Remote Method Invocation

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Remote Method Invocation 
Developing Remote Object Programs  Hea
ther Jenkins Daniel Ferrell University of
North Carolina at Wilmington
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Before Distributed Objects

• Began with monolithic mainframe systems
• Each system contained its own logic
• System was unable to share data with other
  systems
• Monolithic systems were inefficient and costly
• Reusing code was difficult
• Usually it meant copying a segment of code
• Modifying it
• Then deploying the modified copy
• A change to one meant a change to all

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Distributed Objects

• Divides today's still monolithic client/server
  applications into self-managing components, or
  objects, which can interoperate across dissimilar
  networks and operating systems.
• EXAMPLE
• Programs that can be written which enable code on
  other computers to send messages to them
• oldUpdateProxy.adjust (oldVal, newVal)

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Distributed Objects Applications

• CORBA
• lets one build distributed mixed object systems
• DCOM
• is a framework for distributed objects on the
  Microsoft platform
• PYRO
• is a framework for distributed objects using
  the Python programming language
• Distributed Ruby (DRb)
• is a framework for distributed objects using the
  Ruby programming language
• RMI
• is a Java application programming interface for
  performing the object equivalent of remote
  procedure calls.

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RMI Remote Method Invocation

• Pure Java technology that is restricted to the
  development of distributed objects expressed only
  in Java
• A mechanism that allows you to invoke a method or
  an object that is located in another address
  space
• Provides a homogeneous environment
• It supports features such as code portability,
  dynamic code loading, Java security, and garbage
  collection

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RMI Remote Method Invocation

• Registry
• The client communicates to the server and
  requests an object. Once the registry has found
  the object, then it directs the request to the
  correct object.
• Security
• INSECURE with RMI
• Spoofing someone acts as a client and could
  potentially read data that the remote object has
  access to.
• Security Manager RMISecurityManage()
• Garbage Collectors
• Keeps a count of the number of references to
  each object. When the number reaches 0, the
  memory space is de-allocated for future use.

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RMI Layer Terminology

• Stubs/Skeletons
• A stub for a remote object acts as a client's
  local representative or proxy for the remote
  object.
• Remote Reference Layer
• determines whether the request is a Unicast or
  Multicast Protocol.
• Transport
• sets up and manages the request

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Alternatives to RMI

• TCP Sockets
• implemented using TCP sockets to allow
  distributed components of the system to
  communicate with each other.
• CORBA
• - Common Object Request Broker Architecture
  More Expensive. No free development kit. Not
  Java.
• RCP Library
• Remote Procedure Call Better suited for linear
  programming.

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RMI Expensive? Cheap? Easy?

• Java is a widely known OOP
• - Development kits and other resources freely
  available.
• Cheaper than other alternatives of middleware
• - Cheaper than CORBA.
• JDK Development Kit 1.1
• - Development is can easily be started.

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Implementation of RMI

• Remote Objects , Communication, Load Definitions
  for Objects.

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Example A Simple Hello World-
RemoteInterface.Java

• import java.rmi.Remote
• import java.rmi.RemoteException
• public interface RemoteInterface extends Remote
• String REGISTRY_NAME "RMI_Example"
• int REGISTRY_PORT 3273
• String getMessage() throws RemoteException

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Server.Java

• import java.rmi.registry.LocateRegistry
• import java.rmi.registry.Registry
• import java.rmi.server.UnicastRemoteObject
• public class Server implements RemoteInterface
• public String getMessage()
• return "Hello World"
• public static void main(String args)
• try
• Registry registry
  LocateRegistry.getRegistry(RemoteInterface.REGISTR
  Y_PORT)
• RemoteInterface remoteReference
• (RemoteInterface)
  UnicastRemoteObject.exportObject(new Server())
• registry.rebind(RemoteInterface.REGIST
  RY_NAME, remoteReference)

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ClientApplet.Java

• import java.rmi.registry.LocateRegistry
• import java.rmi.registry.Registry
• import javax.swing.JApplet
• import javax.swing.JLabel
• public class ClientApplet extends JApplet
• public void init()
• try
• Registry registry
• LocateRegistry.getRegistry(getCode
  Base().getHost(), RemoteInterface.REGISTRY_PORT)
• RemoteInterface remoteReference
• (RemoteInterface)
  registry.lookup(RemoteInterface.REGISTRY_NAME)
• getContentPane().add(new
  JLabel(remoteReference.getMessage()))
• catch (Exception e)

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How To Put It All Together

• 1. Create HTML File For Applet
• ltHTMLgt
• lttitlegtHello Worldlt/titlegt
• ltcentergt lth1gtHello Worldlt/h1gt lt/centergt
• The message from the HelloServer is
• ltpgt
• ltobject classid"javaClientApplet.class"
• type"application/x-java-applet"
• height"50" width"150" gt
• lt/objectgt
• lt/pgt
• lt/HTMLgt

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Now Run!

• 1. Compile the classes.
• - javac .java
• Run RMI
• - rmic Server
• Start RMI Registry (Open Port)
• start rmiregistry 3273
• (unix) rmiregistry 3273
• Start the server
• start java Server
• (unix) java Server
• Open HTML!

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RMI Questions?

• Questions?