CS4273: Distributed System Technologies and Programming I

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CS4273 Distributed System Technologies and
Programming I

• Lecture 12 Java RMI (Remote Method Invocation)

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Client-Server Programming without RMI (RPC)
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Remote Method Invocation (RMI)
Stub objects are auto-generated by rmic from
interface definition
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An Example of RMI

• A service routine
• public class ProductImpl extends
  UnicastRemoteObject implements Product
• public ProductImpl (String pd) throws
  RemoteException
• descript pd
• public String getDescription ()
• return ProductDescipt descript .
• private String descript
• .
• A client RMI call
• Product p Naming.lookup(url-name)
• String descript p.getDescription ()
• System.out.println(descript)

• An interface
• interface Product extends Remote
• String getDescription () throws
  RemoteException
• String placeOrder () throws RemoteException
• int getOrderNo () throws RemoteException
• N.B. every rmi method must throw exceptions, due
  to the un-reliability of RMI.

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Inheritance Graph of Interface and Remote servers
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Naming Conventions for RMI classes and Files

• Interface def file no suffix (e.g. Product)
  
• Interface impl file Impl suffix (e.g.
  ProductImpl)
• Server program file Server suffix (e.g.
  ProductServer)
• Client program file Client suffix (e.g.
  ProductClient)
• Stub program file _Stub suffix (e.g.
  ProductImpl_Stub)

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Client-Server Binding in RMI

• An RMI registration server (rmiregistry) on a
  site is responsible for locating any appl servers
  on that site. This rmiregistry should run on the
  server site before any appl servers start.
• An appl server needs to register itself to
  rmiregistry on the same site when it is created.
• A client looks up an appl server from rmiregistry
  on that site by specifying the servers URL-addr
• (rmi//svr-site/svr-name).
• A naming class
• java.rmi.server.Naming
• provides an interface for servers
• to reg / de-reg and for clients to look up
  servers.

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An Example of RMI

• A simple interface implementation program
• import java.rmi. import java.rmi.server. //
  file rmi/svr/ProductImpl.java
• public class ProductImpl extends
  UnicastRemoteObject implements Product
• private String descript
• public ProductImpl (String pd) throws
  RemoteException
• descript pd
• public String getDescription ()
• return "ProductDescript"
  descript "."
• A server program // creates an impl obj and
  registers it
• public class ProductSvr // file
  rmi/svr/ProductSvr.java
• public static void main (String args)
• try ProductImpl p1 new
  ProductImpl (Jias Warehouse")
• Naming.rebind
  ("warehouse", p1) // register svr by name
• catch (Exception e)
  System.out.println("Error" e)

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Compile Server Program

• We have the following files in servers
  directory
• Product.java // interface file
• ProductImpl.java // interface implementation
  file
• ProductSvr.java // server program file
• Compile the interface file javac Product.java
• It can be skipped. When compiling
  ProductImpl.java, it is auto-compiled.
• Compile the server implementation javac
  ProductImpl.java
• Generate stub class using RMIC rmic
  ProductImpl
• It generates file ProductImpl_Stub.class (for
  both client and server).
• N.B. ProductImpl java must be compiled before
  using rmic.
• Compile the server program javac
  ProductSvr.java
• N.B. the server links procedures in
  ProductImpl_Stub at run-time.

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A Simple Client Program

• // file java/rmi/clnt/ProductClnt.java
• //usage java ProductClnt server-hostname
• import java.rmi. import java.rmi.server.
• public class ProductClnt
• public static void main (String args)
• String url "rmi//"args0
• try
• Product p1 (Product)
  Naming.lookup (url "/warehouse")
• System.out.println(p1.getDescript
  ion())
• System.out.println(p1.placeOrder(
  ))
• System.out.println("OrderNo "
  p1.getOrderNo())
• catch (Exception e)
  System.out.println("Error" e)

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Build Client Program

• Copy interface file Product.java to local
  directory.
• Copy the client stub file ProductImpl_Stub.class
  (generated by rmic) to the local directory.
• Compile the client source code javac
  ProductClnt.java
• when compiling ProductClnt.java, it automatically
  compiles the interface Product.java if it has not
  been compiled yet. The compiler locates the
  interface Product.java by naming conventions.
• client calls stub procedures in
  ProductImpl_Stub.class at run-time. It is in a
  separate class file, without linking to the
  client at compilation time.

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Run the Example of RMI

• Start RMI registry program on the server site
• rmiregistry
• Start the server program on the server site
• java ProductSvr
• Start the client program on the client site
• java ProductClnt args

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Naming class for client-server Binding

• RMI uses class java.rmi.server.Naming for
  client-server binding
• functions, which includes
• static Remote lookup (String url)
• returns the remote object for the URL
• static void bind(String name, Remote obj)
• binds name to the remote object obj
• static void rebind(String name, Remote obj)
• binds name to remote obj replaces any
  existing one
• static void unbind(String name)
• unbinds the name
• static String list(String url)
• returns an array of strings of urls registered
  at the registry server.

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View Database of RMI Registration Server

• // file java/rmi/showBinding.java
• import java.rmi. import java.rmi.server.
• public class showBindings
• public static void main ( String args)
  
• try
• String bindings
  Naming.list("")
• for (int i 0 i lt
  bindings.length i)
• 
  System.out.println(bindingsi)
• catch(Exception e)
  System.out.println("Error"e)

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Summary of Programming with RMI

• On Server Site
• write an interface file.
• write the interface implementation program, which
  implements all methods in the interface.
• write a server program, which initializes a
  server-impl object and registers the server
  object with the registry.
• compile all the source files on server side.
• generate stub class by using rmic.

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Summary of Programming with RMI (Cont.)

• On Client Site
• copy the interface file and the generated stub
  class file to the local directory (from the
  server site).
• write a client program that uses rmi.
• compile the client program.
• Run the Client Server
• run the registry server on server site.
• run server and client on server and client sites
  respectively.

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Applets Using RMI
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Programming Applets using RMI

• The server (and rmiregistry server) must run on
  the Web server, bcs applets can only communicate
  with their home server!!!
• Copy the interface file (Product.java) and the
  stub class file (ProductImpl_Stub.class) to the
  local directory.
• Write applet "ProductApplet.java" and compile it.
• Run "rmiregistry" and server "ProductSvr" on the
  Web Server site.
• Run applet in Web Browser.
• N.B. No need to change the server program
• when moving a client to an applet.

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An Example of RMI Applet Program

• // file java/rmi/applet/ProductApplet.java
• import java.rmi. import java.rmi.server.
  import java.applet.Applet
• public class ProductApplet extends JApplet
• private String message "Initializing"
• public void init()
• String url "rmi//"
  getCodeBase().getHost() // web-site !!!
• try Product p1 (Product)
  Naming.lookup (url "warehouse")
• message p1.getDescription()
  // rmi
• repaint()
• catch (Exception e)
  System.out.println("Error" e)
• public synchronized void paint(Graphics g)
• g.drawString(message, 5, 50)