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ITEC801Distributed Systems

Remote Method InvocationCoulouris section 5.5

Liu section 7.5, 7.6, 7.7

ITEC 801 RMI 2

Important Things

What is similar between RMI and RPC Most things really

What is different between RMI and RPC Mainly about interfaces and how they are

defined (its OO so interfaces important)

What technologies are based on RMISome understanding of Java RMI and an example

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RMI

Remote Method Invocation

Closely related to RPC

RMI allows an object living in one process to invoke the methods of an object living in another process

Can be thought of as an object-oriented version of RPC

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RMI

Used to allow objects on different machines to communicateobjects on different machines are automatically in different processesmethod invocations between objects in different processes (different machine or not) are remote method invocations

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Distributed Object Paradigm

based on objects existing in a distributed systemIn object-oriented programming objects used to represent an entity significant to an application.Each object encapsulates: the state or data of the entity: in Java, such data is

contained in the instance variables of each object; the operations of the entity, through which the

state of the entity can be accessed or updated.

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Distributed Object Paradigm

network resources are represented by distributed objects

To request service from a network resource a process invokes one of the resource’s

operations or methods passing data as parameters to the method

The method is executed on the remote host the response is sent back to the requesting

process as a return value.

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RPC and RMI

Use location independence

Need interface definitions

Have same problems with data transfer data reference failure

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Location Independence

In RPC, calling code does not know where called code is

In RMI the invoking object does not know where the invoked object is

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Local Objects vs. Distributed Objects

Local objects are those whose methods can only be invoked by a local process, a process that runs on the same computer on which the object exists.

A distributed object is one whose methods can be invoked by a remote process, a process running on a computer connected via a network to the computer on which the object exists.

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Distributed Object SystemObjectregistry

Objectclient

Clientproxy

Runtimesupport

Networksupport

Objectserver

Serverproxy

Runtimesupport

Networksupport

Physical data path

Logical data path

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Distributed Object System

A distributed object is provided, (or exported), by a process (the object server) An object registry must be present in the system architecture for the distributed object to be registered. To access a distributed object, a process – an object client – looks up the object registry for a reference[1] to the object. This reference is used by the object client to make calls to the methods.

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Distributed Object System

Logically, the object client makes a call directly to a remote method.

In reality, the call is handled by a software component, called a client proxy (equivalent to client stub of RPC)

proxy interacts which the software on the client host that provides the runtime support for the distributed object system.

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Runtime support

responsible for the IPC needed to transmit the call to the remote host, including the marshalling of the argument

data that needs to be transmitted to the remote object.

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Distributed Object System

similar on the server side

runtime support for the distributed object system handles the receiving of messages and the

unmarshalling of data forwards the call to a software component

called the server proxy (equivalent to server stub).

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Server Side

The server proxy interfaces with the distributed object to invoke the method call locally, passing in the unmarshalled data for the arguments. The method call results in the performance of some tasks on the server host. The outcome of the method, including the marshalled data for the return value, is forwarded by the server proxy to the client proxy, via the runtime support and network support on both sides.

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Distributed Object Systems/Protocols

The distributed object paradigm has been widely adopted in distributed applications,

a large number of mechanisms based on the paradigm are available.

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Examples

Java Remote Method Invocation (RMI),Common Object Request Broker Architecture (CORBA) systems,Distributed Component Object Model (DCOM), mechanisms that support the Simple Object Access Protocol (SOAP). Of these, the most straightforward is the Java RMI

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Interfaces

Interfaces are an important part of modern programming

The object-oriented approach is partly based on the notion of an object interface

As in RPC, with its server interfaces, we need interface specifications for RMI

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Interfaces

Interfaces in OO allow access to procedures (methods) and variables (attributes)

Modules in one process cannot access variables in another process

Therefore the interface for RMI cannot specify direct access to variables

Some (like CORBA IDL) appear to, but they actually use get and set methods

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Integration with Programming Languages

