Lesson 3 Remote Method Invocation (RMI) Mixing RMI and sockets Rethinking out tic-tac-toe game

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.

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  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

Socket flow of events -- synchronous Await client message … Receive client message Decode client message Perform action Create client message Send to client 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 Client Server Method call on standalone object

Socket flow - asynchronous 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 Client1 Server User input (UI) Decode UI Create srvr msg Send srvr msg Await srvr reply … Receive server message Decode reply Output to user Client2

RMI flow of events -- synchronous 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 Client Server

RMI Cartoon1

RMI Cartoon2

Steps for RMI Application  Implement both client and server on single machine to test  Create two directories –client –server

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)

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 null construct that calls super()

Creating Stubs  The stub or proxy code is generated automatically by using the rmic utility.  Once the interface and implementation classes are complete, generate the stubs as follows: rmic –v1.2 Store This creates Store_Stub.java – the java networking layer that transparently handles the underlying message passing.

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

Steps for RMI, cont.  Create the client –Change to the client dir and copy Store.class and Store_Stub.class (or you can import them but remember that these will ultimately be on different machines). –Create StoreClient.java and import: java.rmi.Naming; java.rmi.RemoteException; java.net.MalformedURLException; java.rmi.NotBoundException;

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).

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

Additional Issues – covered next time  Objects which are not remote are copied (slow!)  Stub and interface codes can be downloaded by client (typical for real distributed systems)  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!

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.