HANDOUT 14

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HANDOUT 14

• Remote Method Invocation (RMI)

THIS DOCUMENT CAN NOT BE REPRODUCED OR
DISTRIBUTED WITHOUT TH E WRITTEN PERMISSION OF
THE AUTHOR
BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• 1.0 DEFINITION
• THE REMOTE METHOD INVOCATION API ENABLES CLIENT
  AND SERVER COMMUNICATIONS OVER THE NET
• RMI ALLOWS JAVA (ONLY) OBJECTS , RESIDING ON THE
  SAME OR DIFFERENT COMPUTERS ,TO COMMUNICATE WITH
  ONE ANOTHER VIA REMOTE METHOD CALLS OVER THE
  NETWORK.
• RMI PROVIDES FOR TRANSFER OF OBJECTS VIA OBJECT
  SERIALIZATION.
• SERIALIZATION ALLOWS OBJECTS TO CARRY ALL THEIR
  ATTRIBUTES (FIELDS) WITH THEM OVER THE NETWORK

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• OBJECT SERIALIZATION IS USED TO SEND THE
  ARGUMENTS (PROPERTIES) OF A METHOD INVOCATION
  FROM THE CLIENT OBJECT TO THE REMOTE OBJECT
• OBJECT SERIALIZATION IS ALSO USED TO RETURN THE
  VALUES FROM THE SERVER OBJECT BACK TO THE CLIENT
  OBJECT.
• OBJECTS ARE CONVERTED INTO A STREAM OF BYTES
  THAT CAN BE TRASMITTED OVER THE NETWORK BY USING
  THE ObjectOutputStream CLASS.
• OBJECTS ARE RECONSTRUCTED AFTER TRANSMISSION BY
  USING THE ObjectInputStream CLASS.

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• THE SERIALIZATION USED BY RMI IS TRANSPARENT TO
  THE CLIENT AND SERVER OBJECTS.
• APPLICATIONS USING TCP SOCKETS ARE RESPONSIBLE
  FOR SERIALIZING AND DESERIALIZING OBJECTS VIA
  INPUT AND OUTPUT STREAMS.
• SECURITY IS AN ISSUE.
• JAVA OFFERS SECURITY VIA THE SECURITY PACKAGE.
• THE IMPLEMENTATION OF THE INTERFACE METHOD AT THE
  SERVER HAS TO IMPLEMENT THE SERIALIZABLE
  INTERFACE.

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• TO OPEN A STREAM AND READ FROM AN INTERNET SITE
• BufferedReader in new BufferedReader ( new
  InputStreamReader ( url.openStream ( ) )
• url is an object of the URL class that carries
  the url of the site you want to connect (URL
  implements serializable interface)
• openStream ( ) is a method of the URL class that
  Opens a connection to this URL and returns an
  InputStream object for reading from that
  connection.
• InputStreamReader class constructor creates an
  object that translates the InputStream bytes to
  UNICODE characters.
• The InputStreamReader object is now passed to
  BufferedReader constructor that creates a stream
  that is buffered so that one line at a time can
  be read out of the buffer.

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• LOCAL OBJECTS ARE OBJECTS THAT EXECUTE ON A
  PARTICULAR HOST.
• REMOTE OBJECTS ARE OBJECTS THAT EXECUTE ON ALL
  OTHER HOSTS.
• REMOTE OBJECTS HAVE TO BE EXPORTED.
• AN OBJECT ITSELF EXPORTS ITSELF BY REGISTERING
  WITH A REMOTE REGISTRY SERVER.
• RMI MAKES USE OF STUBS AND SKELETONS (1.2.2 USES
  ONLY SKELETONS).
• A STUB IS A LOCAL OBJECT THAT ACTS AS A PROXY FOR
  A REMOTE OBJECT

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• THE STUB PROVIDES THE SAME METHODS AS THE REMOTE
  OBJECT.
• LOCAL METHODS INVOKE THE METHODS OF THE STUB AS
  IF THEY WERE METHODS OF THE REMOTE OBJECT.
• THE SKELETON IS THE SAME AS THE STUB BUT ON THE
  REMOTE HOST SIDE (SERVER).

