John Catherino's Blog

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Simple Inter-JVM communication... The Grail!

void export(Object object);
This makes any object's public methods remotely callable

Object[] lookup(Class methodSet);
This finds remote objects matching the given interface

Object proxy(Object reference, Class methodSet);
This creates a local proxy for using the remote object

import gnu.cajo.Cajo; // The cajo implementation of the Grail

public class Server {
   public static class Test { // remotely callable classes must be public
      public String foo(Object bar, int count) {
         System.out.println("foo called w/ " + bar + ' ' + count + " count");
         return "Thanks";
      }
      public Boolean bar(int count) {
         System.out.println("bar called w/ " + count + " count");
         return Boolean.TRUE;
      }
      public boolean baz() {
         System.out.println("baz called");
         return false;
      }
   } // but needn't be defined in the same class

   public static void main(String args[]) throws Exception { // unit test
      Cajo cajo = new Cajo(1198, null, null);
      cajo.export(new Test());
      System.out.println("Server running");
   }
}

import gnu.cajo.Cajo;

interface ClientSet { // client interfaces need not be public
   void baz();
   boolean bar(Integer quantum);
   Object foo(String barbaz, int foobar);
} // the order of the client method set does not matter

public class Client {
   public static void main(String args[]) throws Exception { // unit test
      Cajo cajo = new Cajo(0, null, null); // port does not matter for clients
      Thread.currentThread().sleep(100); // allow a little discovery time
      Object refs[] = cajo.lookup(ClientSet.class);
      if (refs.length > 0) {
         System.out.println("Found " + refs.length);
         ClientSet cs = (ClientSet)cajo.proxy(refs[0], ClientSet.class);
         cs.baz();
         System.out.println(cs.bar(new Integer(77)));
         System.out.println(cs.foo(null, 99));
      } else System.out.println("No servers found");
      System.exit(0);
   }
}

The cajo what?

The java.net community really works!

I was just informed about the logger project; it allows java.net project owners to view access statistics for their projects. What I found out astonished me. So much so, that I thought it important to share this discovery with you.

I started my project after a lot of introspection, (pause for geeky-Java-pun laughter :) For those of you who have not yet visited, the cajo project deals with virtualizing barriers between virtual machines; it allows objects to both interoperate, and travel between them, transparently. While implemented simply, I thought; this is sort-of abstract, and I wondered: Would anybody be interested?

Well, I was amazed to find more than 25 thousand visitors, just this year. On top of that, over 350 thousand page views this year. I never expected anything like this! (Some must be search engine hits, but unless Google got infatuated, it is still a lot of people)

So First I have to sincerely thank all of you who came to visit. Now instead of wondering: Will anybody notice? I am starting to think that this is an idea with some real peer-reviewed merit. I truly hope this project helps you; and I am very glad to be of help. And oh yes, please, spread the word!

Secondly, I am also delighted to have the opportunity to bring this idea to so many interested people! My deepest thanks to Sun, who made this possible; with this site, and through the development of Java; both at no charge! You have given the little developer all the tools of the big-guys, and as result; a real chance to make a difference.

Thirdly, for those of you thinking; is my idea good enough to be a project? There is only one way to find out; please try it! For those of you already running projects, if you have not already; please check out the logger project, it has very interesting information. And finally, as dutiful project leader: for those of you who have not yet visited the cajo project; please give it a look. The odds seem promising that you will find it enlightening.

Inside the World Wide Virtual Machine

The network is a vast collection of objects, ready to be used to create other objects.

An application is an object, which itself is composed of, a collection of objects.

• An object has 1 or more public methods,
• An object has 0 or more non-public member objects,
• An object's public methods interact with 0 or more private member objects, with 0 or more argument objects, to return 0 or 1 result object; or to throw an Exception.

• Local Objects: existing in the local VM address space.
• Remote Objects: existing in the address space of a remote VM, linked by the network.
• Proxy Objects: existing in the local VM address space on behalf of a remote VM, and may (or may not) be communicating with it.

• Local objects are instantiated, either via a new or newInstance method invocation; or are deserialized from an ObjectInputStream.
• Proxy objects are obtained via a remote object method result, or exception.
• Remote objects are obtained via a request from a remote VM; or via a remote VM broadcast. (sometimes called listening to radio station WWVM :-)

• A local object can be passed to a remote object, only if allowed by both sides, where it will exist there, as a proxy.
• A remote object reference can be freely passed between all objects and Virtual Machines.
• A proxy is unique to the runtime in which it exists, and cannot be passed to another VM.

It is expected that all proxy objects will be hosted in a JNLP/Applet-type sandbox.

The World Wide Virtual Machine

Disclaimer: This entry is in no way meant to slight the many other fine distributed frameworks out there. It’s just that this one is designed for us non-rocket scientists. :-)

I host a free project here on java.net called the cajo project. It allows any Virtual Machine to easily expose access to any of its selected objects to Remote Virtual Machines, as well as to send any of its selected objects to RVMs. It offers a very simple yet highly flexible framework for applications to work together. Most importantly, any existing object can be made remotely usable, with no changes to its source; an application simply adds this framework, not restructures around its requirements. I have found this unique amongst distributed frameworks.

A client can obtain a remote reference to an exposed object in one of two ways. It can request one explicitly, using just the name of the server, and the name under which the exposed object is registered, if it knows these things. In the framework, this is called static binding. It can also listen for object reference broadcasts, sent from RVMs. This is called dynamic binding. The references can then be freely shared in turn with other remote objects. Exposing an object for remote reference is done conversely; it can be bound locally under a name, or be broadcast to all listening VMs, even both.

Clients can invoke the remote object’s methods, and receive the resulting return, just as though it was a local object. How does the client know how to use a remote object? The object can implement a getDescription() method, it would provide detailed usage information. The object often implements a getProxy() method, it would generally return a graphical user interface component with which to interact with the remote object. The framework also features a standard generic client, which can be run as applet in a browser, or as an application via JNLP, to remotely host the GUI component.

Any VM can send an object (or remote object reference) to an RVM, as an argument to one of its methods, or by returning it as a method result. The objects will physically exist locally in the RVM, i.e. in the same address space, as its own objects. These objects are known as proxies in the context of this project, as they exist to act in a remote location, on behalf of their sender. The previously mentioned the getProxy() method is used to return a GUI proxy object.

The framework borrows from the best concepts of Jini, CORBA, and DCOM, but it is far smaller, as powerful, and much simpler to use. The core framework consists of only two packages; a total of only 13 small class files. It can be so small and easy to use, as it does not impose any application semantics beyond those outlined above. It comes with a detailed example file, ample documentation, and several support fora. It is completely free, licensed under the GNU LGPL, to allow both free and proprietary applications to operate together.

The framework is currently running on all Java Runtime Environments; J2EE, J2SE, and even J2ME. It is in active use on Linux, Windows, Solaris, and OSX systems. The goal of the project is to foster the creation of a WWVM. This is an exciting shared space of applications and object libraries, where they all appear and are used as though they were local, to foster the creation of new dynamic, interactive application networks. All VMs are free to join, from big mainframes, to tiny PDAs. (You can use any JVM compatible language, even Java)

Please come visit the WWVM community, all are welcome. Participants needed. :-)

This work is licensed under a Creative Commons License.