1. © 1999 Elliotte Rusty Harold 5/16/2015 Java Sockets and Server Sockets Low Level Network Programming Elliotte Rusty Harold elharo@metalab.unc.edu http://metalab.unc.edu/javafaq/SOCKETS.PPT

2. © 1999 Elliotte Rusty Harold 5/16/2015 In this talk: How data is transmitted across the Internet Sockets Server Sockets UDP

3. © 1999 Elliotte Rusty Harold 5/16/2015 I assume you Understand basic Java syntax and I/O Have a user’s view of the Internet No prior socket programming experience Are familiar with the InetAddress class

4. © 1999 Elliotte Rusty Harold 5/16/2015 Applet Network Security Restrictions Applets may: –send data to the code base –receive data from the code base Applets may not: –send data to hosts other than the code base –receive data from hosts other than the code base

5. © 1999 Elliotte Rusty Harold 5/16/2015 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.

6. © 1999 Elliotte Rusty Harold 5/16/2015 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.

7. © 1999 Elliotte Rusty Harold 5/16/2015 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 witha higher level abstraction called a socket.

8. © 1999 Elliotte Rusty Harold 5/16/2015 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.

9. © 1999 Elliotte Rusty Harold 5/16/2015 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.

10. © 1999 Elliotte Rusty Harold 5/16/2015 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.

11. © 1999 Elliotte Rusty Harold 5/16/2015 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

12. © 1999 Elliotte Rusty Harold 5/16/2015 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.

13. © 1999 Elliotte Rusty Harold 5/16/2015 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 65535. Socket webMetalab = new Socket("metalab.unc.edu", 80);

14. © 1999 Elliotte Rusty Harold 5/16/2015 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.

15. © 1999 Elliotte Rusty Harold 5/16/2015 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("A server is listening on port " + port + " of " + hostname); s.close(); } catch (IOException e) { // The remote host is not listening on this port }

16. © 1999 Elliotte Rusty Harold 5/16/2015 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.

17. © 1999 Elliotte Rusty Harold 5/16/2015 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);

18. © 1999 Elliotte Rusty Harold 5/16/2015 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

19. © 1999 Elliotte Rusty Harold 5/16/2015 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.

20. © 1999 Elliotte Rusty Harold 5/16/2015 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 is = s.getInputStream(); InputStreamReader isr = new InputStreamReader(is); BufferedReader br = new BufferedReader(isr); String theTime = br.readLine(); System.out.println(theTime); } catch (IOException e) { return (new Date()).toString(); }

21. © 1999 Elliotte Rusty Harold 5/16/2015 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.

22. © 1999 Elliotte Rusty Harold 5/16/2015 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) {}

23. © 1999 Elliotte Rusty Harold 5/16/2015 Reading and Writing to a Socket It's unusual to only read from a socket. It's even more unusual to only write to a socket. Most protocols require the client to both read and write.

24. © 1999 Elliotte Rusty Harold 5/16/2015 Some protocols require the reads and the writes to be interlaced. That is: –write –read –write –read –write –read

25. © 1999 Elliotte Rusty Harold 5/16/2015 Other protocols, such as HTTP 1.0, have multiple writes, followed by multiple reads, like this: –write –read

26. © 1999 Elliotte Rusty Harold 5/16/2015 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.

27. © 1999 Elliotte Rusty Harold 5/16/2015 try { URL u = new URL(args[i]); if (u.getPort() != -1) port = u.getPort(); if (!(u.getProtocol().equalsIgnoreCase("http"))) { System.err.println("I only understand http."); } Socket s = new Socket(u.getHost(), u.getPort()); OutputStream theOutput = s.getOutputStream(); PrintWriter pw = new PrintWriter(theOutput, false); pw.print("GET " + u.getFile() + " HTTP/1.0\r\n"); pw.print("Accept: text/plain, text/html, text/*\r\n"); pw.print("\r\n"); pw.flush(); InputStream theInput = s.getInputStream(); InputStreamReader isr = new InputStreamReader(theInput); BufferedReader br = new BufferedReader(isr); String theLine; while ((theLine = br.readLine()) != null) { System.out.println(theLine); } catch (MalformedURLException e) { System.err.println(args[i] + " is not a valid URL"); } catch (IOException e) { System.err.println(e); }

28. © 1999 Elliotte Rusty Harold 5/16/2015 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

29. © 1999 Elliotte Rusty Harold 5/16/2015 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()

30. © 1999 Elliotte Rusty Harold 5/16/2015 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 serve, rather than connecting to a remote host, a program waits for other hosts to connect to it.

