Download Java RMI Example

Java EE Programming
UDP & TCP
Java’s RMI
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Topics
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Networking and Internetworking
Inter-process communication
Distributed objects and remote
invocation
UDP & TCP
Java’s RMI
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Networking Issues for
Distributed Systems
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Previous networks had to meet simple
application requirements:
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File transfer, remote login, e-mail and
newsgroups
With the growth of the Internet, more
stringent requirements are necessary:
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New modes of use
Additional and more stringent requirements
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Current Networking
Requirements & Terms
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Performance
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Speed with which individual messages can be
transferred between two interconnected
computers
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Latency – is the delay that occurs after a send operation
is executed before data starts to arrive at the destination
computer (network latency)
Data transfer rate – is the speed at which data can be
transferred between two computers in the network once
the transmission has begun
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Bits per second
Message transmission time =
latency + length/data transfer rate
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Current Networking
Requirements & Terms
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Scalability
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The potential future size of the Internet is
commensurate with the population of the
planet
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Indicates size and load of the Internet
Networks must scale
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Current Networking
Requirements & Terms
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Reliability
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The degree in which an application can recover
from communication failures
Security
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Techniques for achieving security in distributed
systems
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Firewalls – creates a protection boundary between an
organization’s intranet and the rest of the Internet
Distributed systems need a secure network environment
with end-to-end authentication, privacy, and security
Example - encryption
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Current Networking
Requirements & Terms
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Mobility
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Wireless networks provide connectivity to devices
such as laptops, cell phones, PDAs
Addressing and routing schemes of Internet were
developed before the advent of wireless devices
so are not well-adapted to their intermittent
connection to many different subnets
Quality of service
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Applications that transmit multimedia data require
guaranteed bandwidth and latencies
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Current Networking
Requirements & Terms
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Multicasting
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One-to-many communication
Needed in distributed systems
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Types of Networks in Distributed
Systems
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PAN – Personal area network
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Various digital devices carried by a user are connected to a
low-cost, low-energy network
LAN – Local area network
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Carry messages at high speed between computers
connected by a single communication medium
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WAN – Wide area network
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Twisted copper wire, coaxial cable, fiber optics
Carry messages a lower speeds between nodes that are
often in different organizations and may be separated by
large distances
Uses routers to route the messages
MAN – Metropolitan area network
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High speed network installed in towns/cities for transmission
of data and video
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DSL and Cable modem connections
Types of Networks in Distributed
Systems
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WLAN – Wireless local area network
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Used in place of wired LANs for wireless
mobile devices
IEEE 802.11 (WiFi) standard protocol
WMAN – Wireless metropolitan area
network
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Supercedes 802.11
IEEE 802.16 WiMAX protocol
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Types of Networks in Distributed
Systems
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Internetworks – communication
subsystem in which several networks
are linked together to provide common
data communication facilities
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Overlay the technologies and protocols of
the individual component networks and
methods used for their interconnection
Needed for the development of extensible,
open distributed systems
Example – Internet
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Network performance
Example
Range
Bandwidth Latency
(Mbps)
(ms)
LAN
Ethernet
1-2 kms
10-1000
WAN
IP routing
worldwide
0.010-600 100-500
MAN
ATM
250 kms
1-150
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Internetwork
Internet
worldwide
0.5-600
100-500
WPAN
Bluetooth (802.15.1)
10 - 30m
0.5-2
5-20
WLAN
WiFi (IEEE 802.11)
0.15-1.5 km 2-54
5-20
WMAN
WiMAX (802.16)
550 km
1.5-20
5-20
WWAN
GSM, 3G phone nets
worldwide
0.01-02
100-500
Wired:
1-10
Wireless:
Network Protocols
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Protocol – refers to a well-known set of
rules and formats to be used for
communication between processes in
order to perform a given task
Two parts:
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a specification of the sequence of messages
that must be exchanged
a specification of the format of the data in
the messages
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Conceptual Layering of
Protocol Software
Message received
Message sent
Layer n
Layer 2
Layer 1
Sender
Communication
medium
Recipient
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Protocol Layers
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Network software is arranged in a hierarchy
of layers
Each layer presents an interface to the layers
above it and below it
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Sending side – each layer (except top) accepts
items of data in a specified format from the layer
above it and applies transformations to
encapsulate the data in the format specified for
that layer
Receiving side – converse transformations are
applied to data items received from the layer
below before being passed to layer above
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Encapsulation as it is applied
in Layered Protocols
Applic ation-layer mess age
Pres entation header
Sess ion header
Transport header
Netw ork header
The tags in the headers are the protocol type for the layer above,
needed for the receiving protocol stack to correctly unpack the packets
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Protocol Suites
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Complete set of protocol layers is referred to
as a protocol suite or protocol stack
Reference Model for Open Systems
Interconnection (OSI)
Protocol layering brings substantial benefits in
simplifying and generalizing the software
interfaces for access to the communication
services of networks
Also carries significant performance costs
