Chapter 9: Choices for Service Architecture

A client will only be looking for an implementation of an interface. The implementation can be done in many different ways, discussed in this chapter.

One extreme is where the proxy is so fat, that there is nothing left to do on the server side. The role of the server is to register the proxy with service locators, and just to stay alive (renewing leases on the service locators). The service itself runs entirely within the client. A class diagram for the file classifier problem using this method is given in figure 9.1.

The client asks for a FileClassifier. What is uploaded to the service locators, and so what the client gets, is a FileClassifierImpl. The FileClassifierImpl runs entirely within the client, and does not communicate back to its server at all. This can also be done for any service, when the service is purely a software one which does not need any link back to the server. It could be something like a calendar which is independent of location, or a diary which uses files on the client side rather than the server side.

The opposite extreme to this is where all of the processing is done on the server side. The proxy just exists on the client to take calls from the client, invoke the method in the service on the server, and return the result to the client. Java's RMI does this in a fairly transparent way (once all the correct files and additional servers are set up!)

A class diagram for an implementation of the file classifier using this mechanism is shown in figure 9.3.

The class structure for this is much more complex in order to get RMI to work. The interface RemoteFileClassifier has to be defined, and the implementation class has to implement (or call suitable methods from) the UnicastRemoteObject class. The FileClassifierImpl_Stub is generated from FileClassifierImpl by use of the rmic compiler. Implementing the Remote interface allows the methods of the FileClassifierImpl to be called remotely. Inheriting from UnicastRemoteObject allows RMI to export the stub rather than the service, which remains on the server.

Apart from creating the stub class by using rmic, the stub is essentially invisible to the programmer: the server code is written to export the implementation, but the RMI runtime component of Java recognises this and actually exports the stub instead. This can cause a little confusion: the programmer writes code to export an object of one class, but an object of a different one appears in the service locator and in the client.

This structure is useful when the service needs to do no processing on the client side, but does need to do a lot on the server side. For example, a diary that stores all information communally on the server rather than individually on each client. Services that are tightly linked to a piece of hardware on the server are other cases.

If RMI is not used and the proxy and service want to share processing, then both the service and the proxy must be created explicitly on the server side. The proxy is explicitly exported by the server and must implement the interface. But on the server side this requirement does not hold, since the proxy and service are not tightly linked by a class structure any more. The class diagram for the file classifier with this organisation is displayed in figure 9.5.

This doesn't specify how the proxy and the server communicate. They could open up a socket connection, for example and exchange messages using a message structure only they understand. Or they could communicate using a well-known protocol, such as HTTP for example: the proxy could make HTTP requests, and the service could act as an HTTP server handling these requests and returning documents. A version of the file classifier using sockets to communicate is given in the section ``Non-RMI Proxy for FileClassifier''.

This model is good for bringing ``legacy'' client-server applications into the Jini world. Client-server applications often communicate using a specialised protocol between the client and server. Copies of the client have to be distributed out to all machines. If there is a bug in the client, they all have to be updated which is often impossible. Worse, if there is a change to the procotol, then the server must be rebuilt to handle old and new versions while attempts are made to update all the clients. This is a tremendous problem with Web browsers for example, with varying degrees of support for HTML 3.2 and HTML 4.0 features, let alone new protocol extensions such as stylesheets and XML. CGI scripts that attempt to deliver the ``right'' version of documents to various browsers are clumsy but necessary hacks.

What can be done instead is to distribute a ``shell'' client, that just contacts the server and uploads a proxy. The proxy is the real ``heart'' of the client, whereas the service is the server part of the original client-server system. When changes occur, the service and its proxy can be updated together, and there is no need to make changes to the ``shell' out on all the various machines.

The last variation is to have a service, an explicit proxy and an RMI proxy. Both of the proxies are exported: the explicit proxy has to be exported by registering it with lookup services, while the RMI proxy is exported by the RMI runtime mechanisms. The RMI proxy can be used for RPC-like communication mechanism between the explicit proxy and the service. This is just like the last case, but instead of requiring the proxy and service to implement their own communication protocol, it uses RMI instead. The proxy and service can be of any relative size, just like in the last case. What this does is to simplify the task of the programmer.

