Presentation: RMI introduction

Goals of this lesson After this 35 min. lesson you will be: Introduced to Java RMI Ready to present RMI’s position in the Middleware technology family, when and where to use it You will not: Be an RMI expert. More practice and theory is required

Outline Theory: (35x2 min.) Introduction to Java RMI Group work: (35 min.) Pro’s & con’s of Java RMI vs. SOAP vs. ICE vs. CORBA When to use which technology… Differences and equalities

Java RMI In Java 1.0 object communication was confined to objects in one Virtual Machine (VM) Sun decided to introduce inter VM communication Remote Method Invocation (RMI) from Java 1.1 supports communication between different VMs, potentially across the network Provides tight OO integration with Java Work in heterogeneous environment (servers) BUT ONLY with Java (so far) – so no language transparency

Java RMI features Build on Java’s existing object model -> easy No need for IDL – use Java interfaces Arguments & return values can be all types specializing java.io.Serializable or java.rmi.Remote Dynamic loading of classes Use of build-in Java Security Manager Distributed Garbage Collection Integrates with CORBA (later) BUT NOT IN J2ME!!! (use SOAP) J2ME CDC has an RMI profile!

Java RMI position Middleware Transaction-Oriented IBM CICS BEA Tuxedo Encina Message-Oriented IBM MQSeries DEC Message Queue NCR TopEnd (SOAP) RPC Systems ANSA Sun ONC OSF/DCE (SOAP) Object-Oriented OMG/CORBA DCOM Java/RMI (SOAP)

Wire Protocol Java RMI wire protocol: JRMP (Java Remote Method Protocol) OR IIOP (Internet Inter-ORB Protocol) for CORBA connectivity Both build on top of TCP/IP JRMP more advanced than IIOP Other Java RMI specification implementors Historic: BEA Weblogic, Object Voyager, NinjaRMI Object Voyager’s was JRMP compatible Others were not IIOP compatibility can not be guaranteed

Local Java call vs. Java RMI callCalledCalled Stub CallerCalledCalledCaller Caller Transport Layer (e.g. TCP or UDP) Similar to SOAP and CORBA – using Proxy

Interface Definition Design Server Stub Generation Client Stub Generation Server Coding Client Coding Server Registration Development Steps – RMI & CORBA & SOAP SOAP: WSDL Java2WSDL WSDL2JAVA AXIS SOAP RMI: JAVA J2SE JDK Start with Server Interface Coding: JAVA Start with Server Interface Coding: JAVA rmiregistry CORBA CORBA: IDL CORBA: IDLC ORB RMI: JAVA interface C++, Java … RMI: rmic

Team.wsdl included in generates reads WSDL-compiler Teamcl.hh Teamcl.cc Teamsv.cc Teamsv.hh Stub Generation - in SOAP & CORBA Team.idl

package examples.hello; import java.rmi.Remote; import java.rmi.RemoteException; public interface Hello extends Remote { String sayHello() throws RemoteException; void someOther(String param) throws RemoteException; } rmic Compiler HelloImpl_Stub.class HelloImpl_Skeleton.class Stub Generation in Java RMI NOTE: In fact, it is the HelloImpl that is used! NOTE: In fact, it is the HelloImpl that is used! Hello.java Must Extend from Interface Remote Must Extend from Interface Remote From RMI v. 1.2 no skeleton is generated From RMI v. 1.2 no skeleton is generated From Java v. 1.5 no rmic comp is needed From Java v. 1.5 no rmic comp is needed

Java compiler - javac Server HelloClient.java HelloImpl.java Java compiler - javac Client Hello.java included in generates reads rmic Compiler RMI Client and Server Implementation HelloImpl_Stub.class HelloImpl_Skeleton.class

package examples.hello; import java.rmi.Naming; import java.rmi.RemoteException; import java.rmi.RMISecurityManager; import java.rmi.server.UnicastRemoteObject; public class HelloImpl extends UnicastRemoteObject implements Hello { public HelloImpl() throws RemoteException { super(); } public String sayHello() { return "Hello World! ; } public static void main(String args[]) { // Create and install a security manager //if (System.getSecurityManager() == null) { // System.setSecurityManager(new RMISecurityManager()); //} try { HelloImpl obj = new HelloImpl(); // Bind this object instance to the name "HelloServer" Naming.rebind("rmi:// /HelloServer", obj); System.out.println("HelloServer bound in registry"); } catch (Exception e) { System.out.println("HelloImpl err: " + e.getMessage()); e.printStackTrace(); } Server object (HelloImpl.java) Security manager needs a security policy – for access control (i.e. file system). Security manager needs a security policy – for access control (i.e. file system). Instantiate a new object and register (bind it) in the ”rmiregistry” Instantiate a new object and register (bind it) in the ”rmiregistry” Implement all methods from interface Hello.java Implement all methods from interface Hello.java Extend UnicastRemote and implemet Hello Interfacet Extend UnicastRemote and implemet Hello Interfacet ”rmiregistry” is a simpel name server with methods to bind objects (bind/rebind) – and Find them again (lookup) –> client ”rmiregistry” is a simpel name server with methods to bind objects (bind/rebind) – and Find them again (lookup) –> client

package examples.hello; import java.rmi.Naming; import java.rmi.RemoteException; public class HelloClient { public static void main(String args[]) { try { Hello obj = (Hello)Naming.lookup("rmi:// /HelloServer"); String message = obj.sayHello(); System.out.println(message); } catch (Exception e) { System.out.println("HelloApplet exception: " + e.getMessage()); e.printStackTrace(); } ”lookup” the HelloServer – and call Method sayHello() on Stub ”lookup” the HelloServer – and call Method sayHello() on Stub Client object (HelloClient.java) Remember – that the stub and skeleton classes get generated by the ”rmic” compiler Remember – that the stub and skeleton classes get generated by the ”rmic” compiler AND THAT’S IT!

Architecture ServerClient Stub Registry Interfaces Skeleton Activation Interfaces RMI Runtime (rmid,rmiregistry) coded manually rmic generated bind lookup

Things to Remember No attributes / properties in Java Interfaces -> RMI does not support attributes Attributes must be represented as set and get operations by the designer

Things to remember II Parameter passing different than normal Java in single VM Atomic types are passed by value Remote objects are passed by reference Non-Remote objects are passed by value! Reflexive: can return references to other objects And of course – if an object is not on the client – the ByteCode gets transferred (the class incl. implementation) – if a codebase is defined

Key Points True and beautiful OO Middleware Easy to learn – for Java developers No need for a separate IDL (use Java Interfaces) Distributed Garbage Collection ByteCode transfers automatically (if codebase is defined) Works in heterogene environments – but only with Java No build-in services (except for the registry) Depends on other API’s – JavaSpaces, JINI, JDBC, EJB, JDO etc. – integrated into a framework Not ”firewall friendly”