1. 1 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

2. 2 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

3. 3 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

4. 4 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

5. 5 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

6. 6 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

7. 7 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

8. 8 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.

9. 9 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.

10. 10 Remote Method Invocation (RMI) 2.0HOW RMI WORKS BY GEORGE KOUTSOGIANNAKIS Remote object registers its name Client Server Local object Remote object Object registry Access to remote object requested by name Object is invoked Default port 1099

11. 11 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

12. 12 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

13. 13 Remote Method Invocation (RMI) 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 BY GEORGE KOUTSOGIANNAKIS

14. 14 Remote Method Invocation (RMI) 4.0SUMMARY OF TEMPERATURESERVER EXAMPLE : Weather forecast site Info from server retrieved whenever client wishes client Remote serverWeb server internet http rmi Aquire information from 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

15. 15 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 Remote Method Invocation (RMI)

16. 16 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.

17. 17 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/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 BY GEORGE KOUTSOGIANNAKIS

18. 18 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

19. 19 Remote Method Invocation (RMI) –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 BY GEORGE KOUTSOGIANNAKIS

20. 20 Remote Method Invocation (RMI) CLASSES USED: BY GEORGE KOUTSOGIANNAKIS TemperatureClient.class Weatheritem.class TemperatureServerImpl_Stub.class TemperatureClient$1.class CLIENT TemperatureServer.class temperatureServerImpl.class WeatherInfo.class TempeartureServerImpl$timeDelay.class SERVER Rmi registry

21. 21 Remote Method Invocation (RMI) Objects BY GEORGE KOUTSOGIANNAKIS 1) gt (TemperatureClient) passes ip address of server or uses localhost 2) remote method call to object TempServer 3) TemperatureServerImpl_Stub receives 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.