If an RMI mechanism has an adequate means of defining interfaces it can be directly integrated into a programming languageFor example, the Java RMIUseful when all parts of the distributed application to be written in the one language

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IDLs

Sometimes the various parts are written in different languages For example, integrating pre-existing code

Interface definition languages are intended to all objects implemented in different languages to invoke one another

Used to define interfaces for use with RMI

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Input and Output Parameters

An important part of the interface are the parameters

IDLs tend to deal with the problem of parameter passing by using input and output parameters

All parameters in a method definition written in an IDL are defined as either an input parameter or an output parameter

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IDLs and Parameters

An input parameter can be looked on as call by value

Having a single actual parameter used for both an input and and output parameter gives call by value return

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CORBA IDL example

Interface Person List {

readonly attribute string listname

void addPerson(in Person p);

void getPerson(in string name, out Person p);

long number();

}

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Other Example IDLs

IDL – used by OSF’s DCE for RPC

DCOM IDL, based on DCE IDL and used in Microsoft’s Distributed Component Object Model

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Java RMI

Syntactically: A remote object is declared with a remote

interface, an extension of the Java interface. The remote interface is implemented by the object

server. An object client accesses the object by invoking

the remote methods associated with the objects using syntax provided for remote method invocations.

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Remote reference

layer

Transportlayer

The Java RMI ArchitectureDirectoryService

Objectclient

stub

Remote reference

layer

Transportlayer

Objectserver

skeleton

Physical data path

Logical data path

Supports the interface withthe application program

Maps the platform-independent stub/skeleton to the platform-dependent transport layer; carries out remote reference protocols

Sets up, maintains and shuts down connections; carries out the transport protocol

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Object Registry

The RMI API allows a number of directory services to be used for registering a distributed object. Java Naming and Directory Interface

(JNDI) rmiregistry (in Java SDK)

By convention runs on object server’s host machine

TCP port 1099 (by default)

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Interaction between stub and skeleton

s k e le to n

m a rs h a l pa ra m e te rs ;s e n d R e qu e s t

u n m a rs h a l pa ra m e te rsI n v o k e m e th o d

R e m o teM e th o d

e x e cu te co dea n d re tu rn av a lu e

re ce iv e re tu rn v a lu em a rs h a l re plys e n d re ply

u n m a rs h a ll re ply ;re tu rn v a lu e

t im e

(ba s e d o n h t tp: // ja v a . s u n .co m .m a rk e t in g /co lla te ra l/ja v a rim .h tm l)

s tu b

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The API for the Java RMI

The Remote Interface

The Server-side Software The Remote Interface Implementation Stub and Skeleton Generations The Object Server

The Client-side Software

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Java API - Remote Interface

A java remote interface is an interface that inherits from the Java Remote class allows the interface to be implemented using RMI

syntax.

a remote interface has the same syntax as a regular or local Java interface except for Extension of Remote RemoteException specified in each method

signature

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Example

// file: SomeInterface.java // to be implemented by a Java RMI server class.import java.rmi.*public interface SomeInterface extends Remote {

// signature of first remote method public String someMethod1( )

throws java.rmi.RemoteException; // signature of second remote method

public int someMethod2( float ) throws java.rmi.RemoteException;

// signature of other remote methods may follow } // end interface

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Example

The java.rmi.RemoteException must be listed in the throw clause of each method signature. raised when errors occur during the

processing of a remote method call the exception is required to be caught in

the method caller’s program.

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Example

Causes of such exceptions include exceptions that may occur during

interprocess communications, such as access failures and connection failures,

problems unique to remote method invocations, including errors resulting from the object, the stub, or the skeleton not being found.

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Java API - Server-side

An object server is an object that provides the methods of and the interface to a distributed object. Each object server must implement each of the remote methods specified

in the interface, register an object which contains the

implementation with a directory service.

It is recommended that the two parts be provided as separate classes.

