws IOException

Example try { InetAddress ia = InetAddress.getByName(" "); ServerSocket ss = new ServerSocket(80, 50, ia); } catch (IOException e) { System.err.println(e); }

 On a server with multiple IP addresses, the getInetAddress() method tells you which one this server socket is listening to. public InetAddress getInetAddress()  The getLocalPort() method tells you which port you're listening to. public int getLocalPort()

 The accept() and close() methods provide the basic functionality of a server socket. public Socket accept() throws IOException public void close() throws IOException  A server socket can’t be reopened after it’s closed

Reading Data with a ServerSocket  ServerSocket objects use their accept() method to connect to a client.  public Socket accept() throws IOException  There are no getInputStream() or getOutputStream() methods for ServerSocket.  accept() returns a Socket object, and its getInputStream() and getOutputStream() methods provide streams.

Example try { ServerSocket ss = new ServerSocket(2345); Socket s = ss.accept(); PrintWriter pw = new PrintWriter(s.getOutputStream()); pw.println("Hello There!"); pw.println("Goodbye now."); s.close(); } catch (IOException e) { System.err.println(e); }

Better Example try { ServerSocket ss = new ServerSocket(2345); Socket s = ss.accept(); PrintWriter pw = new PrintWriter(s.getOutputStream()); pw.print("Hello There!\r\n"); pw.print("Goodbye now.\r\n"); s.close(); } catch (IOException e) { System.err.println(e); }

Writing Data to a Client try { ServerSocket ss = new ServerSocket(port); while (true) { try { Socket s = ss.accept(); PrintWriter pw = new PrintWriter(s.getOutputStream()); pw.print("Hello " + s.getInetAddress() + " on port " + s.getPort() + "\r\n"); pw.print("This is " + s.getLocalAddress() + " on port " + s.getLocalPort() + "\r\n"); pw.flush(); s.close(); } catch (IOException e) {} } catch (IOException e) { System.err.println(e); }

Interacting with a Client  More commonly, a server needs to both read a client request and write a response.

Multiple Clients

Threading  No more than one server socket can listen to a particular port at one time.  Since a server may need to handle many connections at once, server programs tend to be heavily multi-threaded.  Generally the server socket passes off the actual processing of connections to a separate thread.

Adding Threading to a Server  It's better to make your server multi- threaded.  There should be a loop which continually accepts new connections.  Rather than handling the connection directly the socket should be passed to a Thread object that handles the connection.

Adding a Thread Pool to a Server  Multi-threading is a good thing but it's still not a perfect solution.  Look at this accept loop: while (true) { try { Socket s = ss.accept(); ThreadedEchoServer tes = new ThreadedEchoServer(s) tes.start(); } catch (IOException e) {}

 Every pass through this loop, a new thread gets created. Every time a connection is finished the thread is disposed of.  Spawning a new thread for each connection takes a non-trivial amount of time, especially on a heavily loaded server.  Too many simultaneous threads overload a VM.

Thread Pools  Create a pool of threads when the server launches, store incoming connections in a queue, and have the threads in the pool progressively remove connections from the queue and process them.  The main change you need to make to implement this is to call accept() in the run() method rather than in the main() method.

Setting Server Socket Options  There are three methods to set and get various options. The defaults are generally fine. public synchronized void setSoTimeout(int timeout) throws SocketException public synchronized int getSoTimeout() throws IOException public static synchronized void setSocketFactory(SocketImplFactory fac) throws IOException

Utility Methods  Finally, there's the usual toString() method: public String toString()

UDP  Unreliable Datagram Protocol  Packet Oriented, not stream oriented like TCP/IP  Much faster but no error correction  NFS, TFTP, and FSP use UDP/IP  Must fit data into packets of about 8K or less

The UDP Classes  Java's support for UDP is contained in two classes: java.net.DatagramSocket java.net.DatagramPacket  A datagram socket is used to send and receive datagram packets.

java.net.DatagramPacket  a wrapper for an array of bytes from which data will be sent or into which data will be received.  also contains the address and port to which the packet will be sent.

java.net.DatagramSocket  A DatagramSocket object is a local connection to a port that does the sending and receiving.  There is no distinction between a UDP socket and a UDP server socket.  Also unlike TCP sockets, a DatagramSocket can send to multiple, different addresses.  The address to which data goes is stored in the packet, not in the socket.

