Running a RMI Application Based on ROA
Before you can run the example build in this article, you must develop a RMI-client. This can be an typical RMI client, like those used non-ROA-based RMI applications. Or, you can use the RMI client shown in Listing 3.
The policy file (12.policy), also used in this example, may have the following content:
grant{
permission java.security.AllPermission;
};
The RMI-ROA application is now complete. Before you can test it, build directory structure shown in Figure 1.
Figure 1: Build this directory structure before you test your RMI-ROA application.
Next, start the following command line Windows consoles (if you're testing the application on a network, the first two processes must be started on the server):
//rmiregistry
C:\Data_local\RMIServerActivation>rmiregistry
//rmid
C:\Data_local\RMIServerActivation>rmid -J-Djava.security.policy=12.policy
//start the "setup" program - this will run only once
C:\Data_local\RMIServerActivation>java PrinterServerActivatable
//starting RMI client
C:\Data_local\RMIClientActivation>java PrinterClientActivatable
If all goes as planned, you should see the following message in the client console:
Jobs:1
Notice that after you have started the application for the first time, you can find the Init.txt file it in the C:\Date_local\RMIServerActivation. This file stores information that's used to activate the activable remote object. To test this, stop the rmiregistry and rmid processes and start them again. After that, run the "setup" program one more time. Now, start the client and look at the result. The result should be:
Jobs:2
This confirms that you didn't lose the state of the object when it was destroyed, because the state of the object was saved in the Init.txt file.
The Activator's main concern is: does the requested object have an ActivationGroup? If the Activator discovers that the requested object doesn't have an ActivationGroup, it creates a default group. The Activator is obligated to provide a JVM for the new ActivationGroup and to "start" the ActivationGroup using the information from the ActivationGroupDesc (which is optional). After the Activator solves this problem, it passes a request to this group by calling the ActivatorGroup.newInstance method.
Next, the newInstance method is inherited from the ActivationInstantiator class, which is responsible for the creation of the activable remote objects instances. The Activator provides the ActivationID and the ActivationDesc objects as arguments to the newInstance method. These two objects provide the ActivationGroup with the necessary information for the activation. The first task for the ActivationGroup is to load the class corresponding to the activable remote object (the name of the class is extracted from ActivationDesc). After this class is loaded, the activation/reactivation constructor creates an instance of it. The ActivationID and MarshalledObject objects are then obtained from the newInstance method. The newly created object is then returned to the Activator as a MarshalledObject (in serialized form). Next, the Activator registers the MarshalledObject by associating it with the corresponding ActivationID. After that, it sends the new object to the client stub.
Powerful, but Complex
ROA is a powerful technique that can be implemented by RMI applications that are specially created for a large number of users and which consume many resources. The one drawback with using ROA, as you have seen is that developing applications with it can be fairly complex.
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