Why Java for Network Applications

1
Why Java for Network Applications

• Language features (concurrency, exceptions)
• Built-in objects (information hiding)(better
  than UNIX and building your own)
• Protocol handlers, content handlers
• down load as needed
• loading classes remotely
• Heterogeneous networks work together at Java
  level (all Java homogeneity)
• All the other stuff for modern applicationsGUI,
  databases, security, etc.
• Performanceindividual CPU generally not the
  bottleneck

2
Basic Web Terms

• TCP/IP - Transport Control Protocol / Internet
  Protocol
• UDP - User Datagram Protocol
• URL - Uniform Resource Locator
• URI - Uniform Resource Identifier
• URN - Uniform Resource Name (separate from
  location)
• HTTP - HyperText Transfer Protocol
• HTML - Hypertext Markup Language
• SGML - Standard Generalized Markup Language
• XML eXtensible Markup Language
• MIME - Multipurpose Internet Mail Extensions
• ASCII - American Standard Code for Information
  Interchange

3
Network Layers

• Very simplified view

Application
Application
Logical Communication
Transport(TCP, UDP)
Transport(TCP, UDP)
IP Layer
IP Layer
Physical layer (bits on the wire) ISO Level 1
Net Head IP Head TCP Head App. Head
Data
4
IP Addressing

• Four numbers, each 0..255, e.g.
  123.123.123.123(32 bits for now)
• Class C address block
• first three bytes specified, e.g. 123.123.123.
• room for 254 addresses (0 and 255 special)
• Domain names that are easier to remember and use
• Domain Name Service (DNS) for name resolution
• Multi-hosting (multiple domain names on one IP
  address)multi-homing (multiple IP addresses on
  one machine
• localhost 127.0.0.1 local loopback address
• Java program to find local address

5
URL- Uniform Resource Locator

• protocol//hostnameport/path/filenamesection
• Protocol \file, ftp, http, news, telnet,
  gopher, WAIS
• Hostnamewww.pithecanthropus.com or
  123.123.123.123
• Port 13 (daytime), 20 21 (ftp), 23 (telnet),
  25 (smtp/mail), 37 (time), 43 (whois), 79
  (finger), 80 (http), 110 (pop3/mail), 119
  (nntp/news), 1099 (RMI Registry) defined in
  /etc/services or windows\services

6
URL

• protocol//hostnameport/path/filenamesection
• Pathrelative to document root of server, not
  file system root
• Filenameusual default index.html server has
  other options
• Sectionnamed anchor in an HTML document(text
  bookmark)
• relative and absolute URLs allow moving whole
  collections

7
java.net.URL

• public final class URL extends Object implements
  Serializable
• URL constructor formats match common usepublic
  URL (--) throws MalformedURLException(String
  spec)(String protocol, String host, int
  port, String file)(String protocol, String
  host, String file)(URL context, String spec)

8
URL Constructor Examples

• try URL u1 new URL(http//www.x.com/a.html)
  catch (MalformedURLException e)
  System.err.println(e)
• new URL(http, www.x.com, /a.htmlb)
• new URL(http, www.x.com, 80, /a.html)
• new URL(u1, other.html)/ example / import
  java.applet.AppletURL u new URL(getDocumentBase
  (), b.html)

9
GetURL Copies Files (see code)

• URL url new URL(args0)
• InputStream in url.openStream()
• // InputStream in new FileInputStream(a.b)
• OutputStream out
• new FileOutputStream(args1)
• byte buffer new byte4096
• int bytes_read
• while((bytes_read in.read(buffer)) ! -1)
• out.write(buffer, 0, bytes_read)

10
Idea of sockets

• Computer has (probably) one connection to the net
  
• Logical ports subdivide that connection
• Operating system, network communications driver
  listens and then passes the information to
  waiting programs (sockets)
• Sockets are independent programs registered to
  receive data
• Parallel on senders side
• Sockets can be used for two-way communication
• Form the foundation for network communication
• Popularized in Berkeley UNIX as a unifying concept

11
Copy http with sockets

• Socket soc new Socket(url.getHost(),
  url.getPort())
• InputStream from soc.getInputStream()
• PrintWriter to new PrintWriter(new
  OutputStreamWriter(soc.getOutputStream()))
• to.println("GET " filename) to.flush()
• byte buffer new byte4096
• int bytes_read
• while((bytes_read from.read(buffer)) ! -1)
• to_file.write(buffer, 0, bytes_read)

12
Remote Method Invocation (RMI)

• Distributed systems
• Sockets - flexible and sufficient, but require
  applications-level protocols (cumbersome and
  potentially error-prone)
• Remote Procedure Call (RPC)alternative with
  illusion of calling a local procedure
• RPC not appropriate for distributed object
  systems where remote Method Invocation (RMI) is
  better model (context of execution)
• RMI local surrogate (stub) object manages the
  invocation of surrogate (skeleton) remote object

13
Stubs and Skeletons

client
programstubmarshallnetworknetworkunmarshall
stub returnprogram
14
Java RMI vs. CORBA

• Java RMI integrated in the language (since JDK
  1.1)
• Common Object Request Broker Architecture
• OMG - Object Management Group
• CORBA for a heterogeneous, multi-language
  environment thus a language-neutral object model
• IDL - Interface Description Language
• IIOP - Internet Inter-Orb Protocol
• NDI - Naming Directory Interface
• RMI over IIOP evolving together with CORBA

15
RMI Common Interface

• import java.rmi.
• public interface iCalendar extends Remote
• java.util.Date getDate () throws
  RemoteException
• // look at code examples

16
RMI Client

• public class CalendarUser
• public static void main(String args)
• iCalendar remoteCal
• try remoteCal (iCalendar)
• Naming.lookup("rmi//machine/CalendarImpl")
• t1 remoteCal.getDate().getTime()
• t2 remoteCal.getDate().getTime()
• catch (Exception e) e.printStackTrace()
• System.out.println("This RMI call took "
• (t2-t1) " milliseconds")

17
RMI Server

• public class CalendarImpl extends
  UnicastRemoteObject implements iCalendar
• public CalendarImpl() throws RemoteException
• public Date getDate () throws RemoteException
• return new Date()
• public static void main(String args)
• CalendarImpl cal
• try LocateRegistry.createRegistry(1099)
• cal new CalendarImpl()
• Naming.bind("rmi///CalendarImpl", cal)
• System.out.println("Ready for RMI's")
• catch (Exception e) e.printStackTrace()

18
Stubs and Skeletons

• Java automatically generates stubs and skeletons
  with rmic based on common interface and
  implementation .class file

client
programstubmarshallnetworknetworkunmarshall
stub returnprogram
19
Pulling it together

• Compile client and serve
• Generating stubs skeletons
• Remote Method Invocation Compiler (rmic)
• Start server
• Run client

20
Callbacks, CORBA, J2EE

• X11 Windows
• reversed traditional role of client and server
• routines on display terminal were called back
  by computer
• terminal sent information about these routines to
  the server
• RMI Registry is only one way of objects knowing
  each other
• CORBA Common Object-oriented Request Broker
  Architecture
• Microsoft COM, COM, DCOM
• Java 2 Enterprise Edition (Enterprise Java Beans)

21
Programming Language Implications

• Programming language constructs for distributed
  computing (like looping, testing, subprogram
  definition and calling)
• Event driven style (program set up and waiting
  for event or request)
• Passing by reference and value still issues
• Programming with interfaces (interface
  inheritance v. implementation inheritance)
• More environmental aids for programming, reusable
  classes and frameworks