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Why do you think this is so?
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Protocol layers in the ISO Open
Systems Interconnection (OSI) model
Mess age receiv ed
Mess age s ent
Lay ers
Applic ation
Pres entation
Sess ion
Transport
Netw ork
Data link
Phy sical
Sender
Communic ation
medium
Recipient
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OSI Protocol Summary
Layer
Application
Presentation
Session
Transport
Network
Data link
Physical
Description
Protocols that are designed to meet the communication requirements of
specific applications, often defining the interface to a service.
Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which may
differ. Encryption is also performed in this layer, if required.
At this level reliability and adaptation are performed, such as detection of
failures and automatic recovery.
This is the lowest level at which messages (rather than packets) are handled.
Messages are addressed to communication ports attached to processes,
Protocols in this layer may be connection-oriented or connectionless.
Transfers data packets between computers in a specific network. In a WAN
or an internetwork this involves the generation of a route passing through
routers. In a single LAN no routing is required.
Responsible for transmission of packets between nodes that are directly
connected by a physical link. In a WAN transmission is between pairs of
routers or between routers and hosts. In a LAN it is between any pair of hosts.
The circuits and hardware that drive the network. It transmits sequences of
binary data by analogue signalling, using amplitude or frequency modulation
of electrical signals (on cable circuits), light signals (on fibre optic circuits)
or other electromagnetic signals (on radio and microwave circuits).
Examples
HTTP, FTP , SMTP,
CORBA IIOP
Secure Sockets
(SSL),CORBA Data
Rep.
TCP, UDP
IP, ATM virtual
circuits
Ethernet MAC,
ATM cell transfer,
PPP
Ethernet base- band
signalling, ISDN
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Internet Protocols
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TCP/IP – Transmission Control
Protocol/Internet Protocol
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Grown now to over 60 million hosts
Example application services &
protocols based in TCP/IP:
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Web (HTTP)
email (SMTP, POP)
netnews (NNTP)
file transfer (FTP)
Telnet (telnet)
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TCP/IP Layers
Message
Layers
Application
Messages (UDP) or Streams (TCP)
Transport
UDP or TCP packets
Internet
IP datagrams
Network interface
Network-specific frames
Underlying network
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TCP/IP Layers
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Two transport protocols
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Internet Protocol (IP) is the underlying
‘network’ protocol
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TCP – reliable connection-oriented protocol
UDP – datagram protocol that does not guarantee
reliable transmission
IP datagrams provide the basic transmission
mechanism for the Internet and other TCP/IP
networks
Internet protocols are usually layered over
other network technologies
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i.e, Ethernet
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Encapsulation in a message
transmitted via TCP over an Ethernet
Application message
TCP header
IP header
port
TCP
Ethernet header IP
Ethernet frame
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The programmer's conceptual
view of a TCP/IP Internet
Applic ation
Applic ation
TCP
UDP
IP
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IP Addressing
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Scheme chosen for naming and
addressing hosts and routing IP packets
to their destination
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Assigns an IP address to each host in the
Internet – a 32-bit numeric identifier
containing:
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network identifier – uniquely identifies one of
the subnetworks in the Internet
host identifier – uniquely identifies the host’s
connection to that network
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Internet Address Space
Clas s A:
Clas s B:
0
7
24
Netw ork ID
Host ID
1 0
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16
Netw ork ID
Host ID
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Clas s C:
1 1 0
8
Netw ork ID
Host ID
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Clas s D (multicast ):
1 1 1 0
Multicast address
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Clas s E (reserved):
1 1 1 1 0
unused
Class A is reserved for very large networks (i.e.,US NSFNet)
Class B is used for organizations that operate networks likely to contain more
than 255 computers
Class C is allocated to all other network operators
Class D is reserved for Internet multittask communication
Class E contains a range of unallocated addresses – reserved for future use
TCP and UDP
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Provide the communication capabilities of the
Internet in a form that is useful for application
programs
TCP and UDP are transport protocols and must
provide process-to-process communication
Accomplished by using ports
Port numbers are used for addressing messages to
processes within a particular computer and are only
valid for that computer
Once an IP packet has been delivered to its
destination host, the TCP- or UDP-layer software
dispatches it to a process via a specific port at that
host
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UDP Features
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UDP datagram is encapsulated inside an IP
packet
Short header with source and destination port
numbers, length field and a checksum
UDP does NOT guarantee delivery
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Adds no additional reliability mechanisms except for
checksum (which is optional)
Use is restricted to applications and services
that do not require reliable delivery of single or
multiple messages
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TCP Features
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TCP provides a much more sophisticated
transport service
Provides reliable delivery of a long sequences
of bytes
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Guarantees the delivery to the receiving process of
all the data presented to the TCP software by the
sending process in the same order
Provides retransmission if necessary
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Done when the it does not receive an acknowledgement
from the receiving process within a specified timeout
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Middleware
Applic ations, services
RMI and RPC
reques t-reply protocol
This
c hapter
Middlew are
lay ers
marshalling and ex ternal data representation
UDP and TCP
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Looking at IPC from the Programmer’s point of view
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Remote Method Invocation (RMI)
Remote Procedure Call (RPC)
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Interprocess Communication
(IPC)
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Distributed Systems
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rely on exchanging data and achieving synchronization
amongst autonomous distributed processes
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Inter process communication (IPC)
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message passing in concurrent programming languages
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shared variables
message passing
language extensions
API calls
Principles of IPC
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(Concurrent programs: collections of two or more sequential
programs executing concurrently
Concurrent processes: collection of two or more sequential
programs in operation, executing concurrently
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IPC Characteristics
