CS代写 RMI 433-652 :-)";

Distributed Objects Programming – Remote Invocation
Some concepts are drawn from Chapter 5
Sun Java online tutorials:

Cloud Computing and Distributed Systems (CLOUDS) Laboratory School of Computing and Information Systems
The University of Melbourne, Australia
http://www.cloudbus.org/652
Co-contributors: , , ,
http://java.sun.com/docs/books/tutorial/rmi/

◼ Introduction to Distributed Objects
◼ Remote Method Invocation (RMI) Architecture
◼ RMI Programming and a Sample Example: ◼ Server-Side RMI programming
◼ Client-Side RMI programming
◼ Advanced RMI Concepts ◼ Security Policies
◼ Exceptions
◼ Dynamic Loading
◼ A more advanced RMI application ◼ Math Server
◼ RPC and Summary 2

Introduction
◼ We cover high-level programming models for distributed systems. Two widely used models are:
◼ RemoteProcedureCall(RPC)-anextensionoftheconventional procedure call model
◼ Remote Method Invocation (RMI) – an extension of the object-oriented programming model.
Applications
Remote invocation, indirect communication
Underlying interprocess communication primitives: Sockets, message passing, multicast support, overlay networks
TCP and UDP
Middleware layers

Request-Reply Protocol
◼ Exchange protocol for the implementation of remote invocation in a distributed system.
◼ We discuss the protocol based on three abstract operations: doOperation, getRequest and sendReply
doOperation
(wait) (continuation)
Reply message
getRequest
select object execute method sendReply

Request-Reply Operations
◼ public byte[] doOperation (RemoteRef s, int operationId, byte[] arguments)
◼ Sends a request message to the remote server and returns the reply
◼ The arguments specify the remote server, the operation to be invoked and the arguments of that operation
◼ public byte[] getRequest ()
◼ Acquires a client request via the server port
◼ public void sendReply (byte[] reply, InetAddress clientHost, int clientPort)
◼ Sends the reply message reply to the client at its Internet address and port

Invocation Semantics
◼ Middleware that implements remote invocation generally provides a certain level of semantics:
◼ Maybe: The remote procedure call may be executed once or not at all. Unless the caller receives a result, it is unknown as to whether the remote procedure was called.
◼ At-least-once: Either the remote procedure was executed at least once, and the caller received a response, or the caller received an exception to indicate the remote procedure was not executed at all.
◼ At-most-once: The remote procedure call was either executed exactly once, in which case the caller received a response, or it was not executed at all and the caller receives an exception.
◼ Java RMI (Remote Method Invocation) supports at-most-once invocation.
◼ It is supported in various editions including J2EE.
◼ Sun RPC (Remote Procedure Call) supports at-least-once semantics. ◼ Popularly used in Unix/C programming environments

Distributed Objects
◼ A programming model based on Object-Oriented principles for distributed programming.
◼ Enables reuse of well-known programming abstractions (Objects, Interfaces, methods…), familiar languages (Java, C++, C#…), and design principles and tools (design patterns, UML…)
◼ Each process contains a collection of objects, some of which can receive both remote and local invocations:
◼ Method invocations between objects in different processes are known as remote method invocation, regardless the processes run in the same or different machines.
◼ Distributed objects may adopt a client-server architecture, but other architectural models can be applied as well.

◼ Java Remote Method Invocation (Java RMI) is an extension of the Java object model to support distributed objects
◼ methods of remote Java objects can be invoked from other Java virtual machines, possibly on different hosts
◼ Single-language system with a proprietary transport protocol (JRMP, java remote method protocol)
◼ Also supports IIOP (Internet Inter-Orb Protocol) from CORBA
◼ RMI uses object serialization to marshal and unmarshal
◼ Any serializable object can be used as parameter or method return
◼ Releases of Java RMI
◼ Java RMI is available for Java Standard Edition (JSE), Java Micro Edition (JME), and Java Enterprise Edition (Java EE)

RMI Architecture and Components
◼ Remote reference module (at client & server) is responsible for providing addressing to the proxy (stub) object
◼ Proxy is used to implement a stub and provide transparency to the client. It is invoked directly by the client (as if the proxy itself was the remote object), and then marshal the invocation into a request
◼ Communication module is responsible for networking
◼ Dispatcher selects the proper skeleton and forward message to it
◼ Skeleton un-marshals the request and calls the remote object
client objectA proxyforB
skeleton & dispatcher
for B’s class
remote object B
Remote Communication reference module module
Communication Remote reference module module

Invocation Lifecycle
Client Code
RMI Object
method via 1 stub
Returns response
Calls actual
with args 4
Returns response / exception
Serializes arguments, transmit 2
deserialises arguments 3
Serialises
deserialises 7 response
response, 6 transmit

