PROTOCOL CONTENT HANDLER

1

PROTOCOL / CONTENT HANDLER AND REMOTE METHOD
INVOCATION
2
Agenda

• Part 1 Protocol and Content Handlers
• Part 2 Remote Method Invocation

3
Part 1 Protocol And Content Handler

• Outline
• Protocol Handler
• What is it?
• URLStreamHandler Class
• Writing a protocol Hander
• Example and other techniques
• URLStreamHandlerFactory Interface
• Content Handler
• What is it?
• ContentHandler Class
• ContentHandlerFactory Interface
• FITS Image Format

4
Overview

• When designing an architecture that would allow
  them to build a self-extensible browser, the
  engineers at Sun divided the problem into two
  parts handling protocols and handling content.

5
Part 1 Protocol And Content Handler

• Outline
• Protocol Handler
• What is it?
• URLStreamHandler Class
• Writing a protocol Hander
• Example and other techniques
• URLStreamHandlerFactory Interface

6
What is it?

• Handling a protocol involves the interaction
  between a client and a server
• generating requests in the correct format
• interpreting the headers that come back with the
  data
• acknowledging that the data has been received

7
Protocol Handler classes

• The protocol handler mechanism is implemented by
  four difference class in the java.net package,
  that are
• URL the only concrete class in this group
• URLStreamHandler the abstract class
• URLConnection also the abstract classes
• URLStreamHandlerFactory is the interface

8
How does it work???
9
Part 1 Protocol And Content Handler

• Outline
• Protocol Handler
• What is it?
• URLStreamHandler Class
• Writing a protocol Hander
• Example and other techniques
• URLStreamHandlerFactory Interface

10
URLStreamHandler Class

• The abstract URLStreamHandler class is a
  superclass for classes that handle specific
  protocols
• By overriding the URLStreamHandler methods in
  your own subclass, you teach the URL class how to
  handle new protocols.

11
URLStreamHandler Class

• Constructor

12
URLStreamHandler Class

• Constructor
• Because URLStreamHandler is an abstract class,
  this constructor is never called directly it is
  only called from the constructors of subclasses.
• The single constructor for URLStreamHandler
  doesn't take any arguments
• public URLStreamHandler( )

13
URLStreamHandler Class

• Method
• The first responsibility is to split a string
  representation of a URL into its component parts
  and use those parts to set the various fields of
  the URL object.
• The parseURL( ) method splits the URL into parts,
  possibly using setURL( ) to assign values to the
  URL's fields.

14
protected void parseURL(URL u, String spec, int
start, int limit)

• To parse the String spec into a URL u.
• The task is to set us protocol, host, port,
  file, and ref fields
• Any parts of the String that are before start and
  after limit have already been parsed or can be
  ignored.
• The parseURL( ) method that Java supplies assumes
  that the URL looks more or less like an http or
  other hierarchical URL
• protocol//www.host.comport/directory/another_dir
  ectory/filefragmentID

15
protected void parseURL(URL u, String spec, int
start, int limit)(cont.)

• Work ftp and gopher URLs.
• Not work mailto or news URLs and any new URL
  schemes you define.
• Using URLs that fit this hierarchical form, not
  have to override parseURL() at all
• If the URLs are completely different, must supply
  parseURL( ) method that parses the URL completely
• If your URL looks like a standard URL, make
  parseURL( ) method that handles the nonstandard
  portion of the URL and then calls super.parseURL(
  ) to do the rest of the work

16
Example

• A mailto URL looks like mailtoabc_at_hcm.edu.vn.
• How to map this into the URL class's protocol,
  host, port, file, and ref fields.
• The protocol mailto.
• After the _at_ can be the host.
• what to do with the username.
• Mailto URL really doesn't have a file portion,
  using the URL class's file field to hold the
  username.
• The ref the empty string or null.

17
protected String toExternalForm(URL u)

• Pieces of the URL uprotocol, host, port, file,
  and ref fieldsback together in a String.
• A class that overrides parseURL() should also
  override toExternalForm( )
• return "mailto" u.getFile( ) "_at_"
  u.getHost( )

18
protected void setURL(URL u, String protocol,
String host, int port, String authority, String
userInfo, String path, String query, String
fragmentID)

• Set the protocol, host, port, authority,
  userInfo, path, query, and ref fields of the URL
  u to the given values.
• parseURL( ) uses this method set these fields to
  the values it has found by parsing the URL.
• the host, port, and user info together make up
  the authority, are used in preference to the
  authority when deciding which site to connect to.

