15 - RMI

Java RMI Architecture Example Deployment RMI is a part of J2SE (standard edition), but is used by J2EE) A J2EE server is not nessesary for using RMI

3 NOEA2009Java-kursus – RMI Communikation between different machines Sockets, RPC and RMI –For sockets a common protocol on application level is needed to encode and decode sent and received messages –RPC (Remote Procedure Call) pulls the abstraction level for communication up ti procedural level –RMI (Remote Method Invocation) handles communication between objects in different adress spaces.

4 NOEA2009Java-kursus – RMI Distributed Object-application (DOA) A application, where the server provides remote objects on which methods might be activated from different clients, is called a hvis metoder kan kaldes fra forskellige klienter ”distributed object-application” DOA needs to: –Localize remote objects –Communicate with remote objects –Handle byte code for objects that are send as a parameter or a return value RMI can handle this

5 NOEA2009Java-kursus – RMI Architecture

6 NOEA2009Java-kursus – RMI Example for building a RMI-application The steps: 1.Contruct an interface (remote) 2.Implement the interface 3.Create a server class 4.Create a client class 5.Compile it (javac og rmic) 6.Run the application start rmi-registry (not nessesary from java 1.5), server and klient

7 NOEA2009Java-kursus – RMI Oprettelse af interface import java.rmi.*; public interface Hello extends Remote { public String getGreeting() throws RemoteException; } Java.rmi.Remote is a interface that all RMI-application must inherit from Java.rmi.RemoteException is the superclass for exceptions that RMI can throw and shall always be handled

8 NOEA2009Java-kursus – RMI Implementation af interfacet import java.rmi.*; import java.rmi.server.*; public class HelloImpl extends UnicastRemoteObject implements Hello{ public HelloImpl() throws RemoteException { //Default constructor is implemented because of RemoteException } public String getGreeting() throws RemoteException { return ("Hello there!"); }

9 NOEA2009Java-kursus – RMI Server class //Server. import java.rmi.*; public class HelloServer{ private static final String HOST = "localhost"; public static void main(String[] args) throws Exception{ HelloImpl temp = new HelloImpl(); String rmiObjectName = "rmi://" + HOST + "/Hello"; //Could omit host name, since 'localhost' would be //assumed by default. Naming.rebind(rmiObjectName,temp); System.out.println("Binding complete...\n"); }

10 NOEA2009Java-kursus – RMI Client class import java.rmi.*; public class HelloClient{ private static final String HOST = "localhost"; public static void main(String[] args){ try{ Hello greeting = (Hello)Naming.lookup("rmi://" + HOST + "/Hello"); System.out.println("Message received: " + greeting.getGreeting()); } catch(ConnectException conEx){ System.out.println("Unable to connect to server!"); } catch(Exception e){ e.printStackTrace(); }

11 NOEA2009Java-kursus – RMI From JDK 1.5 this is done automaticly Generate the stub We still missing the piece of code that handles the communication. It is placed in the file HelloImpl_Stub The file is generated by running rmic.exe på impl-filen: rmic –v1.2 HelloImpl

12 NOEA2009Java-kursus – RMI How does it function? - Deployment To make the client run: the HelloClient, Hello and HelloImpl_stub must be present on the client HelloServer, Hello, HelloImpl and HelloImpl_stub must be present on the server The stub is a proxy class for the remote-object. The stub takes care of the communication between the client and the server by marshalling/unmarshalling The client knows how to manipulate the server object because yhe server object implements the interface that is also known on the client

13 NOEA2009Java-kursus – RMI RMIRegistry Rmiregistry is a program that handles the nameservice. Registry shall be started before the server is started (This is done automatically in jdk1.5+) The rebind-method of the server binds a name (URL) to the implementation object on the server. The client can use the lookup method to get a reference to the impl-object Add your server files to classpath in order for rmiregistry to find them. Rmiregistry is started from the command promt by: start rmiregistry

14 NOEA2009Java-kursus – RMI Call semantics When a method is called on a remote object, there is two ways of passing the parameters dependent of whether the parameter (and the return value) is a remote object or not. If remote then is call-by-reference If not then it is call-by-value (copy) Objects that are sent by copy shall implement the interface Serializable