Cup 13: RMI

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Cup 13 RMI

• Special Topics Java

Dr. Tim Margush Department of Mathematics and
Computer Science The University of Akron
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Remote Method Invocation

• Remote - Objects exist on different systems
  (Distributed or Client-Server Computing)
• Remote Method - A method belonging to a class
  called a server
• RMI - A method in a client class calls a method
  in a server class

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RMI Registry

• A separate application running on machine A
  (providing remote objects)
• Links the client and server together
• Server starts and registers in the RMI Registry
  of machine A
• Client on machine B contacts RMI Registry via a
  socket with a request
• If allowed, a reference to the remote object is
  returned

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Stubs

• The client's reference to a remote object allows
  method calls across the internet
• Arguments and return values are passed via object
  serialization
• A Skeleton object is created on the server
  machine to perform the actual method calls and
  communication with the server object
• The Security Manager supervises the communication
  between skeleton and server

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RMISecurityManager

• RMI interactions are governed by some security
  manager
• Client Applets use the Applet Security Manager
• Client Applications require installation of an
  RMI Security Manager
• A default manager is provided in java.rmi
• A bootstrap process is typically used to install
  the security manager before running an application

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Arguments and Parameters

• Passing an object to a local method is
  accomplished by copying the reference to the
  object

• RMI requires copying the object to the remote
  machine where it is reconstructed (in the
  skeleton)
• The skeleton invokes the remote method, passing a
  reference to the reconstructed object

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A Simple RMI Quote Server

• Remote Interface
• Defines the public methods of the remote object
• The remote object will implement this interface
• Remote methods must throw RemoteException, in
  case an error occurs during the RMI process

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The QuoteProtocol

• This is the interface we will implement
• import java.rmi.
• public interface QuoteProtocol extends Remote
• public String getQuote(int qnum)
• throws RemoteException
• public int getMaxQuoteNumber()
• throws RemoteException

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The Remote Object

• import java.rmi.
• import java.rmi.server.UnicastRemoteObject
• //UnicastRemoteObject provides server-type stuff
• public class QuoteOfTheDay
• extends UnicastRemoteObject
• implements QuoteProtocol
• //RMI requires this default constructor
• public QuoteOfTheDay()
• throws RemoteException

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Implement QuoteProtocol

• Q is a private array of Strings containing the
  quotes to be served (declaration omitted)
• public String getQuote(int qnum)
• return Qqnum //no check for limit - sloppy!
• public int getMaxQuoteNumber()
• return Q.length-1

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Remote Bootstrap

• public static void main(String args)
• System.setSecurityManager
• (new RMISecurityManager())
• try
• QuoteOfTheDay qod new QuoteOfTheDay()
• Naming.rebind("quoteoftheday", qod)
• catch (Exception e)
• System.out.println(e.getMessage())

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Creating the Stub and Skeleton

• After compiling the class...
• javac QuoteOfTheDay.java
• Create the stub and skeleton
• rmic QuoteOfTheDay
• QuoteOfTheDay_Stub.class
• QuoteOfTheDay_Skel.class
• (be sure CLASSPATH gives access to java's
  Classes.zip and the QuoteOfTheDay.class file)

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Registering a Remote Object

• Install a Security Manager
• RMISecurityManager
• Instantiate the remote object
• remObjRef new objRemote()
• Bind the remote object to the registry
• Naming.rebind("someName", remObjRef)
• The RMI Registry must be running before binding
  can occur

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Starting the Registry

• The RMI Registry is an application found in the
  jdk's bin directory
• Run it as a background process
• rmiregistry port
• default port is 1099
• Be sure the CLASSPATH variable for this session
  includes the location of the Stub and Skeleton!

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Getting in a Bind

• With the Registry process active, you can now
  register (bind) remote objects
• java QuoteOfTheDay
• Runs main, installs security manager,
  instantiates remote object and registers it
• This will work only on a LAN due to a bug
• Workaround - use this switch to fully qualify the
  hostname of the server
• java -Djava.rmi.server.hostnamesomeHost.edu
  QuoteOfTheDay

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RMI Client

• The client must instantiate the stub for the
  remote object
• The remote interface datatype is used for this
  purpose
• import java.rmi.
• import java.rmi.registry.
• public class Quote
• static QuoteProtocol qod

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Instantiating the Stub

• Naming.lookup finds the registered object by
  name, instantiates a Stub, and returns a
  reference it
• try
• String regName "rmi//130.101.87.25/quoteofthed
  ay"
• qod (QuoteProtocol) Naming.lookup(regName)
• catch (Exception ex)
• System.out.println("Cannot get remote
  object " ex)

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Invoking a Remote Method

• The Stub object allows the client to invoke any
  method in the remote interface
• try
• int num qod.getMaxQuoteNumber()
• String someQuote qod.getQuote(num)
• System.out.println("Quote " num " is "
  aQuote)
• catch (Exception ex)
• System.out.println("Error " ex)

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An Applet Client

• Set up the RMI access in the Applet's Start
  method
• regName "rmi//"
• getCodeBase().getHost() "/quoteoftheday"
• qod (QuoteProtocol)Naming.lookup(regName)
• MQN qod.getMaxQuoteNumber()