Steps for implementing an RMI application
◼ Design and implement the components of your distributed application
◼ Remote interface ◼ Servant program ◼ Server program ◼ Client program
◼ Compile source code and generate stubs ◼ Client proxy stub
◼ Server dispatcher and skeleton
◼ Make classes network accessible
◼ Distribute the application on server side
◼ Start the application 13

RMI Programming and Examples
◼ Application Design
◼ Remote Interface
◼ Exposes the set of methods and properties available ◼ Defines the contract between the client and the server ◼ Constitutes the root for both stub and skeleton
◼ Servant component
◼ Represents the remote object (skeleton) ◼ Implements the remote interface
◼ Server component
◼ Main driver that makes available the servant ◼ It usually registers with the naming service
◼ Client component

proxy
5. method1()
10. Return value
3. lookup(“myRO”)
6. method1()
9. Return value
RMI Registry
<“myRO”, Remote Ref. to RemoteObj>
RemoteObj Dispatcher/ Skeleton
8. return value
7. method1()
RemoteObj
1. new RemoteObj()
4. Return RemoteObj proxy
2. bind(“myRO”, RemoteObj)

Example application – Hello World
◼ Server side
◼ Create a HelloWorld interface
◼ Implement HelloWorld interface with methods
◼ Create a main method to register the HelloWorld service in the RMI Name Registry
◼ Generate Stubs and Start RMI registry
◼ Start Server
◼ Client side
◼ Write a simple Client with main to lookup HelloWorld Service and invoke the methods

1. Define Interface of remote method
//file: HelloWorld.java
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface HelloWorld extends Remote { public String sayHello(String who) throws RemoteException;

2. Define RMI Server Program
// file: HelloWorldServer.java
import java.rmi.Naming;
import java.rmi.Remote;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
public class HelloWorldServer extends UnicastRemoteObject implements HelloWorld { public HelloWorldServer() throws RemoteException {
public String sayHello(String who) throws RemoteException {
public static void main(String[] args) {
String hostName = “localhost”;
String serviceName = “HelloWorldService”;
if(args.length == 2){
hostName = args[0];
return “Hello “+who+” from your friend RMI 433-652 :-)”;
serviceName = args[1];
HelloWorld hello = new HelloWorldServer();
Naming.rebind(“rmi://”+hostName+”/”+serviceName, hello);
System.out.println(“HelloWorld RMI Server is running…”); }catch(Exception e){
e.printStackTrace();

3. Define Client Program
// file: RMIClient.java
import java.rmi.Naming;
public class RMIClient {
public static void main(String[] args) { String hostName = “localhost”;
String serviceName = “HelloWorldService”; String who = “Raj”;
if(args.length == 3){
hostName = args[0];
serviceName = args[1];
who = args[2];
else if(args.length == 1){ who = args[0];
HelloWorld hello = (HelloWorld)Naming.lookup(“rmi://”+hostName+”/”+serviceName);
System.out.println(hello.sayHello(who)); }catch(Exception e){
e.printStackTrace();

Define Access Policy
◼ Example: File HelloPolicy to contain grant { permission java.security.AllPermission “”, “”; };

Java RMI Example
◼ Running the Server and Client
◼ Compile Client and Server classes
◼ Develop a security policy file (e.g., HelloPolicy) ◼ grant { permission java.security.AllPermission “”, “”; };
◼ Start RMI registry ◼ rmiregistry &
◼ Start server
◼ java -Djava.security.policy=HelloPolicy HelloWorldServer
◼ Run a client program
◼ java -Djava.security.policy=HelloPolicy RMIClient
◼ java -Djava.security.policy=HelloPolicy RMIClient Pascal

Security Manager
◼ Java’s security framework
◼ java.security.-
◼ Permissions, Principle, Domain etc.
◼ Security manager, for access control (file, socket, class load, remote code etc)
◼ $JAVA_HOME/jre/lib/security/java.policy
◼ Use security manager in RMI
◼ RMI recommends to install a security manager, or RMI may not work properly while encountering security constraints.
◼ A security manager ensures that the operations performed by downloaded code go through a set of security checks.
◼ Eg. Connect and accept ports for RMI socket and allowing code downloading

Security Manager (cont.)
◼ Two ways to declare security manager
◼ Use System property java.security.manager
java –Djava.security.manager HelloWorldImpl
◼ Explicit declare in the source code public static void main(String[]args){
//check current security manager if(System.getSecurityManager()==null){
System.setSecurityManager(new SecurityManager ()); //lookup remote object and invoke methods.
◼ Use customized policy file instead of java.policy
java -Djava.security.manager -Djava.security.policy=local.policy HelloWorldImpl

File: “local.policy” contents
Specific permissions:
permission java.net.SocketPermission “*:1024-65535″,”connect,accept”; permission java.io.FilePermission “/home/globus/RMITutorial/-“, “read”;
Grant all permissions: grant {
permission java.security.AllPermission; };