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• RMI AVOIDS THE NEED FOR PROGRAMMERS TO LEARN A
  SEPARATE IDL (INTERFACE DEFINITION LANGUAGE).
• SUPPORT FOR CROSS LANGUAGES IS VIA A NEW RELEASE
  OF RMI CALLED RMI OVER IIOP (INTERNET INTER ORB
  PROTOCOL)
• Using RMI-IIOP, objects can be passed both by
  reference and by value over IIOP. With RMI over
  IIOP, developers can write remote interfaces in
  the Java programming language and implement them
  just using Java technology and the Java RMI APIs.
  These interfaces can be implemented in any other
  language that is supported by an OMG mapping and
  a vendor supplied ORB for that language.

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Remote Method Invocation (RMI)

• The Remote Method Invocation (RMI) over Internet
  Inter-Orb Protocol (IIOP) lets existing RMI code
  reference and look up an object using the CORBA
  CosNaming service. This gives you greater
  interoperability between architectures.
• A separate compiler is needed.

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Remote Method Invocation (RMI)

• 2.0 HOW RMI WORKS

Default port 1099
Remote object registers its name
BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• BEFORE THE CLIENT IS STARTED THE RMI REGISTRY HAS
  TO BE STARTED.
• RMI REGISTRY IS A SERVER SIDE NAMING REPOSITORY
  THAT ALLOWS REMOTE CLIENTS TO GET A REFERENCE TO
  A REMOTE SERVER OBJECT.
• ALSO THE SERVER PROGRAM HAS TO BE STARTED BEFORE
  THE CLIENT.

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• RMI IS IMPLEMENTED BY 5 PACKAGES
• java.rmi PROVIDES THE REMOTE INTERFACE, A CLASS
  FOR ACCESSING REMOTE OBJECTS AND SECURITY MANAGER
  FOR RMI.
• Java.rmi.registry PROVIDES CLASSES AND INTERFACES
  THAT ARE USED BY THE REMOTE REGISTRY.
• java.rmi.server PROVIDES CLASSES AND INTERFACES
  USED TO IMPLEMENT REMOTE OBJECTS AND STUBS AND TO
  SUPPORT RMI COMMUNICATIONS.
• java.rmi.activation SUPPORTS PERSISTENT OBJECT
  REFERENCES AND REMOTE OBJECT ACTIVATION
• java.rmi.dgc SUPPORTS RMI DISTRIBUTED GARBAGE
  COLLECTOR.

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• EXAMPLE OF AN RMI APPLICATION (TemperatureServerIm
  pl)
• LET US DEVELOP AN APPLICATION THAT WILL MONITOR
  THE WEATHER INFORMATION OVER THE INTERNET FROM
  THE WEATHER BUREAU WEB SITE TWICE A DAY.
• THE APPLICATION , FOR THE SHAKE OF THE
  PRESENTATION, WILL RUN LOCALLY (I HAVE COPIED THE
  PAGE Traveler.html OF THE SITE TO THE WWWROOT
  DIRECTORY IN MY LAPTOP).
• IT IS ASSUMED THAT WE HAVE A LOCAL NETWORK. THE
  SERVER GETS THE WEATHER INFORMATION FROM THE
  INETRNET AND THE CLIENTS OF THE LOCAL NETWORK CAN
  ACCESS THE INFORMATION FROM THE SERVER VIA REMOTE
  METHOD INVOCATION

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• 4.0 SUMMARY OF TEMPERATURESERVER EXAMPLE

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Remote Method Invocation (RMI)

• 3.0 IMPLEMENTING THE RMI APPLICATION
• AT THE SERVER SIDE
• REMOTE OBJECTS ARE REFERENCED VIA INTERFACES. YOU
  CREATE AN INTERFACE FOR EACH OBJECT.
• THE INTERFACE MUST BE PUBLIC AND MUST EXTEND
  INTERFACE Remote .
• DEFINE THE REMOTE METHODS THAT YOU WANT TO INVOKE
  WITHIN THE INTERFACE. THE METHODS MUST THROW
  RemoteException .
• IN OUR EXAMPLE THERE THE INTERFACE is
• public interface TemperatureServer extends Remote
  
• (See code)

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation(RMI)

• The interface will invoke method getWeatherInfo()
  which returns an array of objects.
• Extending the interface Remote implies that our
  objects can be remotely accessed by any JVM that
  has network connection to the server and the
  appropriate stubs.
• CREATE A CLASS THAT IMPLEMENTS THE INTERFACE
  (TemperatureServerImpl).
• THE CLASS EXTENDS UnicastRemoteObject CLASS
  (SUBCLASS OF RemoteServer CLASS). (SEE
  TemperatureServerImpl extends UnicastRemoteObject
  implements TemperatureServer, Runnable)
• UnicastRemoteObject CLASS ALLOWS THE REMOTE
  OBJECT TO WAIT FOR A CLIENT CONNECTION ON AN
  ANONYMOUS PORT NUMBER.

