Using Activation: Extending `Activatable`

This tutorial describes how to implement an activatable remote object by extending the class java.rmi.activation.Activatable. This tutorial uses a Setup program (described in the tutorial Using Activation: the Setup Program) that registers information about an activatable remote object with the Java^TM Remote Method Invocation (Java RMI) activation system daemon (rmid) and then binds a stub for that remote object in an rmiregistry so that clients can look it up. You may want to read that tutorial before this one.

This tutorial has the following steps:

Implement the activatable remote object
Implement the client
Compile the source files
Run the Setup program
Run the client

The files needed for this tutorial are:

MyRemoteInterface.java - a simple remote interface
ExtendsActivatable.java - an "activatable" implementation of the remote interface
Client.java - a client that uses the remote interface
client.policy - the security policy file for the client

Implement the activatable remote object

There are a few basic ways to implement an activatable remote object. This tutorial describes how to implement an activatable remote object by extending the class java.rmi.activation.Activatable which exports an activatable remote object during construction.

A remote object is activated when a client invokes a remote method on a stub for an activatable remote object. A stub for an activatable remote object contains the remote object's activation ID and information on how to contact the Java RMI activation system daemon (rmid) for the remote object. If the stub cannot connect to the last-known address (i.e., host/port) for the remote object, the stub will contact the remote object's activator (rmid) to activate the object. When rmid receives an activation request, it starts the remote object's activation group (or container) VM if the group is not already executing, and then rmid asks the group to make an instance of the remote object. Once the group constructs the remote object, it returns the remote object's stub to rmid which, in turn, returns the actual stub to the initiating stub so that the initiating stub can update its information on how to contact the remote object in the future.

Before any of this activation can take place, an application must register information about the activatable remote objects it needs to use. The following separate tutorial describes the information needed to activatate a remote object and how to register this information with rmid:

Using Activation: the Setup program

In this example, the activatable remote object implements the following remote interface examples.activation.MyRemoteInterface:

package examples.activation;

import java.rmi.*;

public interface MyRemoteInterface extends Remote {
    Object remoteMethod(Object obj) throws RemoteException;
}

The implementation class, examples.activation.ExtendsActivatable, for the activatable remote object is as follows:

package examples.activation; 

import java.rmi.*;
import java.rmi.activation.*;

public class ExtendsActivatable
        extends Activatable 
        implements MyRemoteInterface
{

    public ExtendsActivatable(ActivationID id, MarshalledObject data) 
        throws RemoteException
    {
        super(id, 0);
    }
        
    public Object remoteMethod(Object obj) {
        return obj;
    }
}

The class ExtendsActivatable extends the class Activatable and implements the remote interface MyRemoteInterface.

The class ExtendsActivatable declares a special "activation" constructor that an activation group calls to construct an instance during the activation process. This special constructor takes two parameters:

The first parameter, ActivationID, is an identifier for the activatable remote object. When an application registers an activation descriptor with rmid, rmid assigns it an activation ID, which refers to the information associated with the descriptor. This same activation ID (also contained in the remote object's stub) is passed to this constructor when the remote object is activated.
The second parameter is a MarshalledObject that contains initialization data pre-registered with rmid. This initialization data may be a filename for the object's persistent state, for example. This example does not require any initialization data to construct the remote object.

The constructor simply calls a constructor in the superclass (Activatable) to export the object on an anoymous port.

Finally, the class implements the remote interface's single method, remoteMethod to return the object passed as an argument.

Implement the client

The Client program looks up a remote object's stub (one that implements the remote interface MyRemoteInterface) in the registry on the host supplied as the optional first argument, and then invokes the stub's remoteMethod method. When this client invokes a remote method on the stub acquired from the registry, the remote object will activate if not already active.

The source for the program is as follows:

package examples.activation; 

import java.rmi.*;
import java.rmi.registry.*;

public class Client {

    public static void main(String args[])  throws Exception {

  
        String hostname = "localhost";
        if (args.length < 1) {
            System.err.println(
                "usage: java [options] examples.activation.Client <hostname>");
            System.exit(1);
        } else {
            hostname = args[0];
        }

        if (System.getSecurityManager() == null) {
            System.setSecurityManager(new SecurityManager());
        }

        String name = System.getProperty("examples.activation.name");
        Registry registry = LocateRegistry.getRegistry(hostname);
        MyRemoteInterface stub =
            (MyRemoteInterface) registry.lookup(name);
        System.err.println("Obtained stub from the registry.");
                
        System.err.println("Invoking remote method...");
        String result = (String) stub.remoteMethod("hello there!");
        System.err.println("Returned from remote call.");
        System.err.println("Result: " + result);
    }
}

This program should be run as follows:

java -cp clientDir                               \
     -Djava.security.policy=client.policy        \
     -Dexamples.activation.client.codebase=clientCodebase   \
     -Dexamples.activation.name=name             \
     examples.activation.Client [host]

where:

clientDir is the root directory for the client program (its class path),
client.policy is the security policy file for this program,
clientCodebase is the location (URL) for this program's class (used in granting permissions to this program in the client.policy file)
name is the name for the object's stub in the registry
host is the server's host

Note: rmid must be running on its default port, and rmiregistry must be running on its default port (both on the remote host) prior to running this program.