In the later section ``RMI and non-RMI Proxies for FileClassifier'' there is a non-RMI proxy FileClassifierProxy implementing the FileClassifier interface. This communicates with an object which implements the ExtendedFileClassifier interface. There is an object on the server of type ExtendedFileClassifierImpl and an RMI proxy for this on the client side of type ExtendedFileClassifierImpl_Stub The class diagram is shown in figure 9.7.

An RMI proxy can be used when all of the work done by the service is done on the server side. Then the server exports a thin proxy that simply channels method calls from the client across the network to the ``real'' service in the server, and returns the result back to the client. The programming for this is relatively simple. The service has to do two major things in its class structure:

1. Implement Remote. This is because methods will be called on the service from the proxy, and these will be remote calls on the service
2. Inherit from UnicastRemoteObject (or Activatable). This means that its constructor will create and export a proxy or stub object, without the programmer having to do anything more. (An alternative to inheritance is for the object to call the UnicastRemoteObject.exportObject() method.)

The client is not concerned about the implementation of the service at all, and doesn't change. The FileClassifier interface doesn't change either, since this is fixed and used by any client and any service implementation. We have already declared its methods to throw RemoteException so a proxy is able to call its methods remotely. And the MIMEType doesn't change since we have already declared it to implement Serializable - it is passed back across the network from the service to its proxy

The implementation will need to implement both the FileClassifier and the Remote interface. It is convenient to define another interface RemoteFileClassifier just to do this. This interface will be used fairly frequently in the rest of this book.

package rmi;

import common.FileClassifier;
import java.rmi.Remote;

/**
 * RemoteFileClassifier.java
 */

public interface RemoteFileClassifier extends FileClassifier, Remote {
        
} // RemoteFileClasssifier

The service itself will look like

package rmi;

import java.rmi.server.UnicastRemoteObject;
import common.MIMEType;
import common.FileClassifier;

/**
 * FileClassifierImpl.java
 */

public class FileClassifierImpl extends UnicastRemoteObject
                                implements RemoteFileClassifier {
    
    public MIMEType getMIMEType(String fileName) 
	throws java.rmi.RemoteException {
	System.out.println("Called with " + fileName);
        if (fileName.endsWith(".gif")) {
            return new MIMEType("image", "gif");
        } else if (fileName.endsWith(".jpeg")) {
            return new MIMEType("image", "jpeg");
        } else if (fileName.endsWith(".mpg")) {
            return new MIMEType("video", "mpeg");
        } else if (fileName.endsWith(".txt")) {
            return new MIMEType("text", "plain");
        } else if (fileName.endsWith(".html")) {
            return new MIMEType("text", "html");
        } else
            // fill in lots of other types,
            // but eventually give up and
            return new MIMEType(null, null);
    }


    public FileClassifierImpl()  throws java.rmi.RemoteException {
	// empty constructor required by RMI
    }
    
} // FileClassifierImpl

The server appears to change very little from one that exports a complete service. It exports a service object by register(), but at that point the RMI runtime intervenes and substitutes an RMI stub object. The other major change is that the server no longer needs to explicitly stay alive. While the RMI system keeps a reference to the RMI stub object, it keeps the JVM alive that contains the stub object. This means that the daemon threads which are looking after the discovery process will continue to run, and in turn, since they have a reference to the server as listener it will continue to exist. A server which creates and manages the RMI service is

package rmi;

import rmi.FileClassifierImpl;
import rmi.RemoteFileClassifier;

import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import net.jini.core.lease.Lease;
// import com.sun.jini.lease.LeaseRenewalManager;
// import com.sun.jini.lease.LeaseListener;
// import com.sun.jini.lease.LeaseRenewalEvent;
import net.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseListener;
import net.jini.lease.LeaseRenewalEvent;
import java.rmi.RMISecurityManager;

