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 servidor JVM cliente método f da classe C em execução chamada remota objeto da classe S método g
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 object reference is published Server Skeleton 1 Registry perceived actual 3 communication naming service get an object reference 2 Client Stub
Componentes em execução cliente servidor f de C g de S chamda remota de g cria S_Stub S_Skel bind main lookup Socket Cliente Socket Servidor Naming Naming Socket Cliente Socket Servidor Registry
Hierarquia de classes e interfaces Interface Remote Classe UnicastRemoteObject extends Interface R assinatura de g extends implements Classe S implementação de g
Compilação S.java javac S.class rmic C.java javac C.class S_Skel.class S_Stub.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", "http://some.where/java/classes/ ); On the server side: Put stubs and classes at the indicated place
Building apps with RMI Interface source code rmic compiler rmic compiler server source code Skeleton Stub client source code javac compiler javac compiler Server bytecode Client bytecode