The following is an example client.policy file that grants the appropriate permissions for the activation examples:

grant codeBase "${examples.activation.client.codebase}" {

    // permissions to read system properties required by the client
    permission java.util.PropertyPermission "examples.activation.name","read";

    // permission to connect to the registry, activation system, and remote host
    permission java.net.SocketPermission "*:1024-","connect";
};

The codebase to which permissions are granted is a file URL specifying the location of the client's classes. This file URL is the value of the examples.activation.client.codebase system property, defined when the client program is run. The client needs two permissions:

java.util.PropertyPermission - to read the system property examples.activation.name that specifies the name for the stub in the registry
java.net.SocketPermission - to connect to the registry, activation system, and remote object's host

Compile the source files

The source files for this example can be compiled as follows:

javac -d implDir MyRemoteInterface.java ExtendsActivatable.java 
javac -d clientDir MyRemoteInterface.java Client.java

where implDir is the destination directory to put the implementation's class files the class files in, and clientDir is the destination directory to put the client's class files in.

Run the `Setup` program

Once your implementation phase is complete, you need to register information about the activatable object so a client can use it. The Setup program, described by the tutorial Using Activation: the Setup Program, registers an activation descriptor for an activatable object with rmid, and then binds the remote object's stub in an rmiregistry so that clients can look it up.

To run the Setup program for this example, see the section Start rmid, rmiregistry, and the Setup program in the setup program tutorial, which describes how to start rmid, rmiregistry, and the Setup program itself.

After you run rmid and rmiregistry as instructed in the Setup tutorial, you will need to run the Setup program to register an activation descriptor for an activatable object that implements the class examples.activation.ExtendsActivatable. The following command line runs the Setup program, supplying an appropriate file URL for each codebase used:

java -cp setupDir:implDir                       \
     -Djava.security.policy=setup.policy                      \
     -Djava.rmi.server.codebase=file:/implDir/                \
     -Dexamples.activation.setup.codebase=file:/setupDir/     \
     -Dexamples.activation.impl.codebase=file:/impDir/        \
     -Dexamples.activation.name=examples.activation.MyRemoteInterface       \
     -Dexamples.activation.policy=group.policy                \
     examples.activation.Setup examples.activation.ExtendsActivatable

where:

setupDir is the root directory for the Setup program's class
implDir is the root directory for the implementation's classes
setup.policy is the security policy file for the Setup program
group.policy is the security policy file for the activation group

Note that the examples.activation.file system property does not need to be specified, because the ExtendsActivatable implementation class does not use it. Also note that each file URL above has the required trailing slash. Examples of group and setup policy files, suitable for this tutorial, are described in the setup tutorial, and are also listed below:

The output from the Setup program should look like this:

Activation group descriptor registered.
Activation descriptor registered.
Stub bound in registry.

Run the client

Once you have successfully registered an activation descriptor for an ExtendsActivatable implementation, you can run the client program, which, during its first execution, will cause the activatable object to activate.

The following command line illustrates how to run the client program, specifying a file URL for the client codebase:

java -cp clientDir                                              \
     -Djava.security.policy=client.policy                       \
     -Dexamples.activation.client.codebase=file:/clientDir/     \
     -Dexamples.activation.name=examples.activation.MyRemoteInterface         \
     examples.activation.Client [host]

where:

clientDir is the root directory for the client program (its class path),
client.policy is the security policy file for this program,
name is the name for the object's stub in the registry
host is the server's host

Notes:

The client codebase must have a trailing slash.
The name must match the name supplied to the Setup program. In this example, we used the name examples.activation.MyRemoteInterface.
rmid and rmiregistry must be running on the server's host. If the server's host is not the local host, the host argument must specify the remote host that they are running on.

The output from the client should look like this:

Obtained stub from the registry.
Invoking remote method...
Returned from remote call.
Result: hello there!

Using Activation: Extending Activatable

Implement the activatable remote object

Implement the client

Compile the source files

Run the Setup program

Run the client

Using Activation: Extending `Activatable`

Run the `Setup` program