Chapter4 RMI 2011

Distributed Objects

In the Good Old Days...

Only local objects existedOnly local objects existed

My MachineMy Machine

My ObjectMy Object

Today’s World...

Network and Distributed ObjectsNetwork and Distributed Objects

My MachineMy Machine Remote Remote MachineMachine

Local Local ObjectsObjects Remote Remote

ObjectsObjects

Overview

• What options do I have for distributed application development?

• Developers who program using the Java programming language can choose several solutions for creating distributed applications programs.

1) Java RMI technology

2) Java IDL technology (for CORBA programmers)

3) Enterprise JavaBeans technology

In this section we shall be talking about Java RMI and IDL (Corba) technologies

JAVA RMI

Remote Method Invocation

Outline

1) RMI Concepts

2) RMI architecture

3) Implementing and running RMI system

4) Implementing activatable RMI server

5) Summary

Introduction

• Remote Method Invocation (RMI) technology was first introduced in JDK1.1.

• RMI allows programmers to develop distributed Java programs with the same syntax and semantics used for non-distributed programs.

• RMI is based on a similar, earlier technology for procedural programming called remote procedure call (RPC)

Introduction

Disadvantages of RPC

a) RPC supports a limited set of data types. Therefore it is not suitable for passing and returning Java Objects

b) RPC requires the programmer to learn a special interface definition language (IDL) to describe the functions that can be invoked remotely

Introduction

The RMI architecture defines

a) How objects behave.

b) How and when exceptions can occur.

c) How memory is managed.

d) How parameters are passed to, and returned from, remote methods.

The remote object model for Enterprise JavaBeans (EJB) is RMI- based.

Introduction

• RMI is designed for Java-to-Java distributed applications. RMI is simpler and easier to maintain than using socket.

• Other options for creating Java-to-non-Java distributed applications are:

a) Java Interface Definition Language (IDL)

b) Remote Method Invocation (RMI) over Internet Inter-ORB Protocol (IIOP) -- RMI-IIOP.

RMI architecture

• RMI allows the code that defines the behavior and the code that implements the behavior to remain separate and to run on separate JVMs.

• At client side, RMI uses interfaces to define behavior.• At server side, RMI uses classes to define

implementation.

RMI Layer

TCPRemote Reference Layer

Transport Layer

Java Virtual Machine

Client Object

Remote Reference Layer

Transport Layer

Java Virtual Machine

Remote Object

Skeleton

Remote Objects

• Remote Objects– Live on server– Accessed as if they were local

Remote Objects

Stubs and Skeletons

• Stub – lives on client– marshals argument data (serialization)– unmarshals results data (deserialization)

• Skeleton– lives on server– unmarshals argument data– marshals results data– receives requests from stub– delivers response to stub

Remote Reference Layer

• This layer provides a RemoteRef object that represents the link to the remote service implementation object.

• Encodes and decodes the on-wire protocol.

• Implements the remote object protocols (Unicast Remote Object/ activatable remote objects ).

Transport Layer

• The Transport Layer makes the connection between JVMs. All connections are stream-based network connections that use TCP/IP.

• Dispatching messages between stub and skeleton.

RMI Flow

Client Virtual Machine

Client

Server Virtual Machine

Remote Object

Skeleton

Registry Virtual Machine

“Fred”

Server

1. Server Creates Remote Object2. Server Registers Remote Object

RMI Flow

Client

Remote Object

Skeleton

“Fred”

Server

3. Client requests object from Registry4. Registry returns remote reference(and stub gets created)

RMI Flow

Client

Remote Object

Skeleton

“Fred”

Server

5. Client invokes stub method6. Stub talks to skeleton7. Skeleton invokes remote object method

The steps...

• Create the Interface to the server

• Create the Server

• Create the Client

• Compile the Interface (javac)

• Compile the Server (javac)

• Compile the Client (javac)

• Generate Stubs and Skeletons (rmic)

Part II: RMI Usage

Creating Remote Objects

• Define a Remote Interface– extends java.rmi.Remote

• Define a class that implements the Remote Interface– extends java.rmi.RemoteObject– or java.rmi.UnicastRemoteObject

Remote Interface Example

import java.rmi.*;

public interface Adder extends Remote

public int add(int x, int y)

throws RemoteException;

Remote Interface Example

• Remark:

• All the interface has to extend the java.rmi.Remote interface

• and all the methods has to declare that it may throw a RemoteException object.

