Creating an Activatable Object

Prior to the release of the JDK1.2, an instance of a UnicastRemoteObject could be accessed from a server program that (1) created an instance of the remote object, and (2) ran all the time. Now with the introduction of the class java.rmi.activation.Activatable and the RMI daemon, rmid, programs can be written to register information about remote object implementations that should be created and execute "on demand", rather than running all the time. The RMI daemon, rmid, provides a JVM from which other JVM instances may be spawned.

Note:  For the remainder of this tutorial, the terms "activatable object implementation", "activatable object," and "implementation" may be used interchangeably to refer to the class, examples.activation.ActivatableImplementation, which implements a remote interface and is activatable.

This tutorial is organized as follows:

• The steps to create the implementation class
• The steps to create the setup class
• The steps to compile and run the example code

The files needed for this tutorial are:

• Client.java, the class which will invoke a method on an activatable object
• MyRemoteInterface.java , the interface that extends java.rmi.Remote, implemented by:
• ActivatableImplementation.java , the class which will be activated
• Setup.java, the class which registers information about the activatable class with the registry and the RMI daemon

You may notice that while the client code is included, it is not discussed in a step-by-step manner, like the implementation and setup classes. The reason for this omission, is that the client code for activatable objects is no different than the RMI client code for accessing non-activatable remote objects. Activation is strictly a server-side implementation decision.

For all of the source code used in the activation tutorials, you may choose from these formats:

• activation.zip
• activation.tar
• activation.tar.Z

Creating the implementation class

For this example, the implementation class will be examples.activation.ActivatableImplementation. There are four steps to create an implementation class:

1. Make the appropriate imports in the implementation class
2. Extend your class from java.rmi.activation.Activatable
3. Declare a two-argument constructor in the implementation class
4. Implement the remote interface method(s)

Step 1:
Make the appropriate imports in the implementation class

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

Step 2:
Extend your class from java.rmi.activation.Activatable

public class ActivatableImplementation extends Activatable
    implements examples.activation.MyRemoteInterface {

Step 3:
Declare a two-argument constructor in the implementation class

public ActivatableImplementation(ActivationID id, MarshalledObject data)
    throws RemoteException {
    // Register the object with the activation system
    // then export it on an anonymous port
    super(id, 0);
}

Step 4:
Implement the remote interface method(s)

public Object callMeRemotely() throws RemoteException {
    return "Success";
}

Creating the "setup" class

The job of the "setup" class is to create all the information necessary for the activatable class, without necessarily creating an instance of the remote object.  For this example the setup class will be examples.activation.Setup.

The setup class passes the information about the activatable class to rmid, registers a remote reference (an instance of the activatable class's stub class) and an identifier (name) with the rmiregistry, and then the setup class may exit. There are seven steps to create a setup class:

1. Make the appropriate imports
2. Install a SecurityManager
3. Create an ActivationGroup instance
4. Create an ActivationDesc instance
5. Declare an instance of your remote interface and register with rmid
6. Bind the stub to a name in the rmiregistry
7. Quit the setup application

Step 1:
Make the appropriate imports in the setup class

import java.rmi.*;
import java.rmi.activation.*
import java.util.Properties;

Step 2:
Install a SecurityManager

System.setSecurityManager(new RMISecurityManager());

Step 3:
Create an ActivationGroup instance

Note: In this example, for simplicity, we will use a policy file that gives global permission to anyone from anywhere. Do not use this policy file in a production environment. For more information on how to properly open up permissions using a java.security.policy file, please refer to to the following documents:

http://java.sun.com/products/jdk/1.2/docs/guide/security/PolicyFiles.html http://java.sun.com/products/jdk/1.2/docs/guide/security/permissions.html

In the setup application,the job of the activation group descriptor is to provide all the information that rmid will require to contact the appropriate existing VM or spawn a new VM for the activatable object.

Note: In order to run this code on your system, you'll need to change the policy file location to be the absolute path to where you've installed the example policy file that came with the source code.

// After JDK1.2Beta4, ActivationGroups are no longer created
// implicitly as a side-effect of creating or registering the
// first Activatable object
//
// Because of the 1.2 security model, a security policy should 
// be specified for the ActivationGroup VM. The first argument
// to the Properties put method, inherited from Hashtable, is 
// the key and the second is the value 
// 
Properties props = new Properties(); 
props.put("java.security.policy", 
   "/home/rmi_tutorial/activation/policy"); 

ActivationGroupDesc.CommandEnvironment ace = null; 
ActivationGroupDesc exampleGroup = new ActivationGroupDesc(props, ace);

// Once the ActivationGroupDesc has been created, register it 
// with the activation system to obtain its ID
//
ActivationGroupID agi = 
   ActivationGroup.getSystem().registerGroup(exampleGroup);

// Now explicitly create the group
//
ActivationGroup.createGroup(agi, exampleGroup, 0);

Step 4:
Create an ActivationDesc instance

The job of the activation descriptor is to provide all the information that rmid will require to create a new instance of the implementation class.

