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Network Programming Chapter 11

Network Programming Chapter 11. Lecture 6. Networks. Client / server. Client. DNS helps in looking up the address. Protocol (http or other). Listens on a “port”. Server. java.net. One of the main strengths of Java is the “embedded” support for network programming

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Network Programming Chapter 11

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  1. Network ProgrammingChapter 11 Lecture 6

  2. Networks

  3. Client / server Client DNS helps in looking up the address Protocol (http or other) Listens on a “port” Server

  4. java.net • One of the main strengths of Java is the “embedded” support for network programming • Includes core classes with which you can communicate with URLs and create TCP/IP and Datagram sockets. • The classes may be distinguished by whether they support URLs, TCP/IP sockets, or datagram sockets.

  5. History: Unix, I/O and IPC • The Unix input/output (I/O) system • Follows the pattern of “open”, “read/write”, “close” • Inter process communication (IPC) in UNIX was designed to be similar to the file I/O interface. Each process has a set of I/O descriptors that may reflect a file, device, or communication channels (sockets). • In Unix all processes has stdin, stdout, stderr as default I/O descriptors, that may be redirected to files or devices or sockets

  6. Java: Pipes and Sockets • Pipe: • is a mechanism by which two programs running in the same machine can talk to one another. It is just a stream of characters that one end writes and the other reads. • Contains a buffer so the producer need not pause if the consumer gets behind • Is in Java used for communication only between threads running in the same JVM • Socket • Is simply a remote pipe. A socket is an endpoint for communication between two machines. • Is in Java used for interprocess communication • May be used between processes on the same machine • Java supports both pipes and sockets

  7. TCP and UDP communications • Stream communication: • The stream communication protocol is known as TCP (transfer control protocol). Unlike UDP, TCP is a connection-oriented protocol. In order to do communication over the TCP protocol, a connection must first be established between the pair of sockets. While one of the sockets listens for a connection request (server), the other asks for a connection (client). Once two sockets have been connected, they can be used to transmit data in both directions (full duplex). • Datagram communication: • The datagram communication protocol, known as UDP (user datagram protocol), is connectionless, meaning that each time you send datagrams, you also need to send the local socket descriptor and the receiving socket's address.

  8. Working with sockets • TCP/IP Socket • Maintains connection for sustained dialog. (like traditional telephone, or a sequential file.) • More overhead • Reliable – no packet lost without notification • Datagram “Socket” • Does not maintain a connection • Very fast – almost no overhead • Unreliable – can lose packets unknowningly • May hog the network. Does not handle network congestion.

  9. Host and Port • Host • The Ip address of the machine we want to talk to. • Port number • A number identifying a process on the host.

  10. TCP/IP Lecture 6

  11. TCP Client Programs • Usually follow the following steps • Open a socket. • Open an input stream and/or output stream to the socket. • Read from and write to the stream. • Close streams. • Close sockets. • Remember to handle Exceptions

  12. TCP Client socket example try { mysocket = new Socket(“some.host.com", 67); os = new DataOutputStream(mySocket.getOutputStream()); is = new DataInputStream(mySocket.getInputStream()); } catch (UnknownHostException e) { System.err.println("Don't know about host: some.host.com"); } catch (IOException e) { System.err.println("Couldn't get I/O for the connection to: some.host.com"); } • Figure 11-3 p719.

  13. TCP server socket • Server-side: • Make a new SocketServer, & call accept on it. • server = new ServerSocket (port);client_socket = server_socket.accept (); • This tells it to wait for a client socket to try to connect, then returns a Socket that is connected to the client socket • You can then ask that connected Socket for an InputStream and an OutputStream, which you can use to send/receive information from the other side of the connection • Figure 11-2 p715.

  14. Datagram (UDP) Lecture 6

  15. Using a Datagram Socket • To receive a packet (server-side) • Call new DatagramSocket( port ); • create a DatagramPacket with an empty byte[] buffer • call receive on the DatagramSocket with the packet as an argument. • The received information will be stored in the DatagramPacket you created. • You can then use DatagramPacket’s access methods to get the data transferred

  16. Using a Datagram Socket • To send a packet (client-side) • Call new DatagramSocket() • create a DatagramPacket complete with data (as a byte[]), length, address, and port. • Call send on your DatagramSocket with the packet as an argument.

  17. URLConnection Lecture 6

  18. Sockets vs. URLConnections • Sockets • Supplies hardware and TCP/IP layers • You can talk any protocol over this connection • URLConnection subclasses are specific to a particular protocol • URL, HttpURLConnection, for example

  19. URLConnections • Returned by URL.getConnection(); • Provides an InputStream to read the content of that URL • Provides an OutputStream to write to the url (i.e. for POST request – parameters are in body, not in the query string) • You can query a URLConnection for meta-data, such as date last modified • Good for one exchange – for more, use sockets

  20. Remote Objects Lecture 6

  21. Java RMI • Java Remote Method Invocation • Makes it possible to access remote objects on another machine, through a network. • You don’t actually get a reference to the remote object, but to a local “stub” object that in turn communicates with the remote object. The stub object is delivered by the ORB (Object Request Broker). • Security is always a big concern in distributed programming, and also in Java RMI. • There is a security framework called “SecurityManager”. • Is not focus in this course, but is briefly mentioned in the end of chapter 11.

  22. Common Object Request Broker Architecture (CORBA) • Like RMI, but may mix Objects from multiple programming languages • Used to be that RMI and CORBA couldn’t communicate • CORBA uses Internet Inter-ORB Protocol (IIOP) • RMI uses Java Remote Method Protocol (JRMP) • As of Java 1.3, support for RMI-IIOP interoperability exists – a.k.a. RMI API • Especially useful with Enterprise JavaBeans (EJBs) • CORBA interfaces are defined with Interface Definition Language (IDL), but you can use an SDK tool called idlj to create the interfaces, stub, and skeleton classes than correspond to the IDL. • Is not focus in this course, but is briefly mentioned in the end of chapter 11.

  23. javax.swing Lecture 6

  24. javax.swing.text.html • Contains network protocol (http) oriented classes for creating HTML text editors.

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