31. © 1999 Elliotte Rusty Harold 5/16/2015 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.

32. © 1999 Elliotte Rusty Harold 5/16/2015 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.

33. © 1999 Elliotte Rusty Harold 5/16/2015 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.

34. © 1999 Elliotte Rusty Harold 5/16/2015 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.

35. © 1999 Elliotte Rusty Harold 5/16/2015 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.

36. © 1999 Elliotte Rusty Harold 5/16/2015 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 bindAddr) throws IOException

37. © 1999 Elliotte Rusty Harold 5/16/2015 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); }

38. © 1999 Elliotte Rusty Harold 5/16/2015 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.

39. © 1999 Elliotte Rusty Harold 5/16/2015 On Unix systems (but not Windows or the Mac) your program must be running as root to bind to a port between 1 and 1023. 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

40. © 1999 Elliotte Rusty Harold 5/16/2015 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); }

41. © 1999 Elliotte Rusty Harold 5/16/2015 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

42. © 1999 Elliotte Rusty Harold 5/16/2015 Example try { InetAddress ia = InetAddress.getByName("199.1.32.90"); ServerSocket ss = new ServerSocket(80, 50, ia); } catch (IOException e) { System.err.println(e); }

43. © 1999 Elliotte Rusty Harold 5/16/2015 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()

44. © 1999 Elliotte Rusty Harold 5/16/2015 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

45. © 1999 Elliotte Rusty Harold 5/16/2015 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.

46. © 1999 Elliotte Rusty Harold 5/16/2015 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); }

47. © 1999 Elliotte Rusty Harold 5/16/2015 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); }

48. © 1999 Elliotte Rusty Harold 5/16/2015 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); }

49. © 1999 Elliotte Rusty Harold 5/16/2015 Interacting with a Client More commonly, a server needs to both read a client request and write a response.

50. © 1999 Elliotte Rusty Harold 5/16/2015 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.

51. © 1999 Elliotte Rusty Harold 5/16/2015 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) {}

52. © 1999 Elliotte Rusty Harold 5/16/2015 Every time you 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. It would be better not to spawn so many threads.

53. © 1999 Elliotte Rusty Harold 5/16/2015 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.

54. © 1999 Elliotte Rusty Harold 5/16/2015 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

55. © 1999 Elliotte Rusty Harold 5/16/2015 Utility Methods Finally, there's the usual toString() method: public String toString()

56. © 1999 Elliotte Rusty Harold 5/16/2015 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 60,000 bytes or less

57. © 1999 Elliotte Rusty Harold 5/16/2015 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.

58. © 1999 Elliotte Rusty Harold 5/16/2015 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.

59. © 1999 Elliotte Rusty Harold 5/16/2015 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.

60. © 1999 Elliotte Rusty Harold 5/16/2015 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.

61. © 1999 Elliotte Rusty Harold 5/16/2015 Two DatagramPacket Constructors public DatagramPacket(byte[] data, int length) public DatagramPacket(byte[] data, int length, InetAddress iaddr, int iport) First is for receiving, second is for sending

62. © 1999 Elliotte Rusty Harold 5/16/2015 For example, String s = "My first UDP Packet" byte[] b = s.getBytes(); DatagramPacket dp = new DatagramPacket(b, b.length);

63. © 1999 Elliotte Rusty Harold 5/16/2015 With a destination: try { InetAddress metalab = new InetAddess("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); }

64. © 1999 Elliotte Rusty Harold 5/16/2015 DatagramPackets are not immutable. public synchronized void setAddress(InetAddress iaddr) public synchronized void setPort(int iport) public synchronized void setData(byte ibuf[]) public synchronized void setLength(int ilength) 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.

65. © 1999 Elliotte Rusty Harold 5/16/2015 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

66. © 1999 Elliotte Rusty Harold 5/16/2015 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

67. © 1999 Elliotte Rusty Harold 5/16/2015 For example, InetAddress metalab = new InetAddess("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);

68. © 1999 Elliotte Rusty Harold 5/16/2015 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.

69. © 1999 Elliotte Rusty Harold 5/16/2015 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.

70. © 1999 Elliotte Rusty Harold 5/16/2015 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); }

71. © 1999 Elliotte Rusty Harold 5/16/2015 To Learn More Java Network Programming –O’Reilly & Associates, 1997 –ISBN 1-56592-227-1