19

• protected int getDefaultPort( )
• returns the default port for the protocol, e.g.,
  80 for HTTP
• protected InetAddress getHostAddress(URL u)
• returns an InetAddress object pointing to the
  server in the URL. If the host can't be located,
  it simply returns null.

20

• protected boolean hostsEqual(URL u1, URL u2)
• determines whether the two URLs refer to the same
  server. This method does use DNS to look up the
  hosts.
• If the DNS lookups succeed, it can tell that
• if the DNS lookup fails for any reason, then
  hostsEqual( ) falls back to a simple
  case-insensitive string comparison, in which case
  it would think these were two different hosts.

21

• protected boolean sameFile(URL u1, URL u2)
• determines whether two URLs point to the same
  file.
• It does this by comparing the protocol, host,
  port, and path.
• The files are considered to be the same only if
  each of those four pieces is the same.

22

• protected boolean equals(URL u1, URL u2)
• Tests almost the entire URL, including protocol,
  host, file, path, and fragment identifier. Only
  the query string is ignored.
• All five of these must be equal for the two URLs
  to be considered equal.
• protected int hashCode(URL u)
• Change the default hash code calculation by
  overriding this method.
• Do this if you override equals( ), sameFile(), or
  hostsEqual( ) to make sure that two equal URL
  objects will have the same hash code, and two
  unequal URL objects will not have the same hash
  code

23
A Method for Connecting

• The second responsibility of a URLStreamHandler
  is to create a URLConnection object appropriate
  to the URL

24
protected abstract URLConnection
openConnection(URL u) throws IOException

• u the URL to connect to.
• It returns an unopened URLConnection, directed at
  the resource u points to. Each subclass of
  URLStreamHandler should know how to find the
  right subclass of URLConnection for the protocol
  it handles.
• The URL u that is passed as an argument is the
  URL that needs a connection.
• The subclass's openConnection( ) method is
  usually extremely simple in most cases, it just
  calls the constructor for the appropriate
  subclass of URLConnection

25
protected URLConnection openConnection(URL u,
Proxy p) throws IOException

• To specify a proxy server for the connection
• Rather than connecting to the host directly, this
  URLConnection connects to the specified proxy
  server, which relays data back and forth between
  the client and the server.
• Protocols that do not support proxies can simply
  ignore the second argument.
• To bypass the usual proxy server and connect
  directly instead, pass the constant
  Proxy.NO_PROXY as the second argument.

26
Part 1 Protocol And Content Handler

• Outline
• Protocol Handler
• What is it?
• URLStreamHandler Class
• Writing a protocol Hander
• Example and other techniques
• URLStreamHandlerFactory Interface

27
Writing a Protocol Handler

• Demonstrate by writing one for the finger
  protocol defined in RFC 1288
• Finger is a relatively simple protocol compared
  to JDK-supported protocols such as HTTP and FTP

28
Writing a Protocol Handler

• Implementing a finger URL like
• finger//hostnameport/usernames
• getContentType() method get the content type
  returned by the finger protocol's
• A finger server returns ASCII text, so the
  getContentType() method should return the string
  text/plain.

29
Writing a Protocol Handler

• New protocols such as HTTP use MIME headers to
  indicate the content type.
• Most protocols precede MIME gt need to specify
  the MIME type explicitly or use the static
  methods
• URLConnection.guessContentTypeFromName(String
  name) and
• URLConnection.guessContentTypeFromStream(InputStre
  am in)

30
Writing a Protocol Handler

• Example

31
Part 1 Protocol And Content Handler

• Outline
• Protocol Handler
• What is it?
• URLStreamHandler Class
• Writing a protocol Hander
• Example and other techniques
• URLStreamHandlerFactory Interface

32
More Protocol Handler Examples and Techniques

• Five basic steps of creating a new protocol
  handler
• Design a URL for the protocol if a standard URL
  for that protocol doesn't already exist.
• Decide what MIME type should be returned by the
  protocol handler's getContentType( ) method.
• Write a subclass of URLConnection that
  understands this protocol.
• Write a subclass of URLStreamHandler with an
  openConnection( ) method
• Implement the URLStreamHandlerFactory interface
  and the createStreamHandler( ) method

33
More Protocol Handler Examples and Techniques

• Daytime Protocol
• Chargen Protocol

34
A daytime Protocol Handler

• URL look like daytime///vision.poly.edu
• Allow for nonstandard port assignments
• Ex daytime///vision.poly.edu2082
• Allow a terminating slash and ignore everything
  following the slash
• Ex daytime///vision.poly.edu/index.html is
  equivalent to daytime///vision.poly.edu.

