1. Overview of RMI Architecture
10/14/2019
Peter Cappello

2. Introduction ... Remote methods have: much greater latency
new failure modes
Do not distribute that which does not need to be
10/14/2019

3. Introduction ...
Remote method invocation is like local method invocation, except:
Arguments & return values must:
implement Serializable, or
be Remote objects.
Arguments & return values are passed by value.
10/14/2019

4. Other differences ...
An RMI client refers to a remote object via a Remote interface (it may have many).
The object methods:
equals()
hashCode(),
toString()
are overridden by java.rmi.RemoteObject.
For example, the toString value includes transport info (network protocol, host name, & port number)
10/14/2019

5. Remote Object Structure
To apply a remote method (y) to a remote object (x): x.y()
x must be a reference to a remote object.
The RMI client gets this reference:
from a rmiregistry or
as the return value of a prior remote method invocation
10/14/2019

6. Reference from Registry
rmiregistry
2. Lookup service
1. Register service
Client
Server
3. Invoke method
10/14/2019

7. Remote Objects Implement the Remote Interface
A remote object must implement at least 1 interface that extends the java.rmi.Remote interface.
Only those methods declared in the interface can be invoked remotely.
A diagram follows ...
10/14/2019

8. The Object Hierarchy Classes Interfaces java.lang.Object
java.rmi.Remote
java.rmi.server.RemoteObject
java.rmi.server.RemoteServer
java.rmi.server.UnicastRemoteObject
YourRemoteInterface
10/14/2019
YourRemoteObject

9. RMI System Architecture
RMI Client
Application
Layer
RMI Server
Stub
Proxy Layer
Skeleton
Remote Reference Layer
Transport Layer
10/14/2019

10. Application Layer No interface description language (IDL)
The server application:
Implements the remote interface that the client uses.
Exports the object whose methods are invoked remotely (implicitly by extending UnicastRemoteObject)
Registers itself with the rmiregistry.
10/14/2019

11. Application Layer ... The client application:
Gets reference to remote object (o)
Casts reference as remote interface (t)
Applies methods (m)
10/14/2019

12. Proxy Layer: Stub
The stub is the client’s proxy for the remote object. It:
marshals arguments
unmarshals returned values
can be typed as any of the remote object’s remote interfaces
10/14/2019

13. Proxy Layer: Skeleton
The skeleton is the server’s proxy for the remote object. It:
Un-marshals arguments
dispatches actual method
marshals returned values
10/14/2019

14. Remote Reference Layer
An abstraction between the proxy layer and the transport layer.
It’s mostly reserved for future development:
replicated objects
persistent objects
connection recovery
10/14/2019

15. Transport Layer
This layer implements machine-to-machine communication.
It defaults to TCP/IP.
It can be overridden if you want to:
encrypt streams
compress streams
perform other security & performance enhancements
10/14/2019

16. Name Service
Remote object registers itself with name server: rmiregistry
Clients get reference to remote object by looking up object’s reference in rmiregistry.
There are 2 ways:
1 rmiregistry/machine for all applications on a well-known port.
Application has its own rmiregistry on its own port.
10/14/2019

17. Garbage Collection
A remote object can implement java.rmi.server.Unreferenced interface.
Method unreferenced is invoked when the object is no longer referenced: You can do “clean up”.
If network fails, an object may be wrongly collected.
Referencing a non-existent object causes a RemoteException to be thrown.
10/14/2019

18. Class Loaders Class loaders dynamically load classes as needed.
The RMIClassLoader loads the stub & skeleton classes, & any utility classes they need.
For stub & skeleton classes, it looks in the directory named in the system property: java.rmi.server.codebase, set by the -D flag of the java interpreter.
10/14/2019

19. Security
Eavesdropping: Secure Sockets Layer (SSL) can be used instead of TCP.
Misbehaving code: RMI requires a security manager.
Stand-alone applications set the security manager in main() .
System.setSecurityManager(new RMISecurityManager());
prohibits stub classes from doing anything over the network except loading necessary classes.
10/14/2019

20. Performance RMI is simple to use. RMI is not as fast as local MI.
RMI is not as fast as special-purpose communication protocols can be.
RMI may not be efficient enough for high-performance real-time applications, such as video streaming.
If you override TCP with a faster protocol, RMI may be fine.
10/14/2019

21. Summary: RMI Server To write an RMI server:
Define interface that extends Remote.
Define a class that extends UnicastRemoteObject & implements your remote interface.
Its main():
Registers itself in the registry.
10/14/2019

22. Summary: RMI Client Execute
System.setSecurityManager( new RMISecurityManager() );
Get a reference to the remote object by looking it up in rmiregistry.
Apply methods as though it were local.
Behind the scenes, object proxies, stubs & skeletons, are communicating.
10/14/2019