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Message passing between pairs of
processes can be supported by two
message communication operations:
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send
receive
In order for one process to communicate
with another, one process sends a
message (sequence of bytes) to a
destination and another process at the
destination receives the message
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Types of Communication
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Synchronous
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Sending and receiving process synchronize
at every message
Both send and receive are blocking
operations
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Whenever a send is issued, the sending
process (or thread) is blocked until the
corresponding receive is issued
Whenever a receive is issued, the process (or
thread) blocks until a message arrives
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Types of Communication
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Asynchronous
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Send is non-blocking – sending process is allowed
to proceed as soon as the message has been
copied to a local buffer
Receive operation can have blocking and nonblocking variants
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Non-blocking – receiving process proceeds with its
program after issuing a receive operation which provides
a buffer to be filled in the background
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Must separately receive notification that is buffer has been
filled by polling or interrupt
Today’s systems do not generally provide the nonblocking form of receive because it adds complexity to
the application
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Sockets
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Both UDP and TCP use socket
abstraction
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Provides an endpoint for communication
between processes
Originated from BSD UNIX but in all Unix
flavors (i.e., Linux)
IPC consists of transmitting a message
between a socket in one process and a
socket in another process
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Sockets and Ports
socket
any port
agreed port
socket
message
client
server
other ports
Internet address = 138.37.94.248
Internet address = 138.37.88.249
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Sockets and Ports
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For a process to receive messages, its socket
must be bound to a local port and one of the
Internet addresses of the computer on which it
runs
Messages sent to a particular Internet address
and port number can be received only by a
process whose socket is associated with that
Internet address and port number
Processes may use the same socket for
sending and receiving messages
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Moving from Interprocess to
Distributed Communication
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A distributed system breaks up a
previously monolithic application into
different components and then proceeds
to place them on different systems (i.e.
tiers).
We are decoupling the consumer and the
supplier; we are spreading the activities
of an application onto team players
(systems)
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Moving from Interprocess to
Distributed Communication
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Analogy
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Let us imagine for a moment a small business run by a single
entrepreneur. This one person, Suzette Sellers, does everything,
much like a monolithic application. She sells to customers, orders
new product, manages the books, pays the bills, and does anything
else. Internally, she has compartmentalized her job function (much
like we program in a modular fashion): selling, ordering, and
administration. There is no overhead associated with external
communication she does everything.
When her store becomes successful, she finds that she simply
cannot do it all and requires assistance. Suzette hires Henry
Helpers, a capable and likeable chap. This is now similar to our
two-tier architecture. Because Suzette and Henry work in the same
physical location, this is a logical two-tier system. Communication
is quick and easy.
Think about two programs on a computer: they can communicate
via files, shared memory, pipes, or other constructs.
If you are familiar with processing on a UNIX- or Linix-based
system, multiple commands are often strung together with pipes.
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Moving from Interprocess to
Distributed Communication
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Suzette’s store does so well that she moves her office
into another building to increase the amount of retail
floor space in her original store. Now Suzette and
Henry are in different physical locations but still need
to complete the same tasks of selling, ordering, and
administration. We have moved into a physical two
tier distributed system and things just become more
complicated. Suzette and Henry still need to
communicate but the previous methods will no longer
work.
The obvious solution is the telephone, but this
changes the dynamic of the entire conversation
In our computer analogy, we now have a client and a
server tier where the previous solutions of files,
shared memory, or pipes are no longer solutions as
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they only work within a single system.
Moving from Interprocess to
Distributed Communication
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Suzette and Henry must now specifically locate
each other (i.e. phone numbers) before
communicating.
Conversations will be slower (network latency).
Data will need to be shared in different ways
(difficult to draw a picture on the phone—the
crayon keeps breaking on the handset).
And finally, the phone lines can go down or
have interference (network errors or
corruption).
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Moving from Interprocess to
Distributed Communication
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Yes, we have gained some benefits by spreading out
Suzette and Henry (more retail floor space) but we
have introduced complexity and latency into the
business.
Similarly, when a distributed application is placed on
different systems (whether physically or logically
separated), we also introduce complexity and latency
into the application.
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However, the ability to take advantage of multiple computers
is such a powerful incentive that we have devised many
ways to accommodate our new problems
The stage is now set to discover how to implement
this distributed communication.
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Middleware Layer
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By choosing Java, we are explicitly ignoring
the platform because Java (in theory) runs
exactly the same on any platform that has a
Java-compatible runtime environment.
We are not yet concerned with what
capabilities an application will exhibit other
than we will create a “server” component that
holds the desired properties and a “client”
component that requests these capabilities.
Assume that our distributed Java-based
application will communicate via the TCP/IP
protocol
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UDP Communication
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Basis of TCP/IP protocol stack is the Internet Protocol (IP)
data packet
IP is a connectionless and stateless protocol (“fire and
forget”)
Once a IP packet is sent, the sender simply does not care
about it anymore
IP packet can become lost, destroyed, misplaced, or
corrupted on its way to its destination
If multiple IP packets are sent – they can arrive out of
order
IP is quick to use but not reliable
Think of IP as simple postal envelopes: they can be easily
damaged, lost, and delivered out of order.