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Example

A class which implements the remote interface

Its syntax is similar to a class that implements a local interface.

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Exampleimport java.rmi.*;import java.rmi.server.*;/** * This class implements the remote interface SomeInterface. */public class SomeImpl extends UnicastRemoteObject implements SomeInterface { public SomeImpl() throws RemoteException { super( ); } public String someMethod1( ) throws RemoteException { // code to be supplied } public int someMethod2( ) throws RemoteException { // code to be supplied }} // end class

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Stub and Skeleton Generation

In RMI, each distributed object requires a proxy for the object server and the object client, known as the object’s skeleton and stub

respectively.

These proxies are generated from the implementation of a remote interface using a tool provided with the Java SDK: the RMI compiler rmic. rmic <class name of the remote interface

implementation>

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For example:

rmic SomeImpl

As a result of the compilation, two proxy files will be generated, each prefixed with the implementation class name: SomeImpl_skel.class SomeImpl_stub.class

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The stub file for the object

The stub file for the object, as well as the remote interface file, must be shared with each object client – these file are required for the client program to compile.

A copy of each file may be provided to the object client by hand.

In addition, the Java RMI has a feature called “stub downloading” which allows a stub file to be obtained by a client dynamically.

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The Object Server

The object server class is a class whose code instantiates and exports an object of the remote interface implementation

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The Object Server

import java.rmi.*;……public class SomeServer { public static void main(String args[]) {

try{ // code for port number value to be supplied

SomeImpl exportedObj = new SomeImpl(); startRegistry(RMIPortNum); // register the object under the name “some” registryURL = "rmi://localhost:" + portNum + "/some"; Naming.rebind(registryURL, exportedObj); System.out.println("Some Server ready."); }// end try} // end main

 

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The Object Server// This method starts a RMI registry on the local host, if it// does not already exists at the specified port number.private static void startRegistry(int RMIPortNum) throws RemoteException{ try {

Registry registry= LocateRegistry.getRegistry(RMIPortNum); registry.list( );

// throw an exception if the registry does not exist } catch (RemoteException ex) { // No valid registry at that port. System.out.println( "RMI registry cannot be located at port " + RMIPortNum); Registry registry= LocateRegistry.createRegistry(RMIPortNum); System.out.println( "RMI registry created at port " + RMIPortNum); }} // end startRegistry

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The Object Server - 3

In our object server template, the code for exporting an object is as follows:// register the object under the name "some"registryURL = "rmi://localhost:" + portNum + "/some";Naming.rebind(registryURL, exportedObj);

The Naming class provides methods for storing and obtaining references from the registry. In particular, the rebind method allows an object reference to be stored in the registry with a URL in the form of rmi://<host name>:<port number>/<reference name>

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Rebind method

will overwrite any reference in the registry bound with the given reference name.

If the overwriting is not desirable, there is also a bind method.

The host name should be the name of the server, or simply “localhost”.

The reference name is a name of your choice, and should be unique in the registry.

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The RMI Registry

A server exports an object by registering it by a symbolic name with a server known as the RMI registry. // Create an object of the Interface

SomeInterfacel obj = new SomeInterface(“Server1”); // Register the object; rebind will overwirte existing

// registration by same name – bind( ) will not. Naming.rebind(“Server1”, obj);

A server, called the RMI Registry, is required to run on the host of the server which exports remote objects.The RMIRegistry is a server located at port 1099 by defaultIt can be invoked dynamically in the server class:

import java.rmi.registry.LocateRegistry; … LocateRegistry.createRegistry ( 1099 );

…

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The RMI Registry

Alternatively, an RMI registry can be activated by hand using the rmiregistry utility which comes with the Java Software Development Kit (SDK), as follows:

rmiregistry <port number>where the port number is a TCP port number. If no port number is specified, port number 1099 is assumed. The registry will run continuously until it is shut down (via CTRL-C, for example)

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The Object Server

When an object server is executed, the exporting of the distributed object causes the server process to begin to listen and wait for clients to request the service of the object. An RMI object server is a concurrent server: each request from an object client is serviced using

a separate thread of the server.

if a client process invokes multiple remote method calls, these calls will be executed concurrently unless provisions are made in the client process to synchronize the calls.

