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CSCE 515:
Computer Network
Programming
------ Remote Method Invocation
reference: Dave Hollinger Wenyuan Xu
Department of Computer Science and Engineering University of South Carolina
Netprog: Java RMI^2
Network Programming Paradigms
Sockets programming: design a protocol
first, then implement clients and servers
that support the protocol.
RMI: Develop an application, then move
some objects to remote machines.
Not concerned with the details of the actual communication between processes – everything is just method calls.
Netprog: Java RMI^3
Call Semantics
Method Call Semantics – what does it mean to make a call to a method?
How many times is the method run?
How do we know the method ran at all?
RMI does a great job of providing natural call semantics for remote objects/methods.
Simply a few additional Exceptions that you need to
handle.
Netprog: Java RMI^4
Finding Remote Objects
It would be awkward if we needed to include a hostname, port and protocol with every remote method invocation. RMI provides a Naming Service through the RMI Registry that simplifies how programs specify the location of remote objects.
This naming service is a JDK utility called
rmiregistry that runs at a well known address (by
default).
RMI Adds a few layers
Server App. Skeleton Remote Reference Transport
Client App. Stubs Remote Reference Transport
Remote Object References
The client acquires a reference to a remote object.
This part is different from creating a local object.
The client calls methods on the remote object
No (syntactic) difference!
Just need to worry about a few new exceptions.
Netprog: Java RMI^7
Overview of RMI Programming
Define an interface that declares the methods that
will be available remotely.
The server program must include a class that
implements this interface.
The server program must create a remote object and
register it with the naming service.
The client program creates a remote object by asking
the naming service for an object reference.
Netprog: Java RMI^8
Java Interfaces
Similar to Class
No implementation! All methods are
abstract (virtual for C++ folks).
Everything is public.
No fields defined, just Methods.
No constructor
an Interface is an API that can be
implemented by a Class.
Netprog: Java RMI^9
Interfaces and Inheritence
In Java a class can only extend a single
superclass (single inheritence).
A class can implement any number of
interfaces.
end result is very similar to multiple inheritence.
Netprog: Java RMI^10
Sample Interface
public interface Shape { public getArea(); public draw(); public fill(Color c); }
Implementing an Interface
public class Circle implements Shape {
private double radius;
private Point center;
// define a constructor and other
// methods
// MUST define the methods:
// getArea();
// draw();
// public fill(Color c);
Server Details – extending Remote
Create an interface the extends the java.rmi.Remote interface.
This new interface includes all the public methods
that will be available as remote methods.
import java.rmi.*; public interface MyRemote extends Remote { public int foo(int x) throws RemoteException; public String blah(int y) throws RemoteException;
... }
Netprog: Java RMI^19
Using The Naming service
Naming.lookup() method takes a string parameter that holds a URL indicating the remote object to lookup. rmi://hostname/objectname
Naming.lookup() returns an Object!
Naming.lookup() can throw
RemoteException
MalformedURLException
Netprog: Java RMI^20
Getting a Remote Object
try { Object o = Naming.lookup(“rmi://localhost/ReMath”);
MyRemote r = (MyRemote) o; //... Use r like any other Java object! } catch (RemoteException re) {
... } catch (MalformedURLException up) { throw up; }
Starting the Server
First you need to run the Naming service
server:
rmiregistry &
Now run the server:
java ServerMain
Sample Code
There is sample RMI code on the course
homepage:
RemoteMathImpl: remote integer arithmetic
