1 handout 14 remote method invocation (rmi) by george koutsogiannakis this document can not be...

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HANDOUT 14

Remote Method Invocation (RMI)

BY GEORGE KOUTSOGIANNAKIS

THIS DOCUMENT CAN NOT BE REPRODUCED OR DISTRIBUTED WITHOUT TH E WRITTEN PERMISSION OF THE AUTHOR

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• 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


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• 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.


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• 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.


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• 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.


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• 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


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– 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).


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• 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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• 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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• 2.0 HOW RMI WORKS


Remote objectregisters its name

ClientServer

Local object

Remote object

Object registry

Access to remote object requestedby name

Object is invoked

Default port 1099

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• 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.


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• 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.


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• EXAMPLE OF AN RMI APPLICATION (TemperatureServerImpl)• 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


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• 4.0 SUMMARY OF TEMPERATURESERVER EXAMPLE:

Weather forecast site

Info from server retrieved

whenever client wishes

client Remote server Web serverinternet

httprmiAquire informationfrom weather forecast site and store it

•Info is stored in an array of objects•Info is downloaded when server program starts and gets repeated twice a day.

Unicast (point-point)communications between objects via method calls.

//host:port/RemoteObjectName

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• 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)



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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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• 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/TempServer”; )

– 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


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– 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).


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– THE IP ADDRESS OF THE MACHINE THAT THE REMOTE OBJECT RESIDES SHOULD BE USED (OR THE REMOTE HOST’S 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


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• CLASSES USED:


TemperatureClient.classWeatheritem.classTemperatureServerImpl_Stub.classTemperatureClient$1.class

CLIENT

TemperatureServer.classtemperatureServerImpl.class

WeatherInfo.classTempeartureServerImpl$timeDelay.class

SERVER

Rmi registry

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• Objects


1) gt (TemperatureClient)passes ip address of server or uses localhost2) remote method call to object TempServer3) TemperatureServerImpl_Stubreceives 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 running2)temp object calls

UnicastRemoteObject which exports the remote object

3)name of remote object TempServer is bound to registry

4)updateWeatherConditions methodstores 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.

1 handout 14 remote method invocation (rmi) by george koutsogiannakis this document can not be...

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