Remote Class Example

• The implementation class may extend from the

java.rmi.server.UnicastRemoteObject to link into the RMI system.

• It must also provide an explicit default constructor throwing RemoteException. When this constructor calls super(), it activates code in UnicastRemoteObject that performs the RMI linking and remote object initialization.

Remote Class Example

import java.rmi.*;

import java.rmi.server.*;

public class AdderImpl extends UnicastRemoteObject implements Adder

public AdderImpl() throws RemoteException

super();

public int add(int x, int y)

throws RemoteException

return x + y;

Extend UnicastRemoteObjectand implemet Adder InterfactExtend UnicastRemoteObjectand implemet Adder Interfact

Implement all methodsfrom interface Hello.javaImplement all methods

from interface Hello.java

Compiling Remote Classes

• Compile the Java class– javac

• reads .java file• produces .class file

• Compile the Stub and Skeleton– rmic

• reads .class file• produces _Skel.class and _Stub.class

Compiling Remote Classes (Diagram)

Adder.java(interface)

Adder.class(interface classfile)

AdderImpl.java(remote class)

AdderImpl.class(classfile)

javacrmic

AdderImpl_Skel.class(skeleton classfile)

AdderImpl_Stub.class(stub classfile)

rmic –v1.1 –classpath “…..\RMI” –d “….\RMI” AdderImpl

Registering Remote Classes

• start the registry– running process

• Windows: start rmiregistry

You can also bind rmiregistry to a different port by indicating the new port number as :

start rmiregistry <new port>

(uses port 1099 by default)

Create the server

• Creates a new instance of the remote object

• Registers it in the registry with a unique name

RMI Server Example

public class RMIServer {

public static void main(String []args) throws RemoteException, MalformedURLException, AlreadyBoundException

// LocateRegistry.createRegistry(1099);

AdderImpl adder = new AdderImpl();

System.out.println("Server has started");

Naming.bind (“rmi://localhost/adder", adder);

System.out.println("Adder bound");

}catch (Exception e)

System.out.println ("Trouble: " + e);

import java.rmi.AlreadyBoundException;import java.rmi.RemoteException;import java.rmi.registry.LocateRegistry;import java.rmi.Naming;import java.net.MalformedURLException;

Instantiate a new object and register (bind it) in the ”rmiregistry”

Registering Remote ClassesRegistering Remote Classes

Start rmicregistry.exe

RMI URLs

rmi://host[:port]/name• default port is 1099

• Specifies hostname of registry

• can also use relative URLs – name only– assumes registry is on local host

Creating an RMI Client

1) In the client’s code, all you need to do is to lookup the object and use, it’s methods as local methods.

2) The client’s code may look like the following:

RMI Client Example

import java.rmi.Naming;

import java.rmi.NotBoundException;

import java.util.Scanner;

import java.io.*;

public class RMIClient {

public static void main(String[] args) throws NotBoundException, IOException {

Adder a = (Adder) Naming.lookup("rmi://localhost/adder");

int sum = a.add(2,2);

System.out.println("2+2=" + sum);

”lookup” the RMIServer – and call Method adder() on Stub

RMI URLs

rmi://host[:port]/name• default port is 1099

• Specifies hostname of registry

• can also use relative URLs – name only– assumes registry is on local host

Remember – that the stuband skeleton classes get generated

by the ”rmic” compiler

Remember – that the stuband skeleton classes get generated

by the ”rmic” compiler

Steps for Developing an RMI System

1. Define the remote interface

2. Develop the remote object by implementing the remote interface.

3. Develop the client program.

4. Compile the Java source files.

5. Generate the client stubs and server skeletons.

6. Start the RMI registry.

7. Start the remote server objects.

8. Run the client

Lab Work: RMI System

1) Please follow what we have discussed to develop a RMI server which hosts a service for calculating the square root of a number.

2) Compile your RMI server and generate the corresponding stub class.

3) Create a client to test the RMI service.

Passing Parameters

• All parameters passed from an RMI client to an RMI server must either be serializable or be a remote object.

• For serializable: a) Data is extracted from the local object and

sent across the network to the remote server. b) Object is then reconstructed in the remote

server. c) Any changes to the object in the

RMIServer will not be reflected in the object held in the RMI client and vice versa.

