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
 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
 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
Socket flow of events -- synchronous
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
Server
•Await client message
•…
•…
•Receive client message
•Decode client message
•Perform action
•Create client message
•Send to client
Method call on standalone object
Socket flow - asynchronous
Client1
•User input (UI)
•Decode UI
•Create srvr msg
•Send srvr msg
•Await srvr reply
•…
•…
•…
•Receive server
message
•Decode reply
•Output to user
Server
•Wait
•…
•…
•Receive client msg
•Decode client msg
•Perform action
•Create client message
•Send to client1
•Send to client2
Client2
•User input (UI)
•Decode UI
•Create srvr msg
•Send srvr msg
•Await srvr reply
•…
•…
•…
•Receive server
message
•Decode reply
•Output to user
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
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 call to super() as first line in
constructor
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
– 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!)
 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.