UDP ports  Separate from TCP ports.  Each computer has 65,536 UDP ports as well as its 65,536 TCP ports.  A server socket can be bound to TCP port 20 at the same time as a datagram socket is bound to UDP port 20.

Two DatagramPacket Constructors public DatagramPacket(byte[] data, int length) public DatagramPacket(byte[] data, int length, InetAddress remote, int port)  First is for receiving, second is for sending

For example, String s = "My first UDP Packet"; byte[] b = s.getBytes(); DatagramPacket dp = new DatagramPacket(b, b.length);

With a destination: try { InetAddress metalab = InetAddress.getByName("metalab.unc.edu"); int chargen = 19; String s = "My second UDP Packet"; byte[] b = s.getBytes(); DatagramPacket dp = new DatagramPacket(b, b.length, metalab, chargen); } catch (UnknownHostException e) { System.err.println(e); }

DatagramPackets are not immutable. public synchronized void setAddress(InetAddress remote) public synchronized void setPort(int port) public synchronized void setData(byte[] data) public synchronized void setLength(int length) public synchronized InetAddress getAddress() public synchronized int getPort() public synchronized byte[] getData() public synchronized int getLength()  These methods are primarily useful when you're receiving datagrams.

java.net.DatagramSocket public DatagramSocket() throws SocketException public DatagramSocket(int port) throws SocketException public DatagramSocket(int port, InetAddress laddr) throws SocketException  The first is for client datagram sockets; that is sockets that send datagrams before receiving any.  The second two are for server datagram sockets since they specify the port and optionally the IP address of the socket

Sending UDP Datagrams  To send data to a particular server –Convert the data into byte array. –Pass this byte array, the length of the data in the array (most of the time this will be the length of the array) and the InetAddress and port to which you wish to send it into the DatagramPacket() constructor. –Next create a DatagramSocket and pass the packet to its send() method

For example, InetAddress metalab = InetAddress.getByName("metalab.unc.edu"); int chargen = 19; String s = "My second UDP Packet"; byte[] b = s.getBytes(); DatagramPacket dp = new DatagramPacket(b, b.length, ia, chargen); DatagramSocket sender = new DatagramSocket(); sender.send(dp);

Receiving UDP Datagrams  Construct a DatagramSocket object on the port on which you want to listen.  Pass an empty DatagramPacket object to the DatagramSocket 's receive() method. public synchronized void receive(DatagramPacket dp) throws IOException  The calling thread blocks until a datagram is received.

 dp is filled with the data from that datagram.  Use getPort() and and getAddress() to tell where the packet came from, getData() to retrieve the data, and getLength() to see how many bytes were in the data.  If the received packet was too long for the buffer, it's truncated to the length of the buffer.  Length is reset when packet is received

For example, try { byte buffer = new byte[65536]; DatagramPacket incoming = new DatagramPacket(buffer, buffer.length); DatagramSocket ds = new DatagramSocket(2134); ds.receive(incoming); byte[] data = incoming.getData(); String s = new String(data, 0, data.getLength()); System.out.println("Port " + incoming.getPort() + " on " + incoming.getAddress() + " sent this message:"); System.out.println(s); } catch (IOException e) { System.err.println(e); }

Receiving Multiple Packets  Watch out for strictly decreasing packet sizes DatagramPacket incoming = new DatagramPacket(new byte[8192], 8192); DatagramSocket ds = new DatagramSocket(2134); while (true) { try { incoming.setLength(8192); ds.receive(incoming); byte[] data = incoming.getData(); String s = new String(data, 0, data.getLength()); System.out.println("Port " + incoming.getPort() + " on " + incoming.getAddress() + " sent this message:"); System.out.println(s); } catch (IOException e) { System.err.println(e); }

