Download printable

11/11/10
Remote Method Invocation
 
Remote Method Invocation
Internet Computing Workshop
Lecture 13
 
 
So far we have look at sending messages
across a network.
Remote Method Invocation (RMI) gives you
access to objects that are implemented at a
remote location.
It's distributed programming in a fully objectoriented Java style.
Advantages of RMI
 
You can program in a purely object-orientated
style.
Disadvantages of RMI
 
Needs additional configurations:
Object registry
  Complied with rmic
 
 
 
Once it's set up you don't need to worry about
handling remote access.
No need to design a protocol for your system.
 
Remote Objects
 
 
In RMI the client has access to an object that is
implemented remotely.
Fields of the object are stored remotely
 
 
Remote data storage.
Hard to tell which objects are local
and which are remote.
Communication Model
 
With sockets: client and
server communicate
directly, exchanging
messages.
Server
Messages
Methods of the object are executed remotely
 
Remote processing.
Client
1
11/11/10
Communication Model
 
 
The client asks the
registry for the object.
 
Bind
Object X
Server
X
This connects the object X
to a stub.
 
Registry
X?
 
Communication Model
The client use the stub as
if it was the object.
Stub
for X
Stub
Client
 
Give me X
The client treats the stub
like the real object.
All methods called on the
stub are forward to the
server.
You must start the a registry running.
 
 
 
 
 
 
Interact with registry using the
java.rmi.Naming class
- 
Adds the object “obj” to the registry under the
name “name”.
lookup (String name)
- 
Returns a stub for the object called “name”
unbind(String name)
- 
Removes the object “name” from the registry
list(String name)
- 
Returns a list of all objects in the registry
rebind (String name, Remote obj)
- 
Method
call
Stub
Client
Both the client and the server must have
access to an interface for the remote object.
This interface must extend the java.rmi.Remote
class.
The methods may throw
java.rmi.RemoteException
java.rmi.Naming
bind(String name, Remote obj)
Registry
Interface for the Object
e.g. type “rmiregistry” from the command line
Usually runs on port 1099, but can run on any
port and even on the same machine as the
server.
Result
The result is then returned
to the client.
The Registry
 
Server
X
Registry URLs
 
Services in registries can be addressed directly
using a URL of the form:
rmi://<host_name>[:<name_service_port>]/<service_name>
rmi://localhost/printObject
or
rmi://68.45.12.7:30099/
process
These are used with the Naming.lookup method:
Naming.lookup(rmi://68.45.12.7:30099/process)
which returns the remote object called “process”
= unbind(name);bind(name,obj)
2
11/11/10
java.rmi.registry.LocateRegistry
 
Communication Model
Binds
You can also refer to a Registry my creating a Registry object
using the LocateRegistry package.
Calc
Server
 
LocateRegistry.getRegistry(h,p) return an reference
to the registry runing on host h and on port p
Registry registry =
LocateRegistry.getRegistry(host);
Calc
Inpl
Generate
using rmic
Registry
Implements
Calc
Interface
Ro o = (Ro)registry.lookup("service");
 
Is the same as:
Ro o = (Ro)Naming.lookup("rmi://host/serv");
Client
Gets
Object
Distributing the Code
 
 
Both the client and the server need the
interface class.
Server
binds
object
Client does need the stub, but can fetch this
dynamically, using the java.rmi.server.codebase
Client
gets
object
To keep things easy you should sending
primitive data types as arguments and return
values.
 
 
You can send Objects that are “serializable”
 
Make your own object serializable by extending the
java.io.serializable class.
 
 
The remote object is a new object: a copy of the one
you sent.
Stub forwards
Calls to remote
stub
Server
Calc
Server
Calc
Impl
Server places
stub on
public host
Stub and impl
communicate
Public Host
Client
Calc
Client
Calc
Interface
Calc
Interface
Calc stub
Calc stub
CORBA vs RMI
Method Arguments and RMI
 
Client uses Stub
Client does not need the implementation
java Calc -Djava.rmi.server.codebase=URLofStub
 
Calc stub
Communication Model
Registry
 
Calc
Client
Common Object Request Broker Architecture
(CORBA) is like RMI but not just in Java.
Only primitive data types can be passed in
CORBA.
CORBA is very useful when new Java code is
combined with legacy systems.
3
11/11/10
CORBA vs RMI
 
CORBA uses a “Object Request Broker” (ORB),
rather than a rmi registory.
CORBA vs RMI
 
 
 
Interfaces in CORBA are written in Interface
Definition Language (IDL).
 
 
You then generate the Java Interface using
“ildj”.
Use CORBA for inter-language working.
Or for legacy Systems
Otherwise use RMI.
For More Information
Summary
RMI gives you access to objects that are
implemented at a remote location.
Use the “org.omg.CORBA.*” to distribution and
access the object.
 
 
 
“ildj” generates all the files you need.
 
For more on RMI see:
http://java.sun.com/docs/books/tutorial/rmi/
 
It's distributed programming in a fully objectoriented Java style.
 
 
 
It needs addition configurations (registry,rmic)
For more on CORBA see:
http://java.sun.com/developer/onlineTraining/corba
Hard to tell which objects are local and which
remote.
Next Week:
 
AJAX: Asynchronous JavaScript and XML
 
Makes really fast, responsive webpages.
 
E.g. gmail
4