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Lesson 3

Lesson 3. Remote Method Invocation (RMI) Mixing RMI and sockets. Distributed Objects. Simple idea – objects existing on one machine (server) may be accessed from another machine through regular method call. Eliminates need to “marshal” and “unmarshal” data sent over sockets

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Lesson 3

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  1. Lesson 3 Remote Method Invocation (RMI) Mixing RMI and sockets

  2. Distributed Objects • Simple idea – objects existing on one machine (server) may be accessed from another machine through regular method call. • Eliminates need to “marshal” and “unmarshal” data sent over sockets • Underlying socket code still exists, but is not programmed by user.

  3. RMI vs. CORBA • RMI is Java framework for creating distributed object applications – Remote Method Invocation • CORBA is alternative technology based on open standard – Common Object Request Broker Architecture • Web Service/SOAP is a remote object framework based on http and XML • RMI is only for pure Java applications; CORBA is language independent • JNI makes this distinction a little less rigid since it allows Java to interact with other languages

  4. Socket flow of events -- synchronous Client Server • Get user input • Decode user input • Create server message • Send message to server • Await server response • … • … • … • Receive server message • Decode reply • Send output to user • Await client message • … • … • Receive client message • Decode client message • Perform action • Create client message • Send to client Method call on standalone object

  5. Socket flow - asynchronous Client1 Server Client2 • Wait • … • … • Receive client msg • Decode client msg • Perform action • Create client message • Send to client1 • Send to client2 • User input (UI) • Decode UI • Create srvr msg • Send srvr msg • Await srvr reply • … • … • … • Receive server message • Decode reply • Output to user • User input (UI) • Decode UI • Create srvr msg • Send srvr msg • Await srvr reply • … • … • … • Receive server message • Decode reply • Output to user

  6. RMI flow of events -- synchronous Client Server • Instantiate object(s) • Bind to registry • … • … • … • … • Get/cache remote obj ref • Get user input • Decode user input • Remote method call • Decode return value • Send output to user

  7. RMI Cartoon1

  8. RMI Cartoon2

  9. Steps for RMI Application • Implement both client and server on single machine to test • Create two directories • client • server

  10. RMI steps, server side • Three files need to be created: • The implementation class (Foo.java) • An interface listing the methods in the implementation class which you want to make remote (FooInterface.java) • A server class, which creates one or more implementation objects and posts them to the registry (FooServer.java)

  11. Creating the interface • Interface file (e.g. StoreInterface.java) • StoreInterface must extend java.rmi.Remote • All methods in interface must throw java.rmi.RemoteException • Implementation file (e.g Store.java) • Store must extend java.rmi.server.UnicastRemoteObject • Store must implement StoreInterface • Program implementations. Be sure to throw RemoteException in remote methods • Explicitly include a call to super() as first line in constructor

  12. Creating the server class • Server class (e.g. StoreServer.java) • Create a new object instance • Call java.rmi.Naming.bind(…) to store the register the object with the naming service • … contains String name associated with bound object

  13. Steps for RMI, cont. • Create the client • Create StoreClient.java and import: java.rmi.Naming; java.rmi.RemoteException; java.net.MalformedURLException; java.rmi.NotBoundException;

  14. Steps for rmi, cont. • Call Naming.lookup() to get remote object reference (be sure to cast to interface type). • Be sure to handle imported exceptions • Once you get remote object reference, handle as regular object (there are some subtle differences that we’ll explore later).

  15. Deploying the Application • Start the rmiregistry • rmiregistry & (Unix) • start rmiregistry (Windows) • Start the StoreServer class • java StoreServer & (Unix) • Run the client • That’s it!

  16. Additional Issues – covered next time • Objects which are not remote are copied (slow!) • Security issues in real system (see ch. 5 in Core Java 2) • Subtleties with Object methods (clone, etc) • Using callbacks with RMI • Synchronization • Registering multiple objects • Bottom Line: Don’t be too fancy!

  17. Examples • tic-tac-toe reorganized as standalone back-end object • single-threaded test of TTT object • multithreaded test of TTT object using polling • client-server TTT using RMI and polling • client-server TTT using RMI over sockets • client-server TTT using RMI callbacks.

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