SE 450 Fall 2001/2002

Week 9 Lecture Notes

• Basic TCP/IP Stack
  

• URLs (Uniform Resource Locators) - address of a resource on the Internet
• 2 parts:
  • Protocol - the part before the :// (ie http, ftp, ...)
  • Resource name - the part after the :// (ie www.depaul.edu/index.html, www.sun.com, ...)
• Create URL's with the java.net.URL class (be careful becuase the constructor can throw a MalformedURLException)
• Example - Read from URL's by getting an InputStream (getInputStream()).

• Sockets are the end points of communication between clients and servers.
  
• 2 types of sockets:
  • Server sockets - listen on a specific port for incoming communication requests
  • Client sockets - used by clients to initiate and propagate communication with a server.
• Sockets are bound to port numbers (16 bit numbers).
• How do you establish a socket connection between a client and a server??
  • The server listens to a socket for a client to make a connection request
  • The client knows the host name of the server and the port number to which the server is connected. The client sends a request to the server
  • The server gets a new socket bound to a different port (it needs a different port so it can continue to listen for other connection requests at the original port)
  • The port number for the new connection does not need to be specified. The server and client will work it out.
  • The client is notified that the connection has been established
  • Client and server communicate via InputStream/OutputStream that they get from the sockets.
• Server example code try{ ServerSocket s = new ServerSocket(port); while(true){ Socket incoming = s.accept(); PrintWriter pw = new PrintWriter (new OutputStreamWriter(incoming.getOutputStream())); // ... do something ... pw.close(); incoming.close(); } } catch (Exception e){}
• Client example code try{ Socket s = new Socket(host, port); BufferedReader br = new BufferedReader (new InputStreamReader(s.getInputStream())); // ... do something ... br.close(); s.close(); } catch (Exception e){}
• Examples - Echo Server, Echo Client, Multi-threaded Echo Server, Broadcast Server

• Simplification of the distributed programming model for distributed applications.
• Allows 2 object that reside on different machines (or different virtual machines) to act as if they were on the same machine (ie via method calls).
• 2 major parts (p 435 in Jia):
  • Client
    • Get reference to remote object - actually a reference to the stub for the remote object, not the actual object itself
    • Call methods on the remote object
    • Clients can treat remote object just like local objects
      • Can be passed as arguments to method calls
      • Can be cast using the same syntax
      • Can use instanceof operator to test them
  • Server
    • Create remote object(s) - these must implement java.rmi.Remote marker interface
    • Make them available to clients using RMI registry via registration (rmi://host:port/nameOfObject)
    • Service method calls from clients

• 6 steps of a RMI method call to server.m() by client
  1. Client calls stub.m() - stub is code located on the client machine
  2. Stub marshals arguments and sends them to the skeleton on the server machine
  3. Skeleton unmarshals arguments and invokes server.m()
  4. Object on server executes method m() and returns results to skeleton
  5. Skeleton marshals results and returns them to the sub
  6. Stub unmarshals the results and returns them to the client

1. Define interface for remote object to act as contract between server and clients
   • Must extend the java.rmi.Remote interface
   • Any methods in interface must throw RemoteException
   • All arguments must be serializable
2. Implement the interface from (1) in another class
   • This class must extend UnicastRemoteObject
3. Create an instance of the server object and register it with the RMI registry using rebind()
4. Generate the stub and skeleton classes using the RMI compiler (rmic)
   • The stub class (*_Stub) must go on the client machine
   • The skeleton class (*_Skeleton) must go on the server machine
5. Develop a client that uses the remote object
   • Get an instance of the Remote object using Naming.lookup(name)
   • Cast the instance above to the interface that you developed in (1)
   • Call the method that you need
6. Compile both your server and client code.
7. Distribute code
8. Start up the RMI registry (rmiregistry)
9. Start up the client code

• Java Database Connectivity - JDBC
• Common Object Request Broker Architecture - CORBA
• JINI