/**
 * FileClassifierServer.java
 */

public class FileClassifierServerRMI implements DiscoveryListener, LeaseListener {

    protected FileClassifierImpl impl;
    protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();
    
    public static void main(String argv[]) {
	new FileClassifierServerRMI();

        // no need to keep server alive, RMI will do that
    }

    public FileClassifierServerRMI() {
	try {
	    impl = new FileClassifierImpl();
	} catch(Exception e) {
            System.err.println("New impl: " + e.toString());
            System.exit(1);
	}

	// install suitable security manager
	System.setSecurityManager(new RMISecurityManager());

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);
    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();
	RemoteFileClassifier service;

        for (int n = 0; n < registrars.length; n++) {
            ServiceRegistrar registrar = registrars[n];

	    // export the proxy service
	    ServiceItem item = new ServiceItem(null,
					       impl,
					       null);
	    ServiceRegistration reg = null;
	    try {
		reg = registrar.register(item, Lease.FOREVER);
	    } catch(java.rmi.RemoteException e) {
		System.err.print("Register exception: ");
		e.printStackTrace();
		// System.exit(2);
		continue;
	    }
	    try {
		System.out.println("service registered at " +
				   registrar.getLocator().getHost());
	    } catch(Exception e) {
	    }
	    leaseManager.renewUntil(reg.getLease(), Lease.FOREVER, this);
	}
    }

    public void discarded(DiscoveryEvent evt) {

    }

    public void notify(LeaseRenewalEvent evt) {
	System.out.println("Lease expired " + evt.toString());
    }
        
} // FileClassifierServerRMI

We have the classes

1. common.MIMEType
2. common.FileClassifier
3. rmi.RemoteFileClassifier
4. rmi.FileClassifierImpl
5. rmi.FileClassifierImpl_Stub
6. rmi.FileClassifierServer
7. client.TestFileClassifier

1. The server machine for FileClassifierServer
2. The HTTP server, which may be on a different machine
3. The machine for the lookup service
4. The machine running the client TestFileClassifier

The server running FileClassifierServer needs to know the following classes and interfaces

1. The common.FileClassifier interface
2. The rmi.RemoteFileClassifier interface
3. The class common.MIMEType
4. The class rmi.FileClassifierServer
5. The class rmi.FileClassifierImpl

The lookup service does not need to know any of these classes. It just deals with them in the form of a java.rmi.MarshalledObject

The client needs to know

1. The common.FileClassifier interface
2. The class common.MIMEType

In addition, the HTTP server needs to be able to load and store classes. It needs to be able to access

1. The rmi.FileClassifierImpl_Stub interface
2. The rmi.RemoteFileClassifier interface
3. The common.FileClassifier interface
4. The class common.MIMEType

What does the phrase ``available'' mean in the last paragraph? The HTTP server will look for files based on the java.rmi.server.codebase property of the application server. The value of this property is a URL. Often, URLs can be file or http references. But for this case, the URL will be used by clients running anywhere, so it cannot be a file reference specific to a particular machine. For the same reason, it cannot be just localhost - unless you are running every part of a Jini federation on a single computer!

If java.rmi.server.codebase is an http reference, then the above class files must be accessible from that reference. For example, suppose the property is set to

java.rmi.server.codebase=http://myWebHost/classes

/home/webdocs/classes/rmi/FileClassifierImpl_Stub.class
/home/webdocs/classes/rmi/RemoteFileClassifier.class
/home/webdocs/classes/common/FileClassifier.class
/home/webdocs/classes/common/MIMEType.class

Again we have a server and a client to run. Calling the client is unchanged from the situation discussed in the previous chapter using the server in the complete package, since the client is independant of any server implementation

java -Djava.security.policy=policy.all client.TestFileClassifier

The server is more complex, because the RMI runtime is manipulating RMI stubs, and these have additional requirements. Firstly, RMI stubs must be generated during compilation. Secondly, security rights must be set since an RMISecurityManager is used.