Broadcasting  Broadcasting allows a single datagram to be sent to a group of listeners  The group consists of all the computers within the local network  The previous code examples can be used for broadcasting  Just change the address: each network has a unique broadcast address

IP addresses revisited  Each 32 bit IP number consists of two components: –The network address The unique international address of the network –The host address The unique address of a specific host in the net  There are three classes of network address denoted class ‘A’, ‘B’ and ‘C’

Class A,B and C addresses Class A Class B Class C Network Address Byte Host Address Byte

Broadcast addresses  CIIPS has a class ‘C’ network which has the address  This portable computer has host address 134 within the CIIPS network  Each network has a single host address which is set aside for broadcasts (either all one bits or all zero bits)  The CIIPS network uses broadcast address  Broadcasts are never routed onto other networks

Multicasting (1)  Described in RFC1112 (August 1989)  Multicasting allows distribution of a datagram to a group of listeners who are not within the local network  Routers between networks need to pass multicast datagrams.. but many do not!  The MBONE is a way of tunneling datagrams across the Internet between islands of multicast activity

Multicasting (2)  Multicasts are also sent to a special address (known as a “group”)  Multicast groups need to be agreed in advance. They are not derived from a specific network/host address –Multicast groups identify a subject area (or stream of content) rather than a specific computer or network. They are more like a TV channel number than a telephone number.  The IETF has set aside addresses from to specifically for multicasting

Multicasting (3)  To send to (or receive from) a multicast group it is first necessary to register interest in the group –This results in an Internet Group Management Protocol (IGMP) message being sent to your router (RFCs 988/1112/2236)  Then a datagram is created, addressed to the group (and the chosen port)  Java has a specialised socket for multicasting: java.net.MulticastSocket

Some multicast groups  All hosts within local subnet  Audio news multicast  Video from IETF meetings  Expts. within local subnet  NBC Professional News  There are 268 million multicast addresses (in IPv4) with 65 thousand ports in each!

java.net.MulticastSocket  Subclass of java.net.DatagramSocket  Constructed the same way  Adds some extra methods: –void joinGroup(InetAddress mcastGroup) Enter the specifies group so that you can send or receive datagrams –void leaveGroup(InetAddress mcastGroup) Leave a group that you previously joined –void setTimeToLive(int ttl) Sets how far your datagrams will travel before routers ignore them –int getTimeToLive()

MulticastSender  Sending similar to the previous example.. ...but must register with the multicast group and decide the longevity  The steps involved are: –Create the MulticastSocket. –Join the multicast group(s) (on startup). –Create the DatagramPacket. –Send the packet through the socket. –Leave the multicast group (on exit).

MulticastSender InetAddress multicastGroup = InetAddress.getByName(multicastGroupAddr); MulticastSocket socket = new MulticastSocket(); socket.joinGroup(multicastGroup); socket.setTimeToLive(5); byte[] data = “This is the message”.getBytes(); DatagramPacket datagram = new DatagramPacket(data, data.length); datagram.setAddress(multicastGroup); datagram.setPort(9876); socket.send(datagram); socket.leaveGroup(multicastGroup);

MulticastReceiver  The steps are: –Create a multicast socket on an agreed port. –Join the multicast group (on startup). –Create an empty datagram. –Wait for datagram to be delivered on the socket. –Unpack and use the datagram. –Leave the multicast group (on exit).