35
A daytime Protocol Handler

• Using the default toExternalForm( ) and
  parseURL() methods.
• Although the content returned by the daytime
  protocol is really text/plain, this protocol
  handler is going to reformat the data into an
  HTML page.

36
A daytime Protocol Handler

• The page can be broken up into three different
  strings
• Everything before the time
• The time
• Everything after the time
• First and the third strings can be calculated
  before the connection is even opened.
• Formulate these as byte arrays of ASCII text and
  use them to create two ByteArrayInputStreams
• Use SequenceInputStream to combine those two
  streams with the data actually returned from the
  server.

37
A chargen Protocol Handler

• Defined in RFC 864
• Is a very simple protocol designed for testing
  clients.
• A chargen URL like chargen//hostnameport
• getContentType() method choose the content type
  to be returned by the chargen protocol handler's
• A chargen server returns ASCII text, so the
  getContentType( ) method should return the string
  text/plain.

38
Part 1 Protocol And Content Handler

• Outline
• Protocol Handler
• What is it?
• URLStreamHandler Class
• Writing a protocol Hander
• Example and other techniques
• URLStreamHandlerFactory Interface

39
The URLStreamHandlerFactory Interface

• The URLStreamHandlerFactory interface declares a
  single method, createURLStreamHandler( )
• This method loads the appropriate protocol
  handler for the specified protocol.
• To use this method, write a class that implements
  the URLStreamHandlerFactory interface and include
  a createURLStreamHandler( ) method in that class.

40
The URLStreamHandlerFactory Interface

• createURLStreamHandler( ) method does not need to
  know the names of all the installed protocol
  handlers.
• To install the stream handler factory, pass an
  instance of the class that implements the
  URLStreamHandlerFactory interface to the static
  method URL.setURLStreamHandlerFactory( ) at the
  start of the program.

41
The URLStreamHandlerFactory Interface

• Example

42
Part 1 Protocol And Content Handler

• Outline
• Content Handler
• What is it?
• ContentHandler Class
• ContentHandlerFactory Interface
• FITS Image Format

43
Idea

• The browser could automatically download the code
  that was needed to view that content type.
• Avoiding to stop, FTP a plug-in, quit the
  browser, install the plug-in, restart the
  browser, and reload the page.

44
What is content handler?

• Handling the content involves converting the raw
  data into a format Java understands
• for example, an InputStream or an AudioClip.
• The content handler would be responsible for
  parsing the content and displaying it to the user
  in the web browser window.

45
Content Handler Classes

• The abstract class that content handlers for
  specific data types such as PNG or RTF would
  extend was java.net.ContentHandler

46
How does it work???
47
Disavantage

• The ContentHandler class was too generic,
  providing too little information about what kind
  of object was being downloaded and how it should
  be displayed.

48
Part 1 Protocol And Content Handler

• Outline
• Content Handler
• What is it?
• ContentHandler Class
• ContentHandlerFactory Interface
• FITS Image Format

49
The ContentHandler Class

• Quite simple A text/plain content handler is
  quite simple
• Quite complex A text/rtf content handler would
  be very complex.
• Depending almost entirely on the complexity of
  the content type to parse.
• public ContentHandler( )Since ContentHandler is
  an abstract class, call only from inside the
  constructors of subclasses.

50
public abstract Object getContent(URLConnection
uc) throws IOException

• Use the URLConnection's InputStream to create an
  object
• Depends on what the application requesting the
  content expects.
• Text-like content bundled with the JDK ? return
  some subclass of InputStream.
• Images ? return ImageProducer objects
• The InputStream includes only the content's raw
  data.
• Any MIME headers or other protocol-specific
  information that come from the server should be
  stripped by the URLConnection before it passes
  the stream to the ContentHandler.
• A ContentHandler is only responsible for content,
  not for any protocol overhead that may be present.

51
Tab-Separated Values

• Produced by many database and spreadsheet
  programs.
• JPE Associates ?341 Lafayette Street, Suite 1025
  ?
• New York ?NY? 10012
• Each line is a record, and the data before each
  tab is a field.
• what kind of Java object should we convert the
  tab- separated values to?
• The simplest and most general way to store each
  record is as an array of Strings. Successive
  records can be collected in a Vector
• public class Address private String name
  private String street private String city
  private String state private String zip

52
Using Content Handlers

• The application that uses the tab-separated-values
  does not need to know about it explicitly.
• Simply call the getContent( ) method of URL or
  URLConnection on a URL with a matching MIME type.
  