Passing Parameters

• For a remote object:

a)Stub information, not a copy of data, is actually sent over RMI.

b)Any call made to the parameter object become a remote calls back to the actual object.

c)Changes made in one JVM are reflected in the original JVM.

Conditions for serializability

If an object is to be serialized:

a) The class must be declared as public

b) The class must implement java.io.Serializable interface are marked as serializable

c) The class must have a default (no-argument) constructor

d) All fields of the class must be serializable: either primitive types or serializable objects.

Remote interfaces and class

Security

• Your program should guarantee that the classes that get loaded do not perform operations that they are not allowed to perform.

• A more conservative security manager than the default should be installed. The following code should be added to the main method of the server and client program:

RMI Server Example

import java.rmi.RMISecurityManager;public class RMIServer {

public static void main(String []args) throws RemoteException, MalformedURLException, AlreadyBoundException

// Create and install a security manager

if (System.getSecurityManager() == null) {

System.setSecurityManager(new RMISecurityManager()); }

RMISecurityManager

• Easiest way to do this is to use java.rmi.RMISecurityManager as:

System.setSecurityManager(new RMISecurityManager());

• This by default restricts all code from making socket connections.

• Obviously this is too strict. Need a policy file to allow client to make network connection to rmi port. It would look something like:grant{ permission java.net.SocketPermission “*:1024-65535”, “connect”}

java Client –Djava.security.policy=“file_policy”

Activatable Server

• Before Java 2, the UnicastRemoteObject could be accessed only from a server program that created instances of the object and ran all the time.

• With Java 2 we got the Activatable and the rmid daemon.

• An activatable class needs only to be registered with the rmid.

• This is an advantage for systems that have many remote object classes but only a few of them are active at any one time. They save memory (and so gain performance).

Activatable Server

• Enable server programs to wake up and start to run when they are needed.

• Java RMI Activation System Deamon (RMID) is introduced to handle this task.

• When a client requests a reference to the server from the rmiregistry, the rmid program, which holds the servers details, will be requested to start up the server and return the reference to the client. After that, the rmiregistry will be able to provide the reference of the server directly.

MyRemoteInterface.java

import java.rmi.*;

public interface MyRemoteInterface extends Remote {

public Object callMeRemotely() throws RemoteException;

Creating the implementation class

• Step 1: Make the appropriate imports in the implementation class import java.rmi.*;

import java.rmi.activation.*; • Step 2:

Extend your class from java.rmi.activation.Activatable

public class ActivatableImplementation extends Activatable implements MyRemoteInterface {

• Step 3: Declare a two-argument constructor in the implementation class

public ActivatableImplementation(ActivationID id, MarshalledObject data) throws RemoteException {

// Register the object with the activation system

// then export it on an anonymous port

super(id, 0);

The super(id, 0) method calls Activatable constructor to pass an activation ID and a port number. In this case, the port number is default 1099.

• Step 4: Implement the remote interface method(s)

public Object callMeRemotely() throws RemoteException {

return "Success";

ActivatableImplementation.java

Activatable Server: Setup

Compile and Run

1. Compile all the classes use javac.

2. Run rmic on the implementation class

3. Start rmi registry use rmiregistry.

Note that if you use jdk version below 1.5.0, you may need to regenerate stub class first by using command

below before start the rmiregistry.

rmic -v1.2 <implement of Interface>

rmic –v1.2 ActivatableImplementation

Compile and Run

Example

http://www.javacamp.org/moreclasses/rmi/rmi23.html

java.rmi.activation.Activatable 1.2

protected Activatable(ActivationID id, int port)• Constructs the activatable object and establishes a listener on

the given port. Use 0 for the port to have a port assigned automatically.

static Remote exportObject(Remote obj, ActivationID id, int port)

• Makes a remote object activatable. Returns the activation receiver that should be made available to remote callers. Use 0 for the port to have a port assigned automatically.

static Remote register(ActivationDescriptor desc)• registers the descriptor for an activatable object and prepares it

for receiving remote calls. Returns the activation receiver that should be made available to remote callers.

java.rmi.MarshalledObject 1.2

MarshalledObject(Object obj)

• constructs an object containing the serialized data of a given object.