However, IP envelopes are cheap and convenient. (The
analogy to a postal service is very appropriate as the cost is
low per message and the service is usually reliable but there44
is no capability to trace a wayward envelope.)
UDP Communication

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
The User Datagram Protocol (UDP) is implemented on top of IP layer.
UDP is still a fairly lightweight protocol which means that most of its
effort is spent on delivering data.
UDP adds two important features: larger “payloads” and data
integrity.
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If a message is larger than the size of a single IP envelope, UDP will split
the message into multiple IP envelopes, taking care to remember how it
split up the original message.
UDP guarantees that the data in a single UDP packet in the will arrive
exactly as it was sent.
What happens if one of the IP packets that made up the UDP packet
is lost? Then UDP discards the rest of the IP packets and thus the
entire UDP packet rather than deliver a damaged UDP packet.
This is analogous to sending a four page letter in separate envelopes;
if envelope #3 does not arrive, then UDP will throw array the
remaining envelopes that did arrive as it has no way to reconstruct
the original four page letter.
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UDP Communication

UDP is also a stateless and connectionless
protocol

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
Stateless means that once a message is sent, the
sender does not care about it anymore.
If the message arrives, then that is fine. If it does
not arrive, we do not feel any grief
Connectionless means that there is no continued
conversation.


The message is complete in and of itself.
Think about sending postcards. If we send one postcard
from Europe and another from South America, neither of
the postcards depend upon each other. They are
separate communications
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UDP Design Considerations

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For client-server applications that transfer relatively small
amounts of data, UDP is acceptable
For the simplest application, a client will send a single
message to the server and expect a response
If the client does not receive a response within a predetermined amount of time, the client may send the
request again.
This is similar to asking a person to repeat an answer. Of
course, after asking for the answer so many times, it is
likely that we (the client) will just give up.
If a DNS server does not respond after a couple of
seconds, a client will simply accept that no answer is
forthcoming and either ask another DNS server or return
an error.
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UDP Example


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

UDP communications can be implemented in Java quite easily
However, it should be pointed out that UDP is simply a service to
ferry data from point A to point B.
UDP does not understand data types or other Java constructs,
such as objects.
To accommodate this understanding, we will use the capability of
the String class to convert itself into an array of bytes and back.
We are implicitly relying on the String class to correctly convert an
array of bytes using the correct code set.
Both the server and client will perform many of the same
operations but the server has the additional task of existing in a
state where it can receive requests at any time and must respond
to them. A client, on the other hand, will only be executed on
demand and exit as soon as it receives a response.
48
UDP Server



The UDP server may look complicated but there is
actually very little to it.
The server first creates a socket, which is a logical
construct that we may attach to an actual port.
We have hard-coded the port upon which our server
will listen, 7777.




You may choose any port between 1024 and 65535
(assuming that it is not already in use).
The server attaches to the port and then performs an
endless loop of listening for any incoming message
and sending the same message (known as a
datagram) back to its sender.
The effect is similar to a mirror
See UDPServerSimple.java
49
UDP Client

The UDP client seems to be much longer in length but
that is because we have added in the ability to specify
the message to send, the server name, and the port on
which the UDP server is listening on the command line,
rather than hard-coding it.



