Java RMI Alcides Calsavara

Objetivos Permitir que um método de uma classe Java em execução em uma máquina virtual JVM chame um método de um objeto (instância de uma classe Java) situado em outra máquina virtual JVM, usando a mesma sintaxe e com a mesma facilidade de se chamar um método local. Transparência de acesso e de localização.

Esquema geral JVM cliente método f da classe C em execução JVM servidor método g objeto da classe S chamada remota

Remote Method Invocation Facility to call methods remotely Similar to RPC - Remote Procedure Call High-level communication abstraction The RMI mechanism provides: control transfer between caller and called parameters cans be exchanged class definitions can be exchanged RMI support in Java provides: A specific API compilation support tools runtime support

General scenario Server Registry ClientStubSkeleton communication actual perceived get an object reference object reference is published naming service 2 1 3

Componentes em execução f de C g de S S_SkelS_Stub Socket Cliente Socket Servidor Naming Registry Socket Cliente Socket Servidor chamda remota de g clienteservidor main cria bind lookup

Hierarquia de classes e interfaces Interface Remote Classe UnicastRemoteObject Interface R assinatura de g Classe S implementação de g extends implements extends

Compilação S.java javac S.class rmic S_Skel.classS_Stub.class C.java javac C.class

Comandos Compilação: javac S.java rmic S javac C.java Execução (3 processos): rmiregistry java S java C

A Java RMI example // file iCalendar.java // specifies a date server interface import java.rmi.* ; public interface iCalendar extends Remote { java.util.Date getDate () throws RemoteException ; }

The RMI date server // file CalendarImpl.java // the date server implementation import java.util.Date; import java.rmi.*; import java.rmi.registry.*; import java.rmi.server.*; //...

The RMI date server public class CalendarImpl extends UnicastRemoteObject implements iCalendar { public CalendarImpl() throws RemoteException {} public Date getDate () throws RemoteException { return new Date (); }

The RMI date server public static void main(String args[]) { CalendarImpl cal; try { cal = new CalendarImpl(); LocateRegistry.createRegistry(1099); Naming.bind("rmi:///CalendarImpl", cal); System.out.println("Ready !"); } catch (Exception e) { e.printStackTrace(); }

The RMI date client // file CalendarUser.java import java.util.Date; import java.rmi.*; public class CalendarUser { // constructor public CalendarUser() {}

The RMI date client public static void main(String args[]) { long t1=0,t2=0; Date date; iCalendar remoteCal; try { remoteCal = (iCalendar) Naming.lookup ("rmi://some.host.com/CalendarImpl"); t1 = remoteCal.getDate().getTime(); t2 = remoteCal.getDate().getTime(); }

The RMI date client catch (Exception e) { e.printStackTrace(); } System.out.println ("This RMI call took + (t2-t1) + " milliseconds"); } // main } // class CalendarUser

RMI naming service Very simple Manages pairs [name, object reference] names are keys to get references there is no domain notion Implementation Only one registry per process Defaults to TCP/IP port 1099 Applications can register names only in a registry running on its host Registry server can be launched manually or inside the program

Naming service method calls To create a registry at port 1099: LocateRegistry.createRegistry (1099); To bind a name to a reference: Naming.bind ("rmi:///someobj", objRef); To resolve a name: obj = (objClass) Naming.lookup ("rmi://some.host/someobj");

Dynamic class loaders To dynamic load classes used by a running program or applet from the local disks or the network Three kinds of class loaders: Default class loader: to load the classes needed by the JVM (CLASSPATH variable). Applet class loader: to download applets and additional classes from the applet server. RMI class loader: to download from the remote server host all the classes (stubs, skeletons, parameters) to allow the RMI calls.

Setting the RMI class loader On the client side: // launch the RMI security manager System.setSecurityManager ( new RMISecurityManager () ); // set a system property System.getProperties ().put ( "java.rmi.server.codebase", " ); On the server side: Put stubs and classes at the indicated place

Building apps with RMI Server bytecode server source code client source code Interface source code Client bytecodeStubSkeleton rmic compiler javac compiler rmic compiler