Exceptions
◼ The only exception that could be thrown out is RemoteException
◼ All RMI remote methods have to throw this exception ◼ The embedded exceptions could be:
◼ java.net.UnknownHostException or java.net.ConnectException: if the client can’t connect to the server using the given hostname. Server may not be running at the moment
◼ java.rmi.UnmarshalException: if some classes not found. This may because the codebase has not been properly set
◼ Java.security.AccessControlException: if the security policy file java.policy has not been properly configured

Passing objects
◼ Restrictions on exchanging objects
◼ Implementing java.io.serializable
◼ All the fields in a serializable object must be also serializable
◼ Primitives are serializable
◼ System related features (e.g. Thread, File) are non-
serializable
◼ How about the socket programming issues?
◼ Where are sockets and corresponding input, output
◼ How to handle object passing?
◼ Who does all the magic?

RMI Dynamic Class Loading
◼ Ability to download bytecode (classes) from Remote JVM
◼ New types can be introduced into a remote virtual machine without informing the client
◼ Extend the behavior of an application dynamically ◼ Removes the need to deploy stubs manually
◼ Explicit set property to support dynamic class load
◼ Specify system property java.rmi.server.codebase to tell the program where to download classes

◼ Introduction to Distributed Objects
◼ Remote Method Invocation (RMI) Architecture
◼ RMI Programming and a Sample Example: ◼ Server-Side RMI programming
◼ Client-Side RMI programming
◼ Advanced RMI Concepts ◼ Security Policies
◼ Exceptions
◼ Dynamic Loading
◼ RPC and Summary 29
◼ A more advanced RMI application ◼ Math Server

A Simple Math Server in RMI
MathServer (multiple operations)
Math Service

Java RMI Example
◼ Specify the Remote Interface
public interface IRemoteMath extends Remote {
double add(double i, double j) throws RemoteException;
double subtract(double i, double j) throws RemoteException;

Java RMI Example
◼ Implement the Servant Class
public class RemoteMathServant extends UnicastRemoteObject implements IRemoteMath { public double add ( double i, double j ) throws RemoteException {
return (i+j);
public double subtract ( double i, double j ) throws RemoteException { return (i-j);

Java RMI Example
◼ Implement the server
public class MathServer {
public static void main(String args[]){
System.setSecurityManager(new RMISecurityManager()); try{
IRemoteMath remoteMath = new RemoteMathServant(); Registry registry = LocateRegistry.getRegistry(); registry.bind(“Compute”, remoteMath );
System.out.println(“Math server ready”); }catch(Exception e) {
e.printStackTrace();

Java RMI Example
◼ Implement the client program
public class MathClient {
public static void main(String[] args) {
if(System.getSecurityManager() == null)
System.setSecurityManager( new RMISecurityManager() );
LocateRegistry.getRegistry(“localhost”);
IRemoteMath remoteMath = (IRemoteMath) registry.lookup(“Compute”);
System.out.println( “1.7 + 2.8 = ” + math.add(1.7, 2.8) );
System.out.println( “6.7 – 2.3 = ” + math.subtract(6.7, 2.3) ); }
catch( Exception e ) { System.out.println( e );

Java RMI Example
◼ Running the Server and Client
◼ Compile Client and Server classes
◼ Develop a security policy file
◼ grant { permission java.security.AllPermission “”, “”; };
◼ Start RMI registry ◼ rmiregistry &
◼ Start server
◼ java -Djava.security.policy=policyfile MathServer
◼ Start client
◼ java -Djava.security.policy=policyfile MathClient

Remote Procedure Call (RPC) – used in C
◼ RPCs enable clients to execute procedures in server processes based on a defined service interface.
Request Reply
server process
client program
client stub procedure
client process
Communication module
Communication
service procedure
dispatcher
server stub procedure

Remote Procedure Call (RPC)
◼ Communication Module
◼ Implements the desired design choices in terms of retransmission of requests,
dealing with duplicates and retransmission of results
◼ Client Stub Procedure
◼ Behaves like a local procedure to the client. Marshals the procedure identifiers
and arguments which is handed to the communication module
◼ Unmarshalls the results in the reply
◼ Dispatcher
◼ Selects the server stub based on the procedure identifier and forwards the
request to the server stub
◼ Server stub procedure
◼ Unmarshalls the arguments in the request message and forwards it to the service
◼ Marshalls the arguments in the result message and returns it to the client

Summary: RMI Programming
◼ RMI greatly simplifies creation of distributed applications (e.g., compare RMI code with socket- based apps)
◼ Server Side
◼ Define interface that extend java.rmi.Remote
◼ Servant class both implements the interface and extends java.rmi.server.UnicastRemoteObject
◼ Register the remote object into RMI registry
◼ Ensure both rmiregistry and the server is running
◼ Client Side
◼ No restriction on client implementation, both thin and rich client can be used. (Console, Swing, or Web client such as servlet and JSP)

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