MulticastReceiver InetAddress multicastGroup = InetAddress.getByName(multicastGroupAddr); MulticastSocket socket = new MulticastSocket(9876); socket.joinGroup(multicastGroup); byte[] data = new byte[1000]; DatagramPacket packet = new DatagramPacket(data, data.length); socket.receive(packet); String message = new String( packet.getData(), 0, packet.getLength()); socket.leaveGroup(multicastGroup);

Case Studies: Distributed TicTacToe Games

Distributed TicTacToe Game

Distributed TicTacToe, cont

URLs  A URL, short for "Uniform Resource Locator", is a way to unambiguously identify the location of a resource on the Internet.

Example URLs file:///Macintosh%20HD/Java/Docs/JDK% %20docs/api/ja va.net.InetAddress.html#_top_ ftp://ftp.info.apple.com/pub/ telnet://utopia.poly.edu rks

The Pieces of a URL  the protocol, aka scheme  the authority –user info user name password –host name or address –port  the path, a.k.a. file  the ref, a.k.a. section or anchor  the query string

The java.net.URL class  A URL object represents a URL.  The URL class 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

Content and Protocol Handlers  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. It gives the content handler only the actual data of the requested resource.  The content handler translates those bytes into a Java object like an InputStream or ImageProducer.

Finding Protocol Handlers  When the virtual machine creates a URL object, it looks for a protocol handler that understands the protocol part of the URL such as "http" or "mailto".  If no such handler is found, the constructor throws a MalformedURLException.

URL Constructors  There are four (six in 1.2) constructors in the java.net.URL class. public URL(String u) throws MalformedURLException public URL(String protocol, String host, String file) throws MalformedURLException public URL(String protocol, String host, int port, String file) throws MalformedURLException public URL(URL context, String url) throws MalformedURLException public URL(String protocol, String host, int port, String file, URLStreamHandler handler) throws MalformedURLException public URL(URL context, String url, URLStreamHandler handler) throws MalformedURLException

Constructing URL Objects  An absolute URL like try { URL u = new URL( " } catch (MalformedURLException e) {}

Constructing URL Objects in Pieces  You can also construct the URL by passing its pieces to the constructor, like this: URL u = null; try { u = new URL("http", " "/schedule/fall97/bgrad.html#cs"); } catch (MalformedURLException e) {}

Including the Port URL u = null; try { u = new URL("http", " 8000, "/fall97/grad.html#cs"); } catch (MalformedURLException e) {}

Relative URLs  Many HTML files contain relative URLs.  Consider the page  On this page a link to “books.html" refers to

Constructing Relative URLs  The fourth constructor creates URLs relative to a given URL. For example, try { URL u1 = new URL(" ); URL u2 = new URL(u1, "books.html"); } catch (MalformedURLException e) {}  This is particularly useful when parsing HTML.

Parsing URLs  The java.net.URL class has five methods to split a URL into its component parts. These are: public String getProtocol() public String getHost() public int getPort() public String getFile() public String getRef()

For example, try { URL u = new URL(" "); 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) { }

Reading Data from a URL  The openStream() method connects to the server specified in the URL and returns an InputStream object fed by the data from that connection. public final InputStream openStream() throws IOException  Any headers that precede the actual data are stripped off before the stream is opened.  Network connections are less reliable and slower than files. Buffer with a BufferedReader or a BufferedInputStream.

Webcat import java.net.*; import java.io.*; public class Webcat { public static void main(String[] args) { for (int i = 0; i < args.length; i++) { try { URL u = new URL(args[i]); InputStream in = u.openStream(); InputStreamReader isr = new InputStreamReader(in); BufferedReader br = new BufferedReader(isr); String theLine; while ((theLine = br.readLine()) != null) { System.out.println(theLine); } } catch (IOException e) { System.err.println(e);} }

Webcat import java.net.*; import java.io.*; public class Webcat { public static void main(String[] args) { for (int i = 0; i < args.length; i++) { try { URL u = new URL(args[i]); InputStream in = u.openStream(); InputStreamReader isr = new InputStreamReader(in); BufferedReader br = new BufferedReader(isr); int c; while ((c = br.read()) != -1) { System.out.write(c); } } catch (IOException e) { System.err.println(e);} }

URLConnections  The java.net.URLConnection class is an abstract class that handles communication with different kinds of servers like ftp servers and web servers.  Protocol specific subclasses of URLConnection handle different kinds of servers.  By default, connections to HTTP URLs use the GET method.

