Dr. Dobb's Journal - December 2008 - (Page 37)

d11haas_p5db 10/13/08 9:51 AM Page 37 Listing One • Persistent storage of the bindings. To allow for a lossless registry reboot, we persistently store the bindings on hard-disk. Each time a binding is added, changed, or removed, the persistent copy is updated. Because of this persistent storage, invalid bindings must be periodically garbage collected. public RemoteRegistry (int port) throws RemoteException { LiveRef lref = new LiveRef (new ObjID(ObjID.REGISTRY_ID),port); new UnicastServerRef (lref).exportObject (this,null); } Listing Two grant { permission java.net.SocketPermission "*:*", "connect,accept,resolve,listen"; permission java.lang.RuntimePermission "accessClassInPackage.sun.rmi.*"; permission java.io.FilePermission".rrrbindings", "read write,delete"; }; Listing Three // Remote interface public interface Hello extends Remote { String getHello() throws RemoteException; } // Implementation class public class HelloImplimplements Hello { public String getHello() throws RemoteException { return "hello, world" ; } } // Startup of RMI serverobject, including registration of // the instantiated server object with remote RMI registry public class HelloSetup { public static void main (String[] args) throws UnknownHostException,RemoteException, MalformedURLException,NotBoundException, InterruptedException,AlreadyBoundException { System.setProperty ("java.rmi.server.codebase", "http://192.168.2.32/hello.jar"); Hello stub = (Hello) UnicastRemoteObject. exportObject (new HelloImpl(),0); //registration with remote RMI-Registry is now possible Registry registry = LocateRegistry.getRegistry ("192.168.2.31"); registry.rebind ("hello",stub); System.out.println ("Successfully registered."); } } Implementation Even though the implementation of the core functionality—that is, the storage and retrieval of the mappings—is rather trivial, the overall registry implementation is not, as the following considerations show: • A remote RMI registry instance is itself an RMI server object (that provides the Registry remote interface). An (ordinary) RMI server or client that needs a reference to the registry uses the getRegistry method of the standard class LocateRegistry. This bootstrapping method is the only one that synthesizes a stub for a remote object, rather than getting it from the remote object itself. Each stub, however, contains the ObjID of the remote object that is created when the object is exported. LocateRegistry assumes the registry to have a special value, REGISTRY ID, as the ObjID. Unfortunately, the Java API does not let you specify the ObjID when exporting a remote server object. As a consequence, we had to take advantage of two classes of Sun’s JRE implementation—sun.rmi.server.UnicastServerRef and sun.rmi.transport.LiveRef to assign the required ObjID to the exported remote registry object. Class RemoteRegistry contains our custom implementation of the standard Registry remote interface. Listing One, the constructor, shows how a newly instantiated RemoteRegistry object is exported by means of the two aforementioned classes. In the first step, a LiveRef object with the required ObjID and the listener port is created. As can be seen, the special value REGISTRY ID is defined as a constant in class ObjID. The port can be set December 2008 l www.ddj.com l Dr. Dobb’s Journal 37 http://www.ddj.com/codetalk http://www.ddj.com/codetalk http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - December 2008

Dr. Dobb's Journal - December 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Conversations The Man Who Sold the Sky Performance on Rails LINQ-to-SQL and T-SQL A Remote Java RMI Registry Beyond B-Trees File Descriptors and Multithreaded Programs Effective Concurrency The Agile Edge Swaine's Flames

Dr. Dobb's Journal - December 2008

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