This approach should allow you to run the program as-is, even
though you will run it on your system (which has a different
network name and IP address than everybody else’s system).
The client, similar to the server, first creates a socket
that can be bound to a network port.
However, we will not bind it to a particular port because
we do not care—just as when you send a letter, the
mailbox you use is irrelevant to the receiver.
50
UDP Client






The client program then resolves the name of the server
(first argument) to an IP address, which is what the UDP
protocol requires.
The second argument is the port that the UDP server is
listening on, and the third argument is the actual message
(remember to enclose it in double quotes if it contains a
space).
A datagram message is then created and sent to the
server.
Immediately after sending the message, the client then
acts like the server, waiting for a reply.
Although the client did not request a specific port, the
port that was automatically assigned is included in the
outgoing message so that the server can successfully
respond back to us.
See UDPClientSimple.java
51
UDP Example In Action



We first start the UDP server which
opens the port and waits for the first
incoming message.
It will wait until the program is
terminated
Although not shown, the UDP server is
running on a system with the name
“Alpha” (as in the first Greek letter).
52
UDP Example In Action


The server gets up and running quickly and then
immediately starts waiting for eternity
Then run the UDP client which sends a message and
then itself waits for a reply.
53
UDP Example In Action


Notice that the client runs and then exits.
This is typical of most client-server
communications where the client makes a
request, receives a reply from the server and is
done.
54
UDP Example In Action

Meanwhile, the server has recorded the
incoming message, responded to it, and
begun waiting for another incoming
request.
55
TCP Communication


TCP network communication is similar to UDP
but it “fixes” the flaws inherent in UDP by
providing message delivery guarantee and
concurrency.
TCP goes a step beyond UDP and is a stateful
and connection-oriented protocol.


Stateful means that the TCP remembers what it has
sent and ensures that it arrives in the same order
Connection-oriented means that each packet sent is
actually part of a larger conversation, or stream.
56
TCP Communication



A TCP-based server differs from a UDP-based
server in that a single thread does not perform
all of the communication.
The usual solution to handle TCP
communications is to create a new thread that
is dedicated to incoming requests from a
particular client.
A server can distinguish multiple requests from
the multiple clients on the same client system
because each client will be forced to use a
different port
57
TCP Example




The programs for the TCP client and server are
similar in nature although the details vary a bit.
Because TCP is stateful, we actually open up two
streams: one inbound and one outbound
These act like conveyor belts for data to flow
between the client and the server.
From a programming standpoint, we no longer
have to use datagrams but can simply read and
write a simple data type or a string object.
58
TCP Server



For the server, the largest change is that
additional of threads in order to maintain
each connection as a separate
conversation.
However, once inside of a thread, the
commands are simply reading and
writing data.
See TCPServer.java
59
TCP Client



The TCP client is very similar to the UDP
client except that instead of sending
datagrams, we hook up to the streamsbased plumbing.
However, the rest of the logic is pretty
much the same.
See TCPClient.java
60
TCP Example in Action


Set CLASSPATH first!
Start with the server first
61
TCP Example in Action


Next we invoke the TCP client to send a simple message.
We can see that the client was able to acquire a port to
communicate out and received a reply that was different than
what was originally sent, proving that “something” happened.
62
TCP Example in Action



The server first received a connection from the remote port of
4304 (the client).
It then created new clientConnection object (the two lines with
the word setup in the brackets) and then went back to listening
for more connection requests.
Meanwhile, the new thread was run and it picked up the message
sent by the client and then sent it back with the prefix
“Received:”.
63
Remote Method Invocation
(RMI)



Provides for communication between
distributed objects by means of remote
method invocation
RMI = objects that can receive remote method
invocations are called remote objects and they
implement a remote interface
Is an extension of local method invocation that
allows an object living in one process to invoke
methods of an object living in another process
64
Middleware Layer for RMI
Applications
RMI, RPC and events
Request reply protocol
Middleware
layers
External data representation
Operating System
65
Interfaces in Distributed
Systems




An interface specifies the procedures and
variables that can be accessed from other
modules
Modules are implemented so as to hide all the
information about them except that which is
available through its interface.
As long as the interface does NOT change,
implementations may change without affecting
the users of the module
In a distributed program, the modules can run
in separate processes.

Cannot access the variables in a module in another
process – not direct variable access
66
Interfaces in Distributed
Systems

Other differences:

Parameter-passing mechanisms (i.e., call by
value, call by reference) used in local
procedure calls do not work in a distributed
system


Uses input and output parameters that are
passed between processes
Pointers in one process are not valid in
another process on a remote computer

No pointer passing allowed
67
Remote and Local Method
Invocations
local
remote
invocation
A
B
C
local E
invocation
invocation
local
invocation
D
remote
invocation
F
68
Java RMI



Java RMI provides an adapter that effectively hides the
network from us, instead allowing us to simply
“lookup” functionality that we want (regardless of
where it actually exists) and use it as if it was locally
available.
The level of complexity definitely increases once we
decide to distribute the objects in a Java system.
Assuming that we started with two Java programs,
with one calling the other, we now introduce the
following elements:



an interface that explicitly states what functionality is remotely
available
a server that provides access to the remote object
the RMI registry that links up requests from a client to a
remote object
69
Overview of RMI
Interfaces and Classes
70
Java RMI Example
Foundation