• Content-type of text/plain, application/octet-stre
  am, or some other value, or not any Content-type
  at all, the server is misconfigured and must be
  fixed before you continue.

53
Choosing Return Types

• public Object getContent(URLConnection uc,
  Class classes) throws IOException
• Allows the caller to request that the content be
  returned as one of the types in the array and
  enables content handlers to support multiple
  types.
• Text/tab-separated-values content handler could
  return data as a Vector, an array, a string, or
  an InputStream.
• The client doesn't request any of the classes
  this ContentHandler knows how to provide, it
  returns null.

54
Choosing Return Types (cont.)

• To call this method, the client invokes the
  method with the same arguments in a URL or
  URLConnection object.
• It passes an array of Class objects in the order
  it wishes to receive the data.
• Thus, if receiving a String but is willing to
  accept an InputStream and will take a Vector as a
  last resort, it puts String.class in the zeroth
  component of the array, InputStream.class in the
  first component of the array, and Vector.class in
  the last component of the array.
• Then it uses instanceof to test what was actually
  returned and either process it or convert it into
  the preferred type.

55
Part 1 Protocol And Content Handler

• Outline
• Content Handler
• What is it?
• ContentHandler Class
• ContentHandlerFactory Interface
• FITS Image Format

56
The ContentHandlerFactory Interfaces

• A ContentHandlerFactory defines the rules for
  where ContentHandler classes are stored.
• The createContentHandler() method in the
  ContentHandlerFactory should return null if no
  ContentHandler appropriates for a MIME type.
• If it is, Java will look for ContentHandler
  classes in the default locations.
• The method setContentHandlerFactory() of the
  URLConnection will initialize a
  ContentHandlerFactory.

57
The createContentHandler() method

• public abstract ContentHandler createContentHandle
  r(String mimeType)
• This method should be called only by the
  getContent() method of a URLConnection object.

58
The createContentHandler() method

• Example TabFactory
• package com.macfaq.net.www.content.
• import java.net.
• public class TabFactory implements
  ContentHandlerFactory
• public ContentHandler createContentHandler(String
  mineType)
• if (mimeType.equals(text/tab-separated-values)
  
• return new com.macfaq.net.www.content.text.tab-sep
  arated-values()
• else
• return null // look for the handler in the
  default locations

59
The createContentHandler() method

• The createContentHandler() method also suggests
  handlers for
• text/tab-separeated-values
• application/x-time
• text/plain
• video/mpeg
• mode/vrml
• ....

60
Install Content Handler Factories

• Use the static URLConnection.setContentHandlerFact
  ory() method to install ContentHandlerFactory.
• public static void setContentHandlerFacotry(
  ContentHandlerFactory fac) throws
  SecurityException, Error

61
Install Content Handler Factories

• Example TabLoader that uses a ContentHandlerFacot
  ry
• import java.io.
• import java.net.
• import java.uitl.
• import com.macfaq.net.www.content.
• public class TabLoader
• public static void main(String args)
• URLConnection.setContentHandlerFactory(new
  TabFacotry())
• for (int i 0 ilt args.length() i)
• try
• URL u new URL(argsi)
• Object content u.getContent()
• Vector v (Vector) content
• for (Enumeration e v.elements()
  e.hasMoreElements() )
• String sa (String) e.nextElement()
• for (int j 0 j lt sa.length() j)
• System.out.print(saj \t)
• System.out,println()

62
Install Content Handler Factories (cont)

• java TabLoader http//metalab.unc.edu/javafaq/ad
  dresses.tab
• JPE Associates
• ?
• 341 Lafayette St, Suite 1025
• ?
• New York
• ?
• NY
• ?
• 10012
• O'Reilly Associates
• ?
• 103 Morris St, Suite A
• ?
• Sebastopol
• ?
• CA
• ?
• 95472

63
Part 1 Protocol And Content Handler

• Outline
• Content Handler
• What is it?
• ContentHandler Class
• ContentHandlerFactory Interface
• FITS Image Format

64
A Content Handler for an Image Format
image.x-fits

• We produce an object that implements the
  java.awt.ImageProducer interface rather than an
  Input Stream object.
• For more details about the FITS format and how to
  handle FITS file, please see Encyclopedia of
  Graphics File Formats, Second Edition, by James
  D. Murray and William vanRyper (OReilly
  Associates, Inc.)

