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The result of the method invocation (such as a return value) is transported back across the network to be delivered to the client by RMI. To better understand what is happening in the remote interface scenario, let s draw an analogy with something familiar to all of us: a television. Think of a television as the remote object to be called. The remote control for our television is the remote interface. The infrared protocol sent by the remote control and picked up by the television is like the RMI protocol. While using the remote control, we don t have to know all of the details of how the infrared signal works; all we need to know is what each of the buttons on the remote does (the interface methods). Figure A.1 shows how a typical RMI invocation scenario works. The parallels between EJB and RMI are obvious. Like the EJB object discussed in chapter 5, RMI works through proxies on both the client and remote object endpoints to transparently provide distributed computing as a service. In the EJB world, the business interface plays the same role as the remote interface, while the EJB bean itself is the remote object. Just as RMI handles the communication details between the remote client and the object, the container handles the communication details between the EJB client and the managed bean. Before EJB 3, the linkages between RMI and EJB were even more obvious for example, remote business interfaces had to put java.rmi.RemoteException in their throws clause. In EJB 3, the fact that EJB uses RMI for remoting is rightfully hidden far behind the API as an implementation detail you can safely ignore.

public class EquityBean implements EntityBean { private JMSPublisher publisher = null; private void publishMessage( String msg ) Encapsulates { calls to the try{ publisher if( publisher=null ) publisher = new JMSPublisher("BookJMSFactory","BookJMSTopic"); System.out.println( "Publishing message: "+msg ); publisher.publish(msg); publisher.close(); } catch(Exception e){ e.printStackTrace(); } } public void ejbRemove()throws RemoveException{ //implementation not shown publishMessage("Removed Equity from Database:"+symbol); } public String ejbCreate(String symbol, String description) throws CreateException { //implementation not shown publishMessage("Inserted Equity into Database:"+symbol); } public void setEquity( EquityVO equity ){ //implementation not shown publishMessage("Changed the Equity in the Database:"+symbol); } }

Figure A.1 Communication between the RMI client and server (remote object). The RMI server binds an object in the RMI registry. The client performs a lookup in the RMI registry to get a reference to the remote object and then invokes the method on the remote object through the remote interface.

When you annotate a session bean business interface using the @Remote annotation as we did in chapters 2 and 3, the container automatically makes the bean callable via RMI as a remote object Similarly, when you inject a remote bean using the @EJB annotation, the container talks to the bean through RMI under the hood The greatest benefit for RMI is that it provides location transparency The client invokes methods on a remote object as if the object were located in the same virtual machine, without having to worry about the underlying plumbing This also means that RMI is generally a lot easier to use and flexible compared to writing TCP/IP sockets by hand for remote communication As compelling as RMI might seem from our very high-level discussion, like all technologies it has its own set of problems Remote object invocation uses pass-by-value semantics.

public class JMXExample { private int port = 8080; private String host = "localhost"; private static Stats statistics = new Stats(); public static void sendCommand(float amount) { ... } public void start() throws JMException, MalformedURLException { MBeanServer server = MbeanServerFactory.createMBeanServer("OrderProcess"); ObjectName serverName = new ObjectName("Http:name=HttpAdaptor"); server.createMBean("mx4j.adaptor.http.HttpAdaptor",serverName,null); server.setAttribute(serverName,new Attribute("Port",new Integer(port))); server.setAttribute(serverName,new Attribute("Host",host)); ObjectName processorName = new ObjectName("Http:name=XSLTProcessor"); server.createMBean("mx4j.adaptor.http.XSLTProcessor",processorName,null); server.setAttribute(processorName, new Attribute("UseCache",new Boolean(false))); server.setAttribute(serverName, new Attribute("ProcessorName",processorName));