Object get()

• deserializes the stored object data and returns the object.

java.rmi.activation.ActivationGroupDesc 1.2

ActivationGroupDesc(Properties props, ActivationGroupDesc.CommandEnvironment env)

• constructs an activation group descriptor that specifies virtual machine properties for a virtual machine that hosts activated objects. The env parameter contains the path to the virtual machine executable and command-line options, or it is null if no special settings are required.

java.rmi.activation.ActivationGroup 1.2

static ActivationSystem getSystem()

• returns a reference to the activation system.

java.rmi.activation.ActivationSystem 1.2

ActivationGroupID registerGroup ( ActivationGroupDesc group)

• registers an activation group and returns the group ID.

java.rmi.activation.ActivationDesc 1.2

ActivationDesc(ActivationGroupID id, String className, String classFileURL, MarshalledObject data)

• constructs an activation descriptor.

Converting UnicastRemoteObject to Activatable

If you already have a UnicastRemoteObject and want to make it Activatable you just have to make some small changes to the implementation class.

1. Make the appropriate imports. 2. Make the class extend Activatable. 3. Remove the no-argument constructor. 4. Declare a two argument constructor.

You then need to create a setup program just like before.

• Using Remote Method Invocation to ImplementCallbacks

Polling vs. Callback

In the absence of callback, a client will have to poll a passive server repeatedly if it needs to be notified that an event has occurred at the server end.

S e rv e r

C lie n t

P o lling

S e rv e r

C lie n t

C a llba c k

A clie n t is s u e s a re qu e s t to th es e rv e r re pe a te dly u n t il th ede s ire d re s po n s e is o bta in e d.

A clie n t re g is te rs it s e lf with th es e rv e r, a n d wa it u n t il th e s e rv e rca lls ba ck .

a re m o te m e th o d ca ll

RMI Callbacks

• 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.

T h e c a llb ac k lis t

c allb ac k

S e rv e rC lie n ts

R M I c a lls

Callback Client-Server 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 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 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 c lie 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 c lie 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 ic ipa 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 .

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

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

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.cla 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.cla 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

Implementation

Defining the Listener Interface

• defines a remote object with methods

• method should be invoked by an event source whenever an event occurs, so as to act as notification that the event occurred.

Object-Oriented Callbacks

Callback notification of event, for every registered listener

Callback implemented by invoking a method on a listening object

RMI Callbacks

• Listener interface and the event source must be implemented as an RMI service.

• For the listener – to register itself with the remote event source, it

must invoke a remote method and pass an object reference to the remote listener interface it defines.

Defining the Listener Interface

Defining the Event Source Interface

• event source must allow a listener to be registered and unregistered, and may optionally provide additional methods

Defining the Event Source Interface

Implementing Event Source Interface

• implement TemperatureSensorServer class– RMI server.

• server must – extend UnicastRemoteObject, to offer a– implement Temperature Sensor interface

Implementing Event Source Interface

Implementing the Listener Interface

• implement the TemperatureListener interface,

• register itself with the remote temperature sensor service,

• client will be notified of changes as they occur, using a remote callback

Implementing the Listener Interface

Running the Callback Example

RMI Components

• java.rmi: client-side RMI classes, interfaces, and exceptions

• java.rmi.server: server-side RMI classes, interfaces, and exceptions

• java.rmi.registry: Classes for naming services

• java.rmi.dgc: distribute garbage collection

• java.rmi.activation: Implements activate on demand RMI services. Objects instantiated on-demand by client-requests. References persistent over server crashes.

• rmic: compiler to generate stubs and skeletons

• rmiregistry: Utility server that provides naming service for RMI. Associates names with objects

• rmid: utility server to RMI activation framework

RMI Server Properties

• java.rmi.server.codebase: a url indicating code base from which classes should be loaded to network clients

• java.rmi.server.disableHttp: if true, RMI will not use Http to try to tunnel through firewalls. Default is false, meaning that Http wraps RMI calls

• java.rmi.server.hostname: sets servers fully qualified host name, if name unavailable via DNS

• java.rmi.server.logCalls: if true, RMI will log information about calls. Default is false.

• java.rmi.dgc.leaseValue: The time in milliseconds until server notices client is no longer connected. Default is 10 minutes.

RMI Server Properties

RMI Naming Methods• Remote lookup(String name): Look up remote object

by URL and return it• bind(String name, Remote obj): bind an object to a

specific URL• unbind(String name): unbind an object at a URL• rebind(String name, Remote obj): Replace the object

currently bound with a new one• String[] list(String name): list of URL’s from specified

Registry

Chapter4 RMI 2011

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