65
A Content Handler for an Image Format
image.x-fits

• FITS files are broken up into blocks of exactly
  2,880 bytes.
• Each FITS file has two parts, the header and the
  primary data unit. The header occupies an
  integral number of blocks, as does the primary
  data unit.
• If the FITS file contains extensions, there may
  be additional data after the primary data unit,
  however we dont care about that.

66
A Content Handler for an Image Format
image.x-fits

• Here is a simple header taken from a FITS file
• SIMPLE T /
• BITPIX 16 /
• NAXIS 2 /
• NAXIS1 242 /
• NAXIS2 252 /
• DATE '19 Aug 1996' /
• TELESC 'NSO/SP - VTT' /
• IMAGE 'Continuum' /
• COORDS 'N29.1W34.2' /
• OBSEND

67
A Content Handler for an Image Format
image.x-fits

• Example an x-fits Content Handler

68
A Content Handler for an Image Format
image.x-fits

• Example the FITS ContentHandlerFacotry

69
Part 2 Remote Method Invocation

• Outline
• What is it?
• Implementation
• Loading class at run time
• The java.rmi Package
• The java.rmi.registry Package
• The java.rmi.server Package

70
Java Remote Object Invocation (RMI)

• Java RMI allowed programmer to execute remote
  function class using the same semantics as local
  functions calls.

71
Idea

• Distribute object across difference machine to
  take advantages of hardware and dedicated
  software.
• Developer builds network service and install it
  on specified machine.
• User requests an instance of class using URL
  syntax.
• User uses object as though it were a regular,
  local object

72
RMI operations

• Stub
• Packet identifier of remote object
• Package method identifier
• Marshall parameter
• Send package to server skeleton
• Skeleton
• Unmarshall parameters
• Calls return value or exception
• Marshall method return
• Send package to client stub

73
The General RMI Architecture

• The server must first bind its name to the
  registry
• The client lookup the server name in the registry
  to establish remote references.
• The Stub serializing the parameters to skeleton,
  the skeleton invoking the remote method and
  serializing the result back to the stub.

74
Three Main Packages

• java.rmi defines the classes, interfaces, and
  exceptions that will be seen on the client side
  access remote objects but are not themselves
  remote objects.
• java.rmi.server defines the classes, interfaces,
  and exceptions that will be visible on the server
  side. Use these classes when you are writing a
  remote object that will be called by clients
• java.rmi.registry defines the classes,
  interfaces, and exceptions that are used to
  locate and name remote objects.

75
Part 2 Remote Method Invocation

• Outline
• What is it?
• Implementation
• Loading class at run time
• The java.rmi Package
• The java.rmi.registry Package
• The java.rmi.server Package

76
The Remote Interface

• In RMI, a common remote interface is the minimum
  amount of information that must be shared in
  advance between client and server machines.
  It defines a high-level protocol through which
  the machines will communicate.
• A remote interface is a normal Java interface,
  which must extent the marker interface
  java.rmi.Remote.
• Corollaries because the visible parts of a
  remote object are defined through a Java
  interface, constructors, static methods and
  non-constant fields are not remotely accessible
  (because Java interfaces cant contain such
  things).
• All methods in a remote interface must be
  declared to throw the java.rmi.RemoteException
  exception.

77
A Simple Example

• A file MessageWriter.java contains the interface
  definition
• import java.rmi.
• public interface MessageWriter extends
  Remote
• void writeMessage(String s) throws
  RemoteException
• This interface defines a single remote method,
  writeMessage().

78
java.rmi.Remote

• The interface java.rmi.Remote is a marker
  interface.
• It declares no methods or fields however,
  extending it tells the RMI system to treat the
  interface concerned as a remote interface.
• In particular we will see that the rmic compiler
  generates extra code for classes that implement
  remote interfaces. This code allows their
  methods to be called remotely.

79
java.rmi.RemoteException

• Requiring all remote methods be declared to throw
  RemoteException was a philosophical choice by the
  designers of RMI.
• RMI makes remote invocations look syntactically
  like local invocation. In practice, though, it
  cannot defend from problems unique to distributed
  computingunexpected failure of the network or
  remote machine.