We will start with two Java classes that are not
RMI-enabled and then adapt them to operate
with RMI so we can see exactly what changes
are needed.
Let us imagine that we have devised an
automated registration process class which,
upon knowing which class for which we want
to register, will tell us if we are registered or if
the class is full.
71
AskTheRegistrar.java
public class AskTheRegistrar
{
private String theClass;
public AskTheRegistrar ( String in_whichClass )
{
theClass = in_whichClass;
}
public String getRegistrationStatus()
{
if ( theClass.toUpperCase().indexOf( "CS483" ) >= 0 )
{
return "You are registered";
}
else
{
return "There are no positions available to be registered";
}
}
}
If the class includes the word “CS483” (case sensitive) then the class
must have some spots open. Otherwise, the class is full.
Java Application to Invoke the Functionality – LocalStudent.java
class LocalStudent
{
public static void main( String args[] )
{
System.out.println();
System.out.println( "LocalStudent running..." );
System.out.println( "--------------------------" );
System.out.println();
String studentClass;
In order to work, both classes
if ( args.length < 1 )
just need to be in a directory within
{
the current CLASSPATH
studentClass = "CS483";
}
environment variable
else
{
studentClass = args[0];
}
AskTheRegistrar currentRegistrationStatus
= new AskTheRegistrar(studentClass);
System.out.println( "The registration status for " +
studentClass + " is:");
System.out.println(" > “
+ currentRegistrationStatus.getRegistrationStatus());
System.out.println();
System.out.println( "--------------------------" );
System.out.println( "LocalStudent finished." );
}
Result when run...

By passing the argument “CS483”, the
AskTheRegistrar service concludes that the
student is registered
74
Java RMI Example
(with a server)


We now seek to move the AskTheRegistrar service to a
remote server so that it can be accessed from multiple
locations within the university.
The first step is to decide what functionality should be
made available.




Currently, the AskTheRegistrar class has two member
functions: the constructor and getRegistrationStatus.
The constructor will not be available remotely and so we need
to create an alternative method to set the location.
These two functions, setClass and
getRegistrationStatus, will be available to
remote invocation and we can now create an interface
detailing this decision.
Need to enhance the object AskTheRegistrar so
that it can be remotely used
75
RMI Server Example –
Sequence Diagram
76
AskTheRegistrarInterface.java
import java.rmi.*;
public interface AskTheRegistrarInterface
extends Remote
{
public void
setClass( String in_whichClass )
throws RemoteException;
public String getRegistrationStatus( )
throws RemoteException;
}
*Remember an interface is a contract that a class can choose to
implement so there is no actual code in the interface itself
77
New Version of AskTheRegistrar



See AskTheRegistrarRemote.java
Imported java.rmi and java.rmi.server packages to
enable remote invocation capability
Class inherits from the UnicaseRemoteObject
parent class



This object means that a single instance (as opposed to one
instance per request) of the AskRemoteRegistrarRemote
object will be enabled to be called remotely
Provides methods outlined in the interface
Added an additional constructor that takes no
arguments and sets the private theClass variable
78
RMI Registry


A client wishing to use a remote object will
need to know exactly where it is located
RMI registry



Coordinated “phone book” that records every
object’s name and address
Publishes object’s info in a well-known location
rmiregistry.exe



In the same directory as other java executables
Can be run by itself as a background process
We will run in it’s own command window
79
RMI Registry



RMI Registry starts off with an empty directory
Has no way of actually invoking any objects
It’s only job is to provide locations to any program that
asks (like directory assistance)
80
RMI Server Example


We now need a Java program similar to
our UDP server that accepts requests for
a remote object
This server program must also
communicate to the RMI registry that it
exists so clients can find it
81
AskTheRegistrarServer.java
import java.rmi.*;
import java.rmi.server.*;
class AskTheRegistrarServer
{
public static void main ( String[] args )
{
try
{
Naming.rebind("//localhost/AskTheRegistrar",
new AskTheRegistrarRemote());
System.out.println("AskTheRegistrar Server is ready.");
System.out.println();
}
catch (Exception e)
{
System.out.println("AskTheRegistrar Server unexpectedly quit. “
+"Exception message: " + e.getMessage());
} //end try
}
//end main
}
//end class
RMI Server Example


This seems like a very small amount of
code compared to our previous UDP
server.
The magic happens on a single line:
. . .
Naming.rebind(
"//localhost/AskTheRegistrar",
new AskTheRegistrarRemote()
);
. . .
83
RMI Server Example


The rebind command is what actually informs
the RMI registry that a remote object is now
available for remote invocation.
The first argument is the remote object’s name in
the directory as well as which RMI registry will be
informed.



In our example, we are contacting the registry on the
local server (i.e. localhost).
If the registry was on a port other than its default 1098,
this could be specified by placing a colon and the new
port number directly after the server name.
The identifier after the server is the name that the
object will be listed as in the registry.

This directory name does not have to be the same as
the actual remote object.
84
RMI Server Example

The second argument creates the actual remote object
that will accept requests from clients.



