What Is Remote Object Activation in Java

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11121 What Is Remote Object Activation
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Remote object activation is a technique that solves the problem of running a large number of idle RMI services It allows services to be registered with the rmiregistry, but not instantiated Instead, they remain inactive, until called upon by a client, when they will awaken and perform their operation A special daemon process called the remote method invocation activation system daemon (rmid) listens for these requests and instantiates RMI services on demand Using some special trickery behind the scenes, a request is forwarded onto the activation daemon process, which creates the service Requests are then forwarded onto the newly created service, transparently to the client, just as if it were a normal RMI service that had been running indefinitely This means that services will lie dormant until invoked, and activated just in time (JIT) for use by a RMI client
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11122 How Does Remote Object Activation Work
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Remote object activation works a little differently from normal RMI servers To be locatable, a service must be available through an RMI registry, but to do that normally requires an object to be instantiated Since the whole point is to avoid instantiating RMI servers and instead to activate them as required, a faulting remote reference is registered in its place A faulting remote reference is a remote reference that acts as a proxy between the remote client and the as-yet-unactivated server Unlike a UnicastRemoteObject server, which runs indefinitely, a server installation program runs for a short duration Its purpose is to notify the activation system of an activatable remote object, and to register the faulting remote reference with the rmiregistry (see Figure 11-9) Once the registration is complete, the server installation program can terminate; creating the remote object is now the responsibility of the faulting reference and the remote method activation system daemon Figure 11-9 Server installation program registers activatable object with rmid, and faulting reference with rmiregistry
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At some point in time in the future, an RMI client will look up the rmiregistry and select the service provided by the faulting remote reference When the faulting reference is activated, it checks to see if it already has a reference to the server The first time the faulting reference receives a call, the reference will be null, indicating that the server must be activated before being used It enlists the aid of the rmid application, to create a new instance of the server, and then
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passes the call to the newly activated object (see Figure 11-10) Method requests to the faulting reference are forwarded onward to the activated object, and then returned back to the client Though there is a slight delay on wakeup, this is not repeated for subsequent requests, as the service is ready for action Figure 11-10 Faulting reference, with the aid of activation system, passes information from activated object back to client
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The elegance of this solution resides in the fact that the client is, at all times, completely unaware of the details of server implementation Although there may be some slight delay in creating the remote object initially, subsequent calls are faster, and the client does not know whether the server has always been running or was only recently activated This means no extra code to compile, and no extra work for client developers
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11123 Creating an Activatable Remote Object
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Creation of an activatable remote object is a little different from that of a UnicastRemoteObject, but the code is fairly similar Let's start with an extremely simple remote interface, and see how it would be implemented as an activatable remote object
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public interface MyRemoteInterface extends javarmiRemote { public void doSomething () throws javarmiRemoteException; }
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The RMI service interface contains a single method, doSomething(), which takes no parameters and may throw a RemoteException There's no difference between a normal remote interface and one that will be implemented by an activatable remote object In fact, an interface can be implemented by both an activatable object and a UnicastRemoteObject The next step is to create an implementation that extends the javarmi activationActivatable class It must implement our RMI interface and provide both a
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constructor and a doSomething() method Unlike classes that extend the UnicastRemoteObject class, a different constructor signature is used This constructor calls the parent constructor and exports the object on any available port The service is then available for use by clients
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public class MyRemoteInterfaceImpl extends javarmiactivationActivatable implements MyRemoteInterface { public MyRemoteInterfaceImpl (javarmiactivationActivationID activationID, javarmiMarshalledObject data) throws javarmiRemoteException { // call the Activatable(ActivationID activationID, // int port) parent constructor super (activationID, 0); } public void doSomething() { Systemoutprintln ("Doing something"); } }
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Comparing this class to one extended from the UnicastRemoteObject class reveals relatively few differences However, there's a little more work to do, as the object must still be registered with the activation system and the rmiregistry
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