URLConnections vs. URLs  Can send output as well as read input  Can post data to CGIs  Can read headers from a connection

URLConnection five steps: 1. The URL is constructed. 2. The URL’s openConnection() method creates the URLConnection object. 3. The parameters for the connection and the request properties that the client sends to the server are set up. 4. The connect() method makes the connection to the server. (optional) 5. The response header information is read using getHeaderField().

I/O Across a URLConnection  Data may be read from the connection in one of two ways –raw by using the input stream returned by getInputStream() –through a content handler with getContent().  Data can be sent to the server using the output stream provided by getOutputStream().

For example, try { URL u = new URL(" URLConnection uc = u.openConnection(); uc.connect(); InputStream in = uc.getInputStream(); // read the data... } catch (IOException e) { //...

Reading Header Data  The getHeaderField(String name) method returns the string value of a named header field.  Names are case-insensitive.  If the requested field is not present, null is returned. String lm = uc.getHeaderField("Last-modified");

Six Convenience Methods  These return the values of six particularly common header fields: public int getContentLength() public String getContentType() public String getContentEncoding() public long getExpiration() public long getDate() public long getLastModified()

try { URL u = new URL(" URLConnection uc = u.openConnection(); uc.connect(); String key=null; for (int n = 1; (key=uc.getHeaderFieldKey(n)) != null; n++) { System.out.println(key + ": " + uc.getHeaderField(key)); } catch (IOException e) { System.err.println(e); }

Writing data to a URLConnection  Similar to reading data from a URLConnection.  First inform the URLConnection that you plan to use it for output  Before getting the connection's input stream, get the connection's output stream and write to it.  Commonly used to talk to CGIs that use the POST method

Eight Steps: 1.Construct the URL. 2.Call the URL’s openConnection() method to create the URLConnection object. 3.Pass true to the URLConnection’s setDoOutput() method 4.Create the data you want to send, preferably as a byte array.

5.Call getOutputStream() to get an output stream object. 6.Write the byte array calculated in step 5 onto the stream. 7.Close the output stream. 8.Call getInputStream() to get an input stream object. Read from it as usual.

HttpURLConnection  java.net.HttpURLConnection is an abstract subclass of URLConnection that provides some additional methods specific to the HTTP protocol.  URL connection objects that are returned by an http URL will be instances of java.net.HttpURLConnection.

Response Codes  The getHeaderField() and getHeaderFieldKey() don't return the HTTP response code  After you've connected, you can retrieve the numeric response code--200 in the above example--with the getResponseCode() method and the message associated with it-- OK in the above example--with the getResponseMessage() method.

getRequestMethod()  The getRequestMethod() method returns the string form of the request method currently set for the URLConnection. GET is the default method.

disconnect()  The disconnect() method of the HttpURLConnection class closes the connection to the web server.  Needed for HTTP/1.1 Keep-alive

For example, try { URL u = new URL( " html" ); HttpURLConnection huc = (HttpURLConnection) u.openConnection(); huc.setRequestMethod("PUT"); huc.connect(); OutputStream os = huc.getOutputStream(); int code = huc.getResponseCode(); if (code >= 200 && < 300) { // put the data... } huc.disconnect(); } catch (IOException e) { //...

Following Redirects HttpURLConnection.setFollowRedirects (true) method says that connections will follow redirect instructions from the web server. Untrusted applets are not allowed to set this. HttpURLConnection.getFollowRedirects () returns true if redirect requests are honored, false if they're not.

To Learn More  Java Network Programming –O’Reilly & Associates, 1997 –ISBN  Java I/O –O’Reilly & Associates, 1999 –ISBN  Web Client Programming with Java – 308/index.html

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