G53ACC Advanced Computer Communications

1
G53ACC Advanced Computer Communications

• Remote Method Invocation
• Dr. Chris Greenhalgh

2
Contents

• Parameter Passing in RMI
• Real RMI Dynamic Loading
• RMI Callback
• RMI Summary

3
Argument Passing in Java RMI

• All arguments must be either
• Primitive data types (passed by value)
• Objects that implement java.io.Serializable
  interface
• an identical copy of the object is made, i.e. all
  fields are copied class can be remotely
  downloaded (e.g. HTTP) if java.rmi.server.codeba
  se property is set properly
• any change to this objects state at the receiver
  is reflected only in the receivers copy, not in
  the original instance
• Remote objects that implement the Remote
  interface
• A remote object reference, a stub is passed
• operations on the object will themselves be
  remote, e.g. callbacks

4
Dynamic Class Downloading

• Stubs can be made available to the clients
  dynamically, normally via a web server using HTTP
  protocol
• Doing so allows changes to be made in the remote
  methods without affecting the client programs
• Security measures are needed to protect both the
  client and server if dynamic class downloading is
  required
• A Java Security Manager should be instantiated
  with the proper security rules set in a java
  security policy file
• java
• -Djava.rmi.server.codebasehttp//sepang.nottingha
  m.edu.my/hsooihock/
• -Djava.security.policyjava.policy TicketServer

5
RMI Callbacks

• A client will have to poll a passive server
  repeatedly if it needs to keep abreast of any
  current changes and updates
• Some applications require the server to actively
  initiate communication to the clients upon
  occurrence of certain events
• Monitoring, Games, Auctioning, Voting/Polling,
  Message/Bulletin Board
• RMI Callbacks can be implemented where registered
  clients can get server notifications (callbacks)
  when needed

6
RMI Callback Requirements

• A remote method for clients registrations for
  callbacks
• A remote interface specifying methods for
  accepting callbacks in addition to the server
  side interface.
• The client must be a subclass of
  UnicastRemoteObject and implements the callback
  interface with the callback methods
• The client registers itself for callback in its
  main method
• The server invokes the clients remote method
  upon the occurrence of the anticipated events

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RMI Callback Example (i)

• import java.rmi.
• /
• _at_()CallCity.java
• _at_author Qusay H. Mahmoud
• /
• public interface CallCity extends Remote
• int getPopulation(String cityName) throws
  RemoteException
• int getTemperature(String cityName) throws
  RemoteException
• void register(Callback cb) throws
  RemoteException

8
RMI Callback Example (ii)

• /
• _at_()Callback.java
• _at_author Qusay H. Mahmoud
• /
• public interface Callback
• void tempChanged(String cityName, int
  temp)throws RemoteException

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RMI Callback Example (iii)

• import java.util.
• import java.rmi.
• import java.rmi.server.UnicastRemoteObject
• /
• _at_()CallCityImpl.java
• _at_author Qusay H. Mahmoud
• /
• public class CallCityImpl extends
  UnicastRemoteObject implements CallCity
• private String name
• Vector list new Vector()
• public CallCityImpl() throws RemoteException
• super()
• public CallCityImpl(String str) throws
  RemoteException

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• public int getPopulation(String cityName)
  throws RemoteException
• if (cityName.equals("Toronto"))
• return 10
• else if (cityName.equals("Ottawa"))
• return 2
• else
• return 100
• public int getTemperature(String cityName)
  throws RemoteException
• return 1
• public void register(Callback cb) throws
  RemoteException
• list.addElement(cb)
• public void setTemp(String cityName, int tmp)
  

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• public static void main(String argv)
• System.setSecurityManager(new
  RMISecurityManager())
• try
• CallCityImpl obj new CallCityImpl("CityS
  erver")
• Naming.rebind("//hamming/CityServer",
  obj)
• System.out.println("CityServer bound in
  registry")
• catch (Exception e)
• e.printStackTrace()

12
RMI Callback Example (iii)

• import java.rmi.
• import java.net.
• /
• _at_()CallCityApp.java
• _at_author Qusay H. Mahmoud
• /
• public class CallCityApp implements Callback
• public static void main(String argv)
• int pop 0
• Callback c (Callback) new
  CallCityApp()
• CallCity obj null
• try
• obj (CallCity)Naming.lookup("//hamming/CityS
  erver")
• pop obj.getPopulation("Toronto")
• catch (Exception e)

13

• // register the callback
• try
• obj.register(c)
• catch(RemoteException e)
• System.out.println("Error ")
• e.printStackTrace()
• //implement Callback interface
• public void tempChanged(String cityName, int
  temp) throws RemoteException
• System.out.println("Dynamic Update ")
• System.out.println("CityName "cityName)
• System.out.println("Temp "temp)