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The Client-side Software

The program for the client class is like any other Java class. The syntax needed for RMI involves locating the RMI Registry in the server host looking up the remote reference for the

server object the reference can then be cast to the remote

interface class and the remote methods invoked.

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The Client-side Software - 2import java.rmi.*;….public class SomeClient { public static void main(String args[]) { try {

String registryURL = "rmi://localhost:" + portNum + "/some"; SomeInterface h = (SomeInterface)Naming.lookup(registryURL); // invoke the remote method(s) String message = h.method1(); System.out.println(message); // method2 can be invoked similarly } // end try catch (Exception e) { System.out.println("Exception in SomeClient: " + e); } } //end main // Definition for other methods of the class, if any.}//end class

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Looking up the remote object

Naming. lookup used to retrieve the object reference previously stored in the registry by the object server.Note that the retrieved reference must be cast to the remote interface (not its implementation) class.

String registryURL = "rmi://localhost:" + portNum + "/some";

SomeInterface h = (SomeInterface)Naming.lookup(registryURL);

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Remote Object/Method

It is a common mistake to cast the object retrieved from the registry to the interface implementation class or the server object class

Instead it should be cast as the interface class.

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Invoking the Remote Method

The remote interface reference can be used to invoke any of the methods in the remote interface,eg: String message = h.method1(); System.out.println(message);

Note that the syntax for the invocation of the remote methods is the same as for local methods.

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Algorithm for developing the server-side software

1. Open a directory for all the files to be generated for this application.

2. Specify the remote-server interface in SomeInterface.java.

3. Implement the interface in SomeImpl.java4. Use the RMI compiler rmic to process the

implementation class and generate the stub file and skeleton file for the remote object:

rmic SomeImpl

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Algorithm for developing the server-side software

The files generated can be found in the directory as SomeImpl_Skel.class and SomeImpl_Stub.class.Steps 3 and 4 must be repeated each time that a change is made to the interface implementation.

5. Create the object server program SomeServer.java.

6. Activate the object server java SomeServer

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Algorithm for developing the client-side software

1. Open a directory for all the files to be generated for this application.

2. Obtain a copy of the remote interface class file.3. Obtain a copy of the stub file for the

implementation of the interface: SomeImpl_Stub.class.

4. Develop the client program SomeClient.java, and compile it to generate the client class.

5. Activate the client. java SomeClient

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Placement of filesObject Client Host

Object Client Directory

SomeInterface.class

SomeClient.class

SomeImpl_Stub.class

Object Server Host

Object Server Directory

SomeInterface.class

SomeServer.class

SomeImpl.class

SomeImpl_Skel.class

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Testing and Debugging

1. Build a template for a minimal RMI program. Start with a remote interface with a single signature, its implementation using a stub, a server program which exports the object, and a client program which invokes the remote method. Test the template programs on one host until the remote method can be made successfully.

2. Add one signature at a time to the interface. With each addition, modify the client program to invoke the added method.

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Testing and Debugging

3. Fill in the definition of each remote method, one at a time. Test and thoroughly debug each newly added method before proceeding with the next one.

4. After all remote methods have been thoroughly tested, develop the client application using an incremental approach. With each increment, test and debug the programs.

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RMI vs. Sockets

RMI API can be used in lieu of the socket API in a network application.There are some tradeoffs The socket API is closely related to the operating

system, and hence has less execution overhead. For applications which require high performance, this may be a consideration.

The RMI API provides the abstraction which eases the task of software development. Programs developed with a higher level of abstraction are more comprehensible and hence easier to debug.

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RMI Callbacks

In outline like any other callback

A callback client registers itself with an RMI server.