Note: In order to run this code on your system, you'll need to change the file URL location to be the location of the directory on your system, where you've installed the example source code.

// The "location" String specifies a URL from where the class   
// definition will come when this object is requested (activated).
// Don't forget the trailing slash at the end of the URL 
// or your classes won't be found.
//       
String location = "file:/home/rmi_tutorial/activation/";

// Create the rest of the parameters that will be passed to
// the ActivationDesc constructor
//
MarshalledObject data = null;

// The second argument to the ActivationDesc constructor will be used  
// to uniquely identify this class; it's location is relative to the  
// URL-formatted String, location.
//
ActivationDesc desc = new ActivationDesc 
    ("examples.activation.ActivatableImplementation", location, data);

Step 5:
Declare an instance of your remote interface and register the activation descriptor with rmid

MyRemoteInterface mri = (MyRemoteInterface)Activatable.register(desc);
System.out.println("Got the stub for the ActivatableImplementation");

Step 6:
Bind the stub, that was returned by the Activatable.register method, to a name in the rmiregistry

Naming.rebind("ActivatableImplementation", mri);
System.out.println("Exported ActivatableImplementation");

Step 7:
Quit the setup application

System.exit(0);

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Compile and run the code

There are six steps to compile and run the code:

1. Compile the remote interface, implementation, client, and setup classes
2. Run rmic on the implementation class
3. Start the rmiregistry
4. Start the activation daemon, rmid
5. Run the setup program
6. Run the client

Step 1:
Compile the remote interface, implementation, client and setup classes

% javac -d . MyRemoteInterface.java
% javac -d . ActivatableImplementation.java
% javac -d . Client.java
% javac -d . Setup.java

Step 2:
Run rmic on the implementation class

% rmic -d . examples.activation.ActivatableImplementation

Step 3:
Start the rmiregistry

% rmiregistry &

Note:  Before you start the rmiregistry, you must make sure that the shell or window in which you will run the registry, either has no CLASSPATH set or has a CLASSPATH that does not include the path to any classes that you want downloaded to your client, including the stubs for your remote object implementation classes.

If you start the rmiregistry, and it can find your stub classes in its CLASSPATH, it will ignore the server's java.rmi.server.codebase property, and as a result, your client(s) will not be able to download the stub code for your remote object.

Step 4:
Start the activation daemon, rmid

% rmid &

Step 5:
Run the setup program

Run the setup, setting the codebase property to be the location of the implementation stubs. There are four things that need to go on the same command line:

1. The "java" command
2. A property name=value pair that specifies the location of the security policy file
3. A property to specify where the stub code lives (no spaces from the "-D" all the way though the last "/")
4. The fully-qualified package name of the setup program.

There should be one space just after the word "java", one between the two properties, and a third one just before the word "examples" (which is very hard to see when you view this as text, in a browser, or on paper).

% java  -Djava.security.policy=/home/rmi_tutorial/activation/policy   -Djava.rmi.server.codebase=file:/home/rmi_tutorial/activation/  examples.activation.Setup

The codebase property will be resolved to a URL, so it must have the form of "http://aHost/somesource/" or "file:/myDirectory/location/" or, due to the requirements of some operating systems, "file:///myDirectory/location/" (three slashes after the "file:").

Please note that each of these sample URL strings has a trailing "/". The trailing slash is a requirement for the URL set by the java.rmi.server.codebase property, so the implementation can resolve (find) your class definition(s) properly.

If you forget the trailing slash on the property, or if the class files can't be located at the source (they aren't really being made available for download) or if you misspell the property name, you'll get thrown a java.lang.ClassNotFoundException. This exception will be thrown when you try to bind your remote object to the rmiregistry, or when the first client attempts to access that object's stub. If the latter case occurs, you have another problem as well because the rmiregistry was finding the stubs in its CLASSPATH.

The server output should look like this:

      Got the stub for the ActivatableImplementation
      Exported ActivatableImplementation

Step 6:
Run the client

The argument to the examples.activation.Client program is the hostname of the implementation server, in this case, "vector".

% java examples.activation.Client vector

The client output should look like this:

      Got a remote reference to the object that extends Activatable.
      Making remote call to the server
      Returned from remote call
      Result: Success

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