Internet Software Development

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Internet Software Development

• Remote Method Invocation
• Paul Krause

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Lecture 10 - RMI

• Introduction to RMI
• Simple Example - DivideServer
• Demo of this example
• Review a more complex example -
  StudentEnrollment

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What is RMI?

• A high-level networking technology for
  distributed Java applications
• Allows a Java object executing on one machine to
  remotely invoke methods in an object executing on
  a second machine
• Such method calls have the appearance of
  operating on objects in the same program

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Skeletons and Stubs
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Stub

• Presents same interfaces as remote object
• Works with JVM and RMI to serialise arguments in
  the remote method call
• Receives results from the remote method call and
  returns them to the client object

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Skeleton

• Receives the remote method call and associated
  arguments
• Works with JVM and RMI to deserialize arguments
  in the remote method call
• Invokes appropriate methods in the remote object
• Receives return value (if any) from the method
  call, serializes it and returns it to the client
• No longer needed in Java 2

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Referencing remote objects

• A Registry object maintains a mapping from Server
  names to remote object references
• We will use the example DivideServer
• The remote object is named via a URL using the
  RMI protocol
• rmi//hostport/server
• host - IP address or name of server machine
• port - optional port number of the Registry on
  above
• server - name of the remote server
• e.g. rmi//127.0.0.1/DivideServer

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Key Classes and Interfaces

• Remote Interface
• Naming Class
• RemoteObject Class
• RemoteServer Class
• UnicastRemoteObject Class
• RemoteException Class

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Remote Interface

• In the java.rmi package
• Contains no constants or methods
• It is used to designate which interfaces are
  remote -
• remote interfaces must extend java.rmi.Remote
• remote objects must implement a remote interface

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Naming Class

• In the java.rmi package
• Three methods to associate names with remote
  objects (used at the Server side)
• static void bind(String name, Remote robj)
• static void rebind(String name, Remote robj)
• static void unbind(String name)
• One method to obtain a name for a remote object
  (used at the Client side)
• static Remote lookup(String url)

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Remote Objects

• RemoteObject
• extends java.lang.Object to provide correct
  behaviour for remote objects
• RemoteServer
• an abstract class that extends RemoteObject.
  Defines the methods needed to create and export
  remote objects
• UnicastRemoteObject
• a concrete subclass of RemoteServer. Extend this
  class to make a remote object

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Simple Client/Server Application

• Client program takes three arguments
• IP address or name of remote server
• two numbers
• The two numbers are passed to the server
• The server divides one into the other and returns
  the result
• From JavaBeans Programming from the Ground Up,
  Joseph ONeil and Herb Schildt

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DivideServer.java

• import java.rmi.
• public interface DivideServer extends Remote
• double divide(double d1, double d2) throws
• RemoteException

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Remote Interfaces

• The remote interface must extend Remote
• Remote methods can throw a RemoteException
• We now require a server that implements the
  interface
• By convention, we name the implementation of
  DivideServer
• DivideServerImpl
• Remote objects must extend UnicastRemoteObject

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DivideServerImpl.java

• import java.rmi.import java.rmi.server.pub
  lic class DivideServerImpl extends
  UnicastRemoteObject implements DivideServer
  public DivideServerImpl( ) throws
  RemoteException public double
  divide(double d1, double d2) throws
  RemoteException return d1/d2

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DivideServerApp.java

• This is the main program for the server side
• Its primary function is to bind the name
  DivideServer to an instance of DivideServerImpl
• Note that we need to start this server program
  running before the client program is invoked

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DivideServerApp.java

• Core part is
• DivideServerImpl divideServerImpl
• divideServerImpl new DivideServerImpl()
• Naming.rebind("DivideServer",
  divideServerImpl)

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DivideServerApp.java

• import java.net.
• import java.rmi.
• public class DivideServerApp
• public static void main(String args)
• try
• DivideServerImpl divideServerImpl
• divideServerImpl new DivideServerImpl()
• Naming.rebind("DivideServer",
  divideServerImpl)
• catch(Exception ex)
• ex.printStackTrace()

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DivideCleint.java

• Looks up the object that is bound to the RMI URL
• The lookup method of the Naming class returns an
  object of type Remote
• This must then be cast to a DivideServer in order
  to be able to access the divide method

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DivideCleint.java

• DivideServer divideServer
• divideServer (DivideServer)Naming.lookup(divideS
  erverURL)
• // Invoke remote method and display results
• double result divideServer.divide(d1, d2)
• System.out.println("The result is " result)

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DivideCleint.java

• import java.rmi.
• public class DivideClient
• public static void main(String args)
• try
• // Make rmi URL to name DivideServer
• String divideServerURL
• divideServerURL "rmi//" args0
  "/DivideServer"
• // Obtain reference to that remote object
• DivideServer divideServer
• divideServer (DivideServer)Naming.lookup(divid
  eServerURL)

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DivideCleint.java

• // Display numbers
• System.out.println("The first number is "
  args1)
• double d1 Double.valueOf(args1).doubleValue(
  )
• System.out.println("The second number is "
  args2)
• double d2 Double.valueOf(args2).doubleValue(
  )
• // Invoke remote method and display results
• double result divideServer.divide(d1, d2)
• System.out.println("The result is " result)
• catch(Exception ex)
• ex.printStackTrace()

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Executing the Application

• Use javac to compile all files
• Open a command prompt and change to the working
  directory
• Use the rmic compiler to produce a stub class for
  the remote server class
• rmic -v1.2 DivideServerImpl
• Copy all files to the client machine
• exercise You dont need all of them on the
  client. Which ones do you need

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Executing the Application

• open a command prompt on the server and start the
  RMI registry
• rmiregistry
• open a second command prompt on the server
  machine and run the server application to bind
  the remote server object to the registry
• java DivideApp
• Start the client (again, use a command prompt)
• java DivideClient server1 8 2

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The result

• You should get the following output at the
  clients command prompt
• The first number is 8
• The second number is 2
• The result is 4.0
• Notes
• Use IP address 127.0.0.1 if experimenting on a
  single machine
• On Windows 2000 you can find out the IP address
  of a machine using hostname and ipconfig/all