1. Remote Method Invocation (RMI) Mixing RMI and sockets
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
User input (UI)
Decode UI
Create srvr msg
Send srvr msg
Await srvr reply …
Receive server message
Decode reply
Output to user
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

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.