Because we are unsure of what location a client will specify,
we invoke the no-argument version of the constructor which
will set the internal location to an empty string (and thus
require the client to specify an actual location later).
Most important point to heed is that in our non-RMI
setup (previous example), each instance of the client
application (LocationStudent) created its own instance
of the AskTheRegistrar service class.
In the RMI version, we have a single instance of the
AskTheRegistrar class that will be used by any and all
clients.
85
RMI Server Example




Before we can run the server, we need to generate a “stub”
class for the remote object that will be used by the server.
We use the rmic utility to do this.
Server does not exit – RMI registry still has an open
reference to it – continuously available to any incoming
requests
Now that we have a server, we run it from the command
line like so:
86
Client Example


Create a client that accesses the
AskTheRegistrar remote object
Two critical lines:
. . .
AskTheRegistrarInterface theRegistrar =
(AskTheRegistrarInterface) Naming.lookup (
"//" + remoteServer + "/AskTheRegistrar"
);
. . .
System.out.println( " > " +
theRegistrar.getRegistrationStatus());
. . .
87
Client Example



The first uses the same class, Naming, that the server did to contact the
RMI registry.
In this case, we are asking the registry for the location of a remote
object that should be known as “AskTheRegistrar”.

Note that what is returned is a reference to the interface we initially
created.

This interface hides the fact that there is no code locally but simply
provides some stubs that will make a call to the remote object to
perform the actual work.
The second line uses the resulting object reference as if it were local—
which was our original intent
. . .
AskTheRegistrarInterface theRegistrar =
(AskTheRegistrarInterface) Naming.lookup (
"//" + remoteServer + "/AskTheRegistrar"
);
. . .
System.out.println( " > " +
theRegistrar.getRegistrationStatus());
. . .
88
Client Example

We can now see the result of running
the client:
89
Server Output

This is the activity on the server:

Server displays the caller’s (client) IP
address
90
Java RMI Example
(with a daemon)





The previous example requires us to run a separate
Java application in a server capacity to register and
make a remote object available.
If we wish to make multiple objects available as
remote objects, we will either need to run a separate
server for each or create “super-server” that registers
one of each object.
To solve this problem use the RMI daemon
(background process), or rmid.
This one process will replace all of the other server
processes and allow any remote object to be
dynamically invoked
No adjustments to the interface or the client code are
91
required.
Java RMI with the Daemon Sequence Diagram
Java RMI Example
(with a daemon)


The remote object that we created in
AskTheRegistrarRemote last time needs
to be modified to inherit from a different,
but related object—the Activatable object
(which implies that the remote object
may be “activated” on demand).
For clarity, we will rename the class
AskTheRegistrarRemote to
AskTheRegistrarActivation.
93
import
import
import
public
java.rmi.*;
java.rmi.server.*;
AskTheRegistrarActivation.java
java.rmi.activation.*;
class AskTheRegistrarActivation
extends Activatable
implements AskTheRegistrarInterface
{
private String theClass;
// + This constructor (which was not required when
//
we used the server-based form of RMI) will
//
be automatically called by the RMI daemon.
//
public AskTheRegistrarActivation(
ActivationID id,
MarshalledObject data
)
throws RemoteException
{
// + Register the object with the activation system
//
then export it on an anonymous port
//
super(id, 0);
// + Here is our additional code.
theClass = "(No class set)";
}
. . .
}
Java RMI Example
(with a daemon)


Instead of inheriting from UnicastRemoteObject,
the remote object now inherits from Activatable.
Both of the constructors that previously existed have
been removed and replaced with one that has two
arguments – Activation ID & MarshalledObject


Any initialization code we need is placed after the parent
constructor is called. Other than those changes, the code for
the remote object is the same.
What has changed the most is that instead of a server
process, we only need to register the object as
available for remote invocation and then create a stub
for use with the new daemon process.
95
Java RMI Example
(with a daemon)
Server Application




See AskTheRegistrarRMIDSetup.java
The server application has undergone some
dramatic change.
The first statement inside the main class
provides the first hint of the alterations.
We are now operating with enhanced security
restrictions that we will need to alter in order
for our remote object to accept any
connections (a remote object that
communicates with nobody is not very useful).
96
Java RMI Example
(with a daemon)
Server Application

To accomplish these alterations, we need
a security manager to be available like
so:
. . .
System.setSecurityManager( new RMISecurityManager());
. . .
97
Java RMI Example
(with a daemon)
Server Application


Next, we need to associate a security policy file with the remote
object when it is activated.
The policy file simply states that the remote object is allowed to
accept connections via the network from any address as long as
the originating port is unprivileged (i.e. above 1023).

