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Introduction to
Java Remote
Method
Invocation
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Remote Objects
Client
Object
Java Virtual Machine
TCP
Remote
Object
Java Virtual Machine
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What is RMI?
• RMI is an RPC system for OOP languages
• RMI allows programs to
– Hold a reference to an object on a remote system
– Invoke methods of the remote object
• Client Server Architecture
– Server holds the object
– Client holds a small stub that accessed the object on the sever
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RMI Layers
Client
Object
Stub
Remote
Object
TCP
Skeleton
Remote Reference Layer
Remote Reference Layer
Transport Layer
Transport Layer
Java Virtual Machine
Java Virtual Machine
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Remote Objects
• Remote Objects
– Live on Server
– Accessed as if they were local
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Remote References and Interfaces
• Remote References
– Refer to remote objects
– Invoked on the client exactly like local object references
• Remote Interfaces
– Declare exposed methods
– Implemented on client
– Like a proxy for the remote object
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Stubs and Skeletons
• Stub
– Lives on client
– Pretends to be remote object
• Skeleton
–
–
–
–
Lives on server
Receives requests from stub
Talks to true remote object
Delivers response to stub
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Remote Interfaces and Stubs
Remote Interface
Client
Stub
Skeleton
Remote Object
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Registries
• Name and look up remote objects
• Servers can register their objects
• Clients can find server objects and obtain a remote
reference
• Registry is a running process on a host machine
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RMI System Architecture
Client
Object
Remote
Object
Stub
Skeleton
Server
Client Virtual Machine
Server Virtual Machine
Registry Virtual Machine
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RMI Flow
Client Virtual Machine
Server Virtual Machine
Remote
Object
Client
Object
1
Skeleton
Stub
Server
1. Server creates remote object
2. Server registers remote
objects
2
myMessage
Registry Virtual Machine
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RMI Flow
Client Virtual Machine
Server Virtual Machine
Remote
Object
Client
Object
Skeleton
Stub
Server
3
4
myMessage
Registry Virtual Machine
3. Client requests object from registry
4. Registry returns remote reference
(and stub gets created)
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RMI Flow
Client Virtual Machine
Client
Object
Server Virtual Machine
7
5
Remote
Object
6
Skeleton
Stub
Server
myMessage
Registry Virtual Machine
5. Client invokes stub method
6. Stub talks to skeleton
7. Skeleton invokes remote object
method
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RMI Usage
• Start registry
• Start server
• Run client
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Creating Remote Objects
• Define a remote Interface
– Extends java.rmi.Remote
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface Message extends Remote {
void sayHello(String name) throws RemoteException;
}
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Creating Remote Objects
• Define a class that implements the remote interface
– Extends java.rmi.RemoteObject or java.rmi.UnicastRemoteObject
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
public class MessageImpl extends UnicastRemoteObject implements Message
{
public MessageImpl() throws RemoteException {
}
@Override
public void sayHello(String name) throws RemoteException {
System.out.println("hello " + name);
}
}
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Create the server
• Create a new instance of the remote object
• Registers it in the registry with unique name
private void startServer() {
try {
// create on port 1099
Registry registry = LocateRegistry.createRegistry(1099);
// create a new service named myMessage
registry.rebind("myMessage", new MessageImpl());
System.out.println("system is ready");
} catch (Exception e) {
e.printStackTrace();
}
}
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Create the Client
• Find the registry
• Lookup for name to obtain a reference
• Cast the reference to the appropriate Remote Interface
private void doTest() {
try {
// fire to localhost port 1099
Registry myRegistry = LocateRegistry.getRegistry(“localhost",
1099);
// search for myMessage service
Message impl = (Message) myRegistry.lookup("myMessage");
// call server's method
impl.sayHello("HND");
System.out.println("Message Sent");
} catch (Exception e) {
e.printStackTrace();
}
}
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Limitations of RMI
• Java Only
– Can use JNI on the server
• Uses TCP, not UDP
• At least two sockets per connection