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Remote Method Invocation (RMI)

• THE CLASS SHOULD HAVE A CONSTRUCTOR THAT CREATES
  , INITIALIZES THE REMOTE OBJECT
• IT SHOULD IMPLEMENT ALL THE METHODS DEFINED IN
  THE REMOTE INTERFACE (SEE public WeatherInfo
  getWeatherInfo () )
• IT SHOULD HAVE A main METHOD THAT CAN BE EXECUTED
  AS A REMOTE CLASS.
• IT SHOULD REGISTER A NAME BY WHICH IT CAN BE
  REMOTELY REFERENCED WITH THE REMOTE REGISTRY. (
  SEE String serverObjectName //localhost/TempServ
  er )
• THE NAME OF THE OBJECT USED BY THE MAIN SHOULD BE
  BOUND TO THE NAME USED BY THE CLIENT. (SEE
  Naming.rebind ( serverObjectName, temp) ).
• USE rmic COMPILER TO CREATE A STUB AT THE SERVER
• rmic -v1.2 TemperatureServerImpl.
• THIS COMMAND GENERATED THE FILE
  TemperatureServerImp_stub.class

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• THIS FILE MUST BE AVAILABLE TO THE CLIENT
• START THE REMOTE REGISTRY SERVER. THIS PROGRAM
  LISTENS ON THE DEFAULT PORT 1099 FOR INCOMING
  REQUESTS TO ACCESS NAMED OBJECTS. ON THE COMMAND
  WINDOW TYPE
• start rmiregistry
• LEAVE REGISTRY SERVER RUNNING.
• BOUND REMOTE SERVER OBJECT TO THE REGISTER. RUN
  THE COMPILED OBJECT IMPLEMENTATION CLASS
• java TewmperatureServerImpl
• This is a server and shpuld be left running
• AT THE CLIENT
• WRITE A CLIENT PROGRAM THAT WILL CALL THE REMOTE
  OBJECT ( THE IMPLEMENTATION OF THE INTERFACE).

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• THE IP ADDRESS OF THE MACHINE THAT THE REMOTE
  OBJECT RESIDES SHOULD BE USED (OR THE REMOTE
  HOSTS NAME).
• Naming.lookup ( serverObjectName) IS USED TO
  INTERACT WITH THE RMI REGISTRY TO HELP THE CLIENT
  OBTAIN A REFERENCE TO THE REMOTE OBJECT.
• A COPY OF THE INTERFACE STUB SHOULD BE COPIED TO
  THE SAME DIRECTORY AS THE CLIENT PROGRAM.
• Start the client prgram by calling
• java TemperatureClient

BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• CLASSES USED

SERVER
CLIENT
TemperatureServer.class temperatureServerImpl.cla
ss WeatherInfo.class TempeartureServerImpltimeDel
ay.class
Rmi registry
TemperatureClient.class Weatheritem.class Temperat
ureServerImpl_Stub.class TemperatureClient1.class
BY GEORGE KOUTSOGIANNAKIS
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Remote Method Invocation (RMI)

• Objects

1) gt (TemperatureClient) passes ip address of
server or uses localhost 2) remote method call to
object TempServer 3) TemperatureServerImpl_Stub r
eceives method call and passes it to the java
RMI system which performs the network
connections 4) requets a reference to
TempServer object from rmi registry
1)rmi registry server is running 2)temp object
calls UnicastRemoteObject which exports the
remote object 3)name of remote object TempServer
is bound to registry 4)updateWeatherConditions
method stores data from internet site into
WeatherInfo objects (array) 5) client calls
getWeatherInfo method via the interface which in
turn returns a reference to theWeatherInfo
array. 6) array objects are passed to client.
BY GEORGE KOUTSOGIANNAKIS