Using Activation: Persistence

This tutorial describes how to implement an activatable remote object that uses persistent state in its implementation. 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 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:

Counter.java - a remote interface for counting
CounterImpl.java - an "activatable" implementation of the remote interface
CounterClient.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 that uses persistent state in its implementation.

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) virtual machine (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

This example defines the remote interface Counter with a single operation, increment. The implementation class CounterImpl persistently stores the result of the operation increment in a file, so that the result of the increment operation will survive across an object activation/deactivation cycle or even survive a machine reboot or crash. To make all this work, a CounterImpl object, when activated, needs the name of the file it will use to read and store counter values. A user can register an activation descriptor that contains this file name (in a MarshalledObject) as the "initialization data". The object's activation group will pass this pre-registered marshalled data (the file name) to the object's constructor when the group constructs the object during activation.

The remote interface examples.activation.Counter is defined as follows:

package examples.activation;

import java.rmi.Remote;
import java.io.IOException;

public interface Counter extends Remote {

    int increment() throws IOException;
}

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

package examples.activation; 

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.rmi.MarshalledObject;
import java.rmi.activation.Activatable;
import java.rmi.activation.ActivationID;

public class CounterImpl implements Counter {
    private RandomAccessFile raf;
    private int count;
    private final Object countLock = new Object();
    
    public CounterImpl(ActivationID id, MarshalledObject data) 
        throws Exception
    {
        if (data != null) {
            String filename = (String) data.get();
            synchronized (countLock) {
                count = openFile(filename);
            }
            System.err.println("count upon activation = " + count);
        }

        Activatable.exportObject(this, id, 0);
    }

    private int openFile(String filename) throws IOException {

        if (filename != null && !filename.equals("")) {
            File file = new File(filename);
            boolean fileExists = file.exists();
            raf = new RandomAccessFile(file, "rws");
            return (fileExists) ? raf.readInt() : writeCount(0);
            
        } else {
            throw new IOException("invalid filename");
        }
    }

    private int writeCount(int value) throws IOException {
        raf.setLength(0);
        raf.writeInt(value);
        return value;
    }
            
    public int increment() throws IOException {
        synchronized (countLock) {
            return writeCount(++count);
        }
    }
}

The class CounterImpl implements the remote interface Counter, but does not extend any class.

The class 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. In this example, the marshalled data is the name of a file containing the object's persistent state which is the latest counter value.

The constructor obtains the file name contained in the MarshalledObject which was passed as the second parameter. Next, the constructor calls the local method openFile to open the file and return the current counter value. If the file exists, the openFile method reads the value last saved to the file; otherwise, it creates a new file and initializes the count to zero. The constructor then calls the static method Activatable.exportObject, passing the implementation itself (this), the activation ID, and the port number 0, indicating that the object should be exported on an anonymous TCP port. While this implementation does not use the activation ID passed as a parameter to the constructor, another implementation may wish to save the activation ID for future use, in order to deactivate the object, for example.

Finally, the class implements the remote interface's single method, increment to increment the count, save the count to the file, and return the incremented count. Note that there is a flaw in the implementation of the writeCount method. If the machine crashes between the calls to setLength and writeInt, the value of the counter will be lost. Making this implementation more robust to a crash within that small window is an exercise left to the reader.

Implement the client

The CounterClient program looks up a remote object's stub (one that implements the remote interface Counter) in the registry on the host supplied as the optional first argument, and then invokes the stub's increment method and displays the result. 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.registry.LocateRegistry;
import java.rmi.registry.Registry;

public class CounterClient {

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

        String hostname = "localhost";
        if (args.length < 1) {
            System.err.println(
                "usage: java [options] examples.activation.CounterClient [host]");
            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);
        Counter stub = (Counter) registry.lookup(name);
        System.err.println("Obtained stub from the registry.");

        System.err.println("Invoking increment method...");
        int count = stub.increment();
        System.err.println("Returned from increment remote call.");
        System.err.println("count = " + count);
    }
}

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.CounterClient [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 Counter.java CounterImpl.java 
javac -d clientDir Counter.java CounterClient.java

where implDir is the destination directory to put the implementation's 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.CounterImpl. 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.Counter                 \
     -Dexamples.activation.policy=group.policy                \
     -Dexamples.activation.file=file                          \
         examples.activation.Setup examples.activation.CounterImpl

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
file is the name of a file containing the object's persistent state which is registered as the data in the object's activation descriptor

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 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 a CounterImpl 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.Counter                   \
     examples.activation.CounterClient [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.Counter.
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 increment method...
Returned from increment remote call.
count = 1

Using Activation: Persistence

Implement the activatable remote object

Implement the client

Compile the source files

Run the Setup program

Run the client

Run the `Setup` program