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Distributed Objects
And A Jini Use Case
1. 
RMI Overview: Example taken from Java
a)  “Java” is not important – here for a use case of the concept of
distributed objects (code not examinable, nor is it in labs).
b)  What is RMI
c)  Serialization: to disk, sockets and in RMI
d)  RMI in Action: A Broad Overview
2. 
Jini
a) 
b) 
c) 
d) 
What is Jini
Jini Scenario: An Overview
Types of Jini Services
Jini in Operation: Jini Protocols
a)  Discovery
b)  Join
c)  Lookup
e)  Jini Scenario: Filling in the Gaps
f)  Jini Scenario: More Detail …
3. 
Jini: other
a)  Jini Event Model
b)  Jini Groups
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What is RMI?
To communicate across the network in Java, can use:
• 
Sockets – one to one, duplex connections
•  Need to pack data for socket and unpack other side
•  i.e. need to agree on format and internal protocol up front
• 
RPC (Remote Procedure Call)
•  abstracts communication interface to the level of a procedure
call
•  programmer appears to be calling a local procedure
•  encodes arguments and returns values using an external
data representation e.g. XDR
• 
RMI: Remote Method Invocation
• 
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Similar to RPC but uses Java Serialization to pack objects
Serialization in Java
java.io.Serializable & java.io.Externalizable – Java Interfaces
•  For storing and retrieving objects
•  represents the state of objects sufficient for reconstruction
•  Objects to be saved in the stream may support:
•  Serializable interface: default serialization mechanism
•  Externalizable interface: custom serialization
Java
Object
Serialize
(ByteStream/String)
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Java
Object
Disk File
TCP Socket
RMI Argument/
Parameter
Deserialize
(ByteStream/String)
Serialize/Deserialize to Disk File
Serialize today's date to a file:
FileOutputStream f = new FileOutputStream("tmp");
ObjectOutputStream s = new ObjectOutputStream(f);
s.writeObject(new Date());
s.flush();
Deserialize today's date from a file:
FileInputStream in = new FileInputStream("tmp");
ObjectInputStream s = new ObjectInputStream(in);
Date date = (Date)s.readObject();
Same for Sockets … - RMI does this for you automatically
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Serializing a Class
To Implement
Default
Serialization
import java.io.Serializable;
public class PersistentClass implements Serializable {
transient private String notToCopy;
private String name;
To tag
Instance
variables
which
should not
be copied
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public PersistentClass() {
notToCopy = whatever ;
name = PersitentObject ;
}
}
RMI In Action: A Broad Overview
Known as a Java
Proxy Object
1. Create a Remote
Interface e.g.
public void sendMessage(String message);
4. Call Remote
Function by using
Proxy
RMI Service
5. Returns data from remote call (if any)
RMI Client
the network (TCP/IP)
2. Create Implementation of
Remote Interface
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3. Create Client Side that
wants to call remote method
What is Jini?
Historically, operating systems rely on disk drives …
Jini’s goal is to shift this reliance back to the network
Key Features:
• 
• 
Written in Java
Uses RMI and Java Object Serialization to enable Java objects
to move around the network
• 
Offers network plug and play of services (java objects)
Differences with RMI:
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• 
Provides Discovery of Jini Services
•  Published RMI stub -> RMI proxy
• 
Distributed Service concepts e.g.:
• 
Searching,
• 
Leasing and Grouping
• 
Distributed Events
Jini Concepts and Architecture
1.  Jini Players
2.  Broad Overview
3.  Joining and Using a Jini Network
a.  Discovery
b.  Join
–  Leasing
c.  Lookup
4.  A Jini Scenario: Filling in the gaps
5.  Jini Scenario: More Detail..
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Jini Players
Jini defines a runtime infrastructure that provides mechanisms that enable you to add,
remove, locate, and access services. There are three main players:
a service, such as a printer,
scanner, storage device, a
software service etc.
a client which would like to
make use of this service.
a lookup service
(LUS) - a service
locator
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… and yes, and the network connecting all three - generally be running TCP/IP
Jini Scenario: Broad Overview
4. Jini client uses
proxy to contact Jini
service directly
3. Jini client
receives Java
proxy for Jini
Service
Jini Service
1. Jini service
discovers LUS
and registers its
service
LUS –
Lookup Service
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the network (TCP/IP)
Jini Client
(Consumer)
2. Jini client
discovers LUS
and locates the
desired Jini
service
Brokered
Architecture
Using Jini Services
Three ways proxies interact with the service:
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• 
Proxy performs the service itself:
• 
Java proxy code is completely self-contained – like an applet
• 
does not require any remote functionality
• 
Proxy is an RMI stub for a remote service:
• 
proxy is a minimal piece of code - interface to the remote object
• 
client makes a call on the proxy object, then make remote invocation (by
transferring the arguments to the remote object)
• 
Proxy is a Smart Adapter: Proxy contains code that decides where to execute
it e.g.: execute code elsewhere (e.g. Web) or contact services written in other
programming languages
Jini Federations
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•  Federation of services available on the Jini network
•  cleints enlist the help of services to accomplish their tasks
Joining and Using a Jini Network
Jini defines one network-level protocol -discovery
and two object-level protocols - join and lookup
Discovery - enables clients and services to locate lookup
services (LUS)
Join - enables a service to register itself in a lookup
service
Lookup - enables a client to query a lookup service for
available services.