The server makes a callback to each registered client upon the occurrence of a certain event.

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RMI Callbacks

Server Client 1

Callback list

Client 2

Client 3

Client 3

Callback

RMI call

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Callback InteractionsC lie n t h o s t

S e rv e r h o s t

R M I r eg is tr y

S o m eS erv er . c las s

S o m eIn ter f ac e_ s tu b .c las s

S o m eIn ter f ac e_ s k el. c las s

C lien t. c las s12

1 . C lie n t lo o k s u p th e in te rfa ce o bje ct in th e R M I re g is try o n th e s e rv e r h o s t .2 . Th e R M I R e g is try re tu rn s a re m o te re fe re n ce to th e in te rfa ce o bje ct .3 . V ia th e s e rv e r s tu b, th e clie n t pro ce s s in v o k e s a re m o te m e th o d to re g is te r it s e lf fo r ca llba ck , pa s s in g a re m o te re fe re n ce to it s e lf to th e s e rv e r. Th e s e rv e r s a v e s th e re fe re n ce in it s ca llba ck lis t .4 . V ia th e s e rv e r s tu b, th e clie n t pro ce s s in te ra ct s with th e s k e le to n o f th e in te rfa ce o bje ct to a cce s s th e m e th o ds in th e in te rfa ce o bje ct .5 . W h e n th e a n t icipa te d e v e n t ta k e s pla ce , th e s e rv e r m a k e s a ca llba ck to e a ch re g is te re d c lie n t v ia th e ca llba ck in te rfa ce s tu b o n th e s e rv e r s ide a n d th e ca llba ck in te rfa ce s k e le to n o n th e c lie n t s ide .

X

C allb ac k In ter f ac e_ s k el. c las s

C allb ac k In ter f ac e_ s tu b .c las s5

3 ,4

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Placement of filesObject Client Host

Object Client Directory

ClientInterface.class

Client.class

ClientImpl_Stub.class

Object Server Host

Object Server Directory

ServerInterface.class

Server.class

ServerImpl.class

ServerImpl_Skel.class

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Callback application files

C lie n t .c la s s

C l i e n tIn te rface .cl as s

S e rve rIn te rface .cl as s

C lie n t I m pl.c la s s

S e rve rIm pl _S tu b.cl as s

C l i e n tIm pl _s k e l .cl as s

O bje ct c lie n t h o s t

o bje ct c lie n t dire cto ry

S e rv e r.c la s s

S e rve rIn te rface .cl as s

C l i e n tIn te rface .cl as s

S e rv e rI m pl.c la s s

C l i e n tIm pl _S tu b.cl as s

S e rve rIm pl _s k e l .cl as s

O bje ct s e rv e r h o s t

o bje ct s e rv e r dire cto ry

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RMI Callback file placements

jav a .p o lic y

S o m eC lien t. c las s

jav a .p o lc y

S o m eS er v er . c las s

S o m eI n ter f ac e _ s tu b .c las s

S o m eI n ter f ac e .S k ele to n .c las s

S o m eI n ter f ac e _ s tu b .c las s

C lie n t h o s t

c lien t d ir ec to r y

S e rv e r h o s t

s er v er d ir ec to r y

H TTP S e rv e r

C allb ac k I n ter f ac e _ s tu b .c las s

C allb ac k I n ter f ac e _ s k el. c las s

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Remote Interface for Server

public interface HelloInterface extends Remote { // remote method public String sayHello() throws

java.rmi.RemoteException; // method to be invoked by a client to add itself to

the callback list public void addCallback( HelloCallbackInterface CallbackObject) throws java.rmi.RemoteException;}

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Remote Interface for Callback Client

// an interface specifying a callback method

public interface HelloCallbackInterface extends java.rmi.Remote

{

// method to be called by the server on callback

public void callMe (

String message

) throws java.rmi.RemoteException;

}

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Other things to look up

Stub downloading

Security manager