Download Java RMI overview I

INF5040
Work group meeting no. 1
Åshild Grønstad Solheim
(aashig@simula.no)
Outline
•
•
Some smaller exercises (voluntary)
Mandatory programming project:
– EJB / JBoss
– Changed from CORBA to EJB, new exercises
– No regular work group meetings during project period
•
Student presentations followed by discussions
•
Work in groups of 3: indicate preferences on who to work with
and what subject to present
•
No work group meeting next week
•
Today: Java RMI - overview, code example, exercise
4/5/2001
2
Java RMI overview I
•
Java RMI (Remote Method Invocation) in lecture next week
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Middleware:
– A software layer
– Provides a convenient programming model for application
programmers
– Masks heterogenity of underlying networks, hardware, operating
systems and programming languages
•
Java RMI is middleware
– Provides components (e.g proxies, dispatchers) that hide the details
of marshalling, message passing and locating remote objects from
client and server programmers
– Only for the Java language
•
Java only => Interface Definition Language (IDL) not necessary. Ref.
CORBA (Common Object Request Broker Architecture)
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Java RMI overview II
• Request – reply protocol:
– Message structure : MessageType, CallId,
Remote Object Reference, Method,
Arguments
• Java RMI extends Java object model to support
distributed objects
• Invoke remote methods using familiar syntax
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Java RMI overview III
• Remote interface
– Extends Remote
– Public methods which throw RemoteExceptions
• Servant class
– Implements the remote interface
– Extends UnicastRemoteObject (or Activatable)
• Parameter and result passing
– Non-remote objects passed by value
– Remote objects passed by reference
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Java RMI overview IV
• Marshalling
– Translate data structures and primitive values to a form
suitable for transmission (external data representation)
– Java object serialization
– Unmarshalling: the reverse
• RMIregistry
– Non-persistent name server
– Rebind(String name, Remote obj), bind (String name,
Remote obj), unbind(String name, Remote obj),
lookup(String name), list()
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Java RMI overview V
• Client invokes remote method
• Server (contains main () method and servant
class (implements remote interface))
• Callback to client possible (ref. exercise)
– Server notifies client about event
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Java RMI architecture I
• NB! Skeleton no longer required.
• Generic dispatcher used instead
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Java RMI architecture II
• Stub/Proxy
– Client side stand-in for the remote object, same interface
– Marshals method and arguments into request message
– Unmarshals reply message
•
Generic dispatcher
– Reflection allows same dispatcher for all classes of remote objects
– Skeleton not required
– Unmarshals the request message
– Invokes the method
– Marshals result or exeption into reply message
• Remote Reference Module
– ROR => local object reference
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Java RMI development process
•NB! Skeleton no longer required.
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10
A distributed Hello World application
- The remote interface:
import java.rmi.*;
public interface HelloInterface extends Remote {
public String say() throws RemoteException;
}
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The servant class
(implements the remote interface):
import java.rmi.*;
import java.rmi.server.*;
public class Hello extends UnicastRemoteObject implements HelloInterface {
private String message;
public Hello (String msg) throws RemoteException {
message = msg;
}
public String say() throws RemoteException {
return message;
}
}
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The server:
import java.rmi.*;
public class HelloServer {
public HelloServer () { }
public static void main (String[] argv) {
try {
Naming.rebind ("Hello", new Hello ("Hello, world! Welcome to INF5040!"));
System.out.println ("Hello Server is ready.");
} catch (Exception e) {
System.out.println ("Hello Server failed: " + e);
}
}
}
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The client:
import java.rmi.*;
public class HelloClient {
public HelloClient () { }
public static void main (String[] argv) {
try {
HelloInterface hello = (HelloInterface) Naming.lookup ("//"+argv[0]+"/Hello");
System.out.println (hello.say());
} catch (Exception e) {
System.out.println ("HelloClient exception: " + e);
}
}
}
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Compilation
1. javac *.java
2. rmic Hello
•
rmic generates client proxy from compiled
server class, not from remote interface
definition.
•
If rmic does not find the class files, set
CLASSPATH to current directory:
setenv CLASSPATH .
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Running the distributed application
•
Server:
1. rmiregistry &
2. java HelloServer &
•
Client:
java HelloClient <server name – ex. kolme.ifi.uio.no>
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Discussion in groups
• How to implement callback in the Hello world
application?
• Server side: Suggest an additional method for printing
a greeting and performing a callback to the client.
– NB! Unrealistic test-application!
– Normally register to receive callback when event occurs
• Client side: Suggest an implementation for receiving
the callback from the server.
• Present and discuss proposals
• Hint: p. 214 in Coulouris..
4/5/2001
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Until next meeting
• Find source code and Java RMI tutorial on
web
• Run the distributed Hello World application,
extend with callback to client
• Hopefully the subjects to present will be ready
• Email preferences on group and subject to
present
• Remember: no work group meeting next week
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