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Discovery
•  Unicast Interaction
–  Client knows location of Jini LUS e.g. jini://spectra.astro.cf.ac.uk
–  Use when static connections are needed
•  Multicast dynamic discovery
–  Uses the multicast request protocol
•  Sends multicast request message to a well known multicast address
•  LookUp service gets the request and connects to the requesting service.
•  Sends a unicast message which contains proxy for the lookup service.
…whichever the method, when
LUS is contacted:
LUS return a ServiceRegistrar
Object (using RMI)
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Service
Registrar
The Join Process
Allows services to register as Jini services with the LUS
register(ServiceItem, long)
Service
Registrar
Jini Service
•  Pass register() a ServiceItem object + lease duration (next….)
ServiceItem Class:
•  ID for the service (automatically generated by LUS)
•  object implementing the actual Jini Service
•  attributes for service - list of objects
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Leasing in Jini
•  used to keep track of active Jini services
•  can be set by service or controlled by LUS
•  services use leasing to register that they are alive, but allow themselves to be
``timed out'' if they have failed or if they are unreachable.
•  lookup service acts as the granter of the lease.
Setting Lease Duration:
Services can use defaults or set manually:
•  Lease.ANY - the service lets the lookup service decide on the time
•  Lease.FOREVER - the request is for a lease that never expires
Grant Lease
4 Second
Lease
Scenario
Jini Service
LUS
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2
4
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Lookup
•  When a service has registered - its available for use by clients
•  Clients use lookup to find services
Jini Client
(Consumer)
Lookup(ServiceTemplate t)
Service
Registrar
(Obtain ServiceRegistrar
using Discovery)
ServiceTemplate:
•  contains the search query
•  typically search using Java type (i.e. interface)
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public interface JiniPrinter {
// print a document contained in String or throw PrinterException
public void print(String text) throws PrinterException;
}
public Class LaserPrinter implements JiniPrinter {
public int print(String text) throws PrinterException {
// implement Laser-specific code here or throw exception
}
}
Jini Scenario: Filling the Gaps
Jini Service
4. Jini client uses
proxy to contact Jini
service directly
3. Jini client
receives Java
proxy for Jini
Service
1. Jini service
discovers LUS
(Discovery) and
registers its
service (Join)
LUS –
Lookup Service
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the network (TCP/IP)
Jini Client
(Consumer)
2. Jini client
discovers LUS
(Discovery) and
locates the
desired Jini
service (Lookup)
Jini Scenario: More Detail….
7. Jini client uses Java proxy
to communicate and use the
Jini service
Jini Service
3. invokes
register() on
ServiceRegistrar
object to
register
service
6. invokes
lookup() on
ServiceRegistrar to
locate desired service
(returns
Java proxy)
5. receives a
ServiceRegistrar object
2. receives a
ServiceRegistrar
object
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1. Jini service
discovers LUS
Jini Client
(Consumer)
LUS –
Jini Lookup Service
4. Jini service
discovers LUS
Jini Event Model
•  Jini objects may also be interested in changes in other Jini
objects i.e. want to be listeners for such changes.
•  Networked nature of Jini means need a different event model:
–  Network delivery is unreliable
–  Network delivery is time-dependent
–  A remote listener may have disappeared by the time the event
occurs. Listeners have to be allowed to ``time out'', like services
do.
•  Uses one event type RemoteEvent and small number of
sublclasses – are serializable
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Organization of Jini Services
•  Jini Services are organized as a federations of services
i.e. communities
•  Grouping
•  similar to communities – groups are the names used
to represent communities
•  Based around LUS -> client can search for specific
groups
•  LUS can be members of many groups
•  not globally unique
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Distributed Objects
1. 
RMI Overview
a)  What is RMI, Serialization, Broad Overview
2. 
Jini
a) 
b) 
c) 
d) 
What is Jini
Jini Scenario: An Overview
Types of Jini Services
Jini in Operation: Jini Protocols
a)  Discovery
b)  Join
c)  Lookup
e)  Jini Scenario: Filling in the Gaps
f)  Jini Scenario: More Detail …
3. 
Jini: other
a)  Jini Event Model
b)  Jini Groups
4. 
Focus
a)  To provide a context for distributed objects
b)  E,g, what are the difference between distributed objects and
services?
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