It should be noted that although the policy file kind of looks like Java
code, it is a configuration file and not compiled; it remains a text file
called AskTheRegistrarActivation.policy
grant
{
// + This allows the remote AskTheRegistrar object to accept
//
incoming connections from anywhere, as long as it is on
//
an unprivileged port (which is normally the case).
//
permission java.net.SocketPermission "*:1024-", "accept";
};
98
Java RMI Example
(with a daemon)





The RMI registry is still operating as it was previously
therefore it must be run first of all the server-based
processes.
The RMI daemon must be run next.
However, it will default to being very restrictive with
any of the remote objects that will be registered with it
as available to be activated.
So, we must define a security policy for the RMI
daemon itself that allows future remote objects to be
allowed to register their own security policy.
This could potentially open the door for arbitrary
permissions which is why this step is explicitly
required.
99
Java RMI Example
(with a daemon)


Our security policy will look like the following: RMID.policy
This policy has only one purpose: future remote objects that will
be registered may use the security policy located in a file
Z:\CS483\AskTheRegistrarActivation.policy. Any other request for
elevated permissions will be expressly denied.
grant
{
// + This allows the remote AskTheRegistrar object to
//
request the security policy in the file
//
Z:\CS483\AskTheRegistrar.policy.
// + If this is not allowed, then the remote AskTheRegistrar
//
object will either not be allowed to registered, or
//
will be registered with no ability to make network
//
connections.
//
permission com.sun.rmi.rmid.ExecOptionPermission
"-Djava.security.policy=Z:/CS483/AskTheRegistrarActivation.policy";
};
100
Run the daemon

By default, the RMI daemon uses a security
policy of “none” (which is like selling a padlock
with no key) so we must change this when
running the daemon process.
C:> %JAVAHOME%\rmid –J-Djava.security.policy=RMID.policy

Start the daemon:
101
Java RMI Example
(with a daemon)



Next, we must register the AskTheRegistrar
remote object with the daemon and registry.
The registration program itself needs permission to
contact both the daemon and the registry.
Here is its policy file:
grant
{
// + This allows the AskTheRegistrar setup to connect to
//
the RMI registry.
//
permission java.net.SocketPermission "Alpha:1098", "connect";
// + This allows the AskTheRegistrar setup to connect to
//
the RMI daemon (background process).
//
permission java.net.SocketPermission "Alpha:1099", "connect";
};
102
Java RMI Example
(with a daemon)


Assuming that the remote AskTheRegistrar object will reside on
the server known as “Alpha”, the policy file allows the setup
program to contact the registry on the default port 1098 and the
daemon on the default port of 1099
If the registration succeeds like we sincerely hope it will, we
should get two status messages:
C:> java -Djava.security.policy=AskTheRegistrarRMIDSetup.policy
AskTheRegistrarRMIDSetup
103
Java RMI Example
(with a daemon)


It is worthwhile to note here that the
remote object,
AskTheRegistrarActivation, has
not yet been instantiated and will remain
so until the first request.
This is one of the main benefits of the
daemon implementation in that it
conserves resources until they are
required.
104
Java RMI Example
(with a daemon)

We need to make two more adjustments.


First, we need to provide the client with a “stub” copy of the
remote object (in addition to the compiled interface
definition).
After compiling the remote object (which is
AskTheRegistrarActivation in our example), we employ the
remote invocation compiler to whittle it down to just a stub.
C:> %JAVAHOME%\rmic AskTheRegistrarActivation
105
Java RMI Example
(with a daemon)


This will produce a file named
AskTheRegistrarActivation_stub.class that
needs to be distributed with the client application (in
addition to the AskTheRegistrarInterface.class and
LocalStudentClient.class files).
Secondly, while the client is unchanged, it now requires a
security policy file in order to access the RMI daemon and
registry
grant
{
// + This allows the local client permission to use the network
//
and make outbound connections to the "Alpha" server on any
//
unprivileged port.
//
permission java.net.SocketPermission
"Alpha:1024-", "connect,resolve";
};
106
Java RMI Example
(with a daemon)


The policy file for the client says that connections may
be made to any non-privileged port on the server that
contains the remote object.
When executing the client application, we now need to
include a reference to the security policy file.
C:> java -Djava.security.policy=LocalStudentClient.policy
LocalStudentClient Alpha "CS483"
107
Executed Client Application
with Reference to Policy File
108
LocalStudent Client



Our invocation of a remote object is now
functional again!
That was certainly a lot of work just to
learn that we are registered for CS483.
If we look back at the RMI daemon, we
note that the status messages are
displayed, with some extra logging
information about execution groups
(which we did not cover).
109
RMI Daemon

Again, the benefit to this setup is that we can
run with just the daemon and dynamically
register objects with the daemon and the
object will be immediately available for use by
clients.
110
Java RMI Resources




http://java.sun.com/docs/books/tutori
al/rmi/index.html
http://java.sun.com/j2se/1.5.0/docs/g
uide/rmi/index.html
http://java.sun.com/j2se/1.5.0/docs/g
uide/rmi/spec/rmiTOC.html
http://java.sun.com/j2se/1.5.0/docs/t
ooldocs/windows/rmic.html
111