Java RMI

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Chapter 13

• Java RMI

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What is RMI?

• Remote Method Invocation (RMI) allows a Java
  object to call public methods on another java
  object which may exist on another machine.
• This protocol supports a client/server
  architecture (where a client is the calling
  object and the server is the object being called).

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How RMI Works

• Objects that want to communicate via RMI will use
  classes from java.rmi package.
• A server object that wants to publish its public
  methods, making them available to remote client
  objects, must create a Remote Stub class.
• A client will then use this stub to instantiate
  an object that is actually a pointer to the real
  object on the server.
• Methods called on this local object will invoke
  the real methods of that object on the server.

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RMI Architecture

• Client Object Any Java class (servlet, bean,
  applet, etc)
• Stub Object Remote interface for the server
  object, thats compiled and shipped as part of
  the client package.
• Server (Remote) Object Actual instantiated
  object that client is calling remotely.
• Skeleton Translates the call from the client
  (retired in JDK 2).

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RMI Architecture

• A message sent by a client is Marshalled over the
  network and recd by the server object. In JDK
  1.1, the Skeleton object receives the marshalled
  message. In JDK 2.0, the RMI libraries handle it.
• Transport protocols such as Java Remote Method
  Protocol (JRMP) are used, and will sometimes
  serialize an object before marshalling.

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RMI Architecture

• All network related code is contained within the
  RMI framework. Developers should not worry about
  how marshalling is done, for instance.
• Security is outside the scope of this lecture,
  but is important for real world applications that
  are accessible outside a firewall (client is
  outside, server is inside).

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Client Architecture

• Clients that need to access a server object must
  have the server objects stub class as part of
  its package.
• Once this is imported, a client application can
  instantiate an object of that type by doing an
  RMI or JNDI Lookup for that remote object, and
  then use this object instance as if it were just
  another local object.
• Client must be prepared to catch RMI-level
  exceptions, network errors or custom server
  exceptions.

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Client Architecture

• Example Stub Interface
• public interface DateTimeRemote extends
  Remote static String SERVER_NAME
  objDayTime
• String getDateTime() throws RemoteException
• void setDateTime( Date dt )
• throws RemoteException

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Client Architecture

• Example RMI Client
• ...
• static DateTimeRemote objDayTime
• public myClient extends Applet
• objDateTime (DateTimeRemote)
  Naming.lookup(DateTimeRemote.SERVER_NAME)
• println(objDateTime.getDateTime())
• ...

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Server Architecture

• To make a server object available remotely, you
  must
• have the RMIREGISTRY program running on the
  server
• create a java runnable class which extends a
  remoteobject class and implements the remote
  interface for that object
• A small class with a main() can be run which acts
  as the server, and receives requests by remote
  clients.

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Server Architecture

• Example RMI Server
• public class Server extends UnicastRemoteObject
• implements DateTimeRemote
• public Server() throws RemoteException
  super()
• public String getDateTime() throws
  RemoteException
• // code here ..
• public void setDateTime( Date dt ) throws
  RemoteException
• // code here
• public static void main(String args)
• DateTimeRemote objDateTime new Server()
• Naming.rebind(DateTimeRemote.SERVER_NAME,
  objDateTime)

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Example of Bidirectional Messaging via RMI

• This is a chat program, where each client
  registers with a single server, and each time any
  client sends a message, that message is
  broadcasted back to all registered clients.
• Components of this application are
• Message Receiver
• Message Server
• Server Applications
• Client Application(s)

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Message Receiver

• Extends the RMI Remote interface class
• Contains a print method that is used by the
  server to print each incoming message to each
  client that is registered.
• A client will IMPLEMENT this class (meaning itll
  have a public method called print).
• A server will instantiate each registered client
  and cast them to this class so it can use this
  method to make the client print. This is done by
  RMI.

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Message Server

• Extends the RMI Remote interface class.
• Contains the send method that is used by the
  client to send a message to the server.
• A server will implement this interface, and
  define the send() message that clients will use.
• A client will instantiate this class, and via
  RMI, will call the send method.

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Server Application

• Java application that takes requests from all
  registered client applications.
• Implements the MessageServer.send and
  MessageServer.register methods to be used by each
  client.
• The server will connect to each client via RMI
  and store that pointer in an enumerated class
  (assuming itll be connected to more then one
  client).
• Server will then use each clients
  MessageReceiver.print method to broadcast each
  message received to ALL clients.

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Server Application
Public class Server extends UnicastRemoteObject i
mplements MessageServer // c enumeration
of each registered client ... public void
send() RMI MessageReceiver m
(MessageReceiver) c.nextClient() m.print(
message ) ... public int register()
c.addClient() ...
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Client Application

• Java application, with a single frame containing
  a
• TestArea object used for receiving messages from
  the server
• TestField object used for sending messages to the
  server
• Implements the MessageReceiver.print() method so
  the server can call it.
• Connects to the server via RMI and uses the
  servers send message to deliver a new message to
  the server.

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Client Application
Public class Client extends Frame implements
MessageReceiver, etc public static void
main() RMI MessageServer s (MessageServer)
Naming.lookup( MessageServer.SERVER_NAME)
... when finished entering new
message s.send( clientname, message )
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Homework

• Implement the Bidirectional Messaging server with
  the following enhancements
• User can specify how message is sent, either
  broadcasted or point-to-point.
• For broadcasting, the algorithm in the book
  applies.
• For point-to-point, another client name will be
  entered and only that client will see the message
  that is sent.