80
java.rmi.RemoteException

• Forcing the programmer to handle remote
  exceptions helps to encourage thinking about how
  these partial failures should be dealt with.
• See the influential essay A Note on Distributed
  Computing by Waldo et al, republished in The
  Jini Specification
• http//java.sun.com/docs/boo
  ks/jini

81
class UnicastRemoteObject

• Extend java.rmi.server.UnicastRemoteObject,
  either directly or indirectly
• public class UnicastRemoteObject extends
  RemoteServer
• Marshalling is the process by which arguments and
  return values are converted into a stream of
  bytes that can be sent over the network.
• Unmarshalling is the reverse the conversion of a
  stream of bytes into a group of arguments or a
  return value.)
• A UnicastRemoteObject exists only as long as the
  server that created it still runs.
• When the server dies, the object is gone forever.
  Activatable objects allow clients to reconnect to
  servers at different times across server
  shutdowns and restarts and still access the same
  remote objects.

82
The Remote Object

• A remote object is an instance of a class that
  implements a remote interface.
• Most often this class also extends the library
  class java.rmi.server.UnicastRemoteObject. This
  class includes a constructor that exports the
  object to the RMI system when it is created, thus
  making the object visible to the outside world.

83
The Remote Object

• Usually you will not have to deal with this class
  explicitlyyour remote object classes just have
  to extend it.
• One fairly common convention is to name the class
  of the remote object after the name of the remote
  interface it implements, but append Impl to the
  end.

84
A Remote Object Implementation Class

• The file MessageWriterImpl.java contains the
  class declaration
• import java.rmi.
• import java.rmi.server.
• public class MessageWriterImpl extends
  UnicastRemoteObject
• 
  implements MessageWriter
• public MessageWriterImpl() throws
  RemoteException
• public void writeMessage(String s) throws
  RemoteException
• System.out.println(s)

85
Compiling the Remote Object Class

• To compile classes that implement Remote, you
  must use the rmic compiler. The reasons will be
  discussed later. For example
• sirah rmic MessageWriterImpl

86
Client and Server Programs

• We have completed the Java files for the remote
  object class itself, but we still need the actual
  client and server programs that use this class.
• In general there are some pieces of administrivia
  one has to deal withpublishing class files and
  installing security managers.

87
Client and Server Programs

• We initially make the simplifying assumption that
  both client and server have copies of all class
  files for MessageWriter (e.g., they may share
  access through shared NFS directories).
• Then publishing class files is not an issue,
  and we also dont need a security manager,
  because all code is local, and therefore
  trusted.

88
A Server Program

• We assume the file HelloServer.java contains the
  class declaration
• import java.rmi.
• public class HelloServer
• public static void main(String args)
  throws Exception
• MessageWriter server new
  MessageWriterImpl()
• Naming.rebind(messageservice,
  server)

89
Remarks

• This program does two things
• It creates a remote object with local name
  server.
• It publishes a remote reference to that object
  with external name MessageWriter.
• The call to Naming.rebind() places a reference to
  server in an RMI registry running on the local
  host (i.e., the host where the HelloServer
  program is run).
• Client programs can obtain a reference to the
  remote object by looking it up in this registry.

90
A Client Program

• We assume the file HelloClient.java contains the
  class declaration
• import java.rmi.
• public class HelloClient
• public static void main(String args)
  throws Exception
• MessageWriter server
• (MessageWriter)
  Naming.lookup(
• 
  rmi//sirah.csit.fsu.edu/messageservice)
• server.writeMessage(Hello, other
  world)

91
Remarks

• Again the program does two things
• It looks up a reference to a remote object with
  external name MessageWriter, and stores the
  returned reference with local name server.
• Finally (!), it invokes the remote method,
  writeMessage(), on server.
• The call to Naming.lookup() searches in a remote
  RMI registry. Its argument is a URL, with
  protocol tag rmi.
• This example assumes the remote object lives on
  the host sirah, and has been registered in the
  default RMI registry (which happens to listen on
  port 1099) on that machine.

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Compiling and Running the Example

• Compile HelloServer and HelloClient on their
  respective hosts, e.g.
• sirah javac HelloServer
• merlot javac HelloClient
• Either ensure client and server share the current
  directory, or copy all files with names of the
  form MessageWriter .class to the clients
  current directory.

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Part 2 Remote Method Invocation

• Outline
• What is it?
• Implementation
• Loading class at run time
• The java.rmi Package
• The java.rmi.registry Package
• The java.rmi.server Package

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Loading class at run time

• All the client really has to know about the
  remote object is its remote interface.
• Everything else it needsfor instance, the stub
  classescan be loaded from a web server (though
  not an RMI server) at runtime using a class
  loader.
• Indeed, this ability to load classes from the
  network is one of the unique features of Java.

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Solution

• The solution is that we can use Applet.
• The reason is that whenever a web page including
  an applet is loaded, all of the necessary classes
  will be loaded too.

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Thank you for your listening