Although the FileClassifierImpl is created explicitly by the server, it is not this class file that is moved around. It continues to exist on the server machine. Rather, a stub file is moved around, and will run on the client machine. This stub is responsible for sending the method requests back to the implementation class on the server. This stub has to be generated from the implementation class by the stub compiler rmic:

rmic -v1.2 -d /home/webdocs/classes rmi.FileClassifierImpl

The value of java.rmi.server.codebase must specify the protocol used by the HTTP server to find the class files. This could be the file protocol or the http protocol. If the class files are stored on my Web server's pages under classes/rmi/FileClassifierImpl_Stub.class. the codebase would be specified as

java.rmi.server.codebase=http://myWebHost/classes/

The server also sets a security manager. This is a restrictive one, so it needs to be told to allow access. This can be done by setting the property java.security.policy to point to a security policy file such as policy.all.

Combining all these points leads to startups such as

java -Djava.rmi.server.codebase=http://myWebHost/classes/ \
     -Djava.security.policy=policy.all \
     rmi.FileClassifierServer

Many client-server programs communicate by message passing, often using a TCP socket. The two sides need to have an agreed protocol, that is they must have a standard set of message formats, and know what messages to receive and what replies to send at any time. Jini can be used in this sort of case by providing a wrapper around the client and server, and making them available as a Jini service. The original client then becomes a proxy agent for the server, and is distributed to Jini clients for execution. The original server runs within the Jini server and performs the real work of the service, just as in the thin proxy model. What differs is the class structure and how the components communicate.

The proxy and the service do not need to belong to the same class, or even share common superclasses. Unlike the RMI case, the proxy is not derived from the service so does not have a shared class structure. Both are written independently, using their own appropriate class hierarchies. The proxy still has to implement the FileClassifier interface, since that is what the client is asking for and the proxy is delivering.

If RMI is not used, then any other distributed communication mechanism can be employed. Typically client-server systems will use something like reliable TCP ports, but this is not the only choice. It is the one used in this example, though. So the service listens on an agreed port, the client connects to this port, and they exchange messages.

The message format adopted for this problem is really simple:

1. The proxy sends a message giving the file extension that it wants classified. This can be sent as a newline terminated string
2. The service will either succeed or fail in the classification. If it fails, it sends a single line of the string "null" followed by a newline. If it succeeds, it sends two lines, the first being the content type, the second the subtype.

The proxy object will be exported completely to a Jini client such as TestFileClassifier. When this client calls the getMIMEType() method, the proxy opens up a connection on an agreed TCP port to the service and exchanges messages on this port. It then returns a suitable result. The code is

package socket;

import common.FileClassifier;
import common.MIMEType;
import java.net.Socket;

import java.io.Serializable;
import java.io.IOException;
import java.rmi.Naming;

import java.io.*;

/**
 * FileClassifierProxy
 */

public class FileClassifierProxy implements FileClassifier, Serializable {

    static public final int PORT = 2981;
    protected String host;

    public FileClassifierProxy(String host) {
	this.host = host;
    }

    public MIMEType getMIMEType(String fileName) 
	throws java.rmi.RemoteException {
	// open a connection to the service on port XXX
	int dotIndex = fileName.lastIndexOf('.');
	if (dotIndex == -1 || dotIndex + 1 == fileName.length()) {
	    // can't find suitable index
	    return null;
	}
	String fileExtension = fileName.substring(dotIndex + 1);

	// open a client socket connection
	Socket socket = null;
	try {
	     socket = new Socket(host, PORT);
	} catch(Exception e) {
	    return null;
	}

	String type = null;
	String subType = null;

	/* 
	 * protocol:
	 * Write: file extension
	 * Read: "null" + '\n' 
         *       type + '\n' + subtype + '\n'
	 */
	try {
	    InputStreamReader inputReader = 
		new InputStreamReader(socket.getInputStream());
	    BufferedReader reader = new BufferedReader(inputReader);
	    OutputStreamWriter outputWriter = 
		new OutputStreamWriter(socket.getOutputStream());
	    BufferedWriter writer = new BufferedWriter(outputWriter);

	    writer.write(fileExtension);
	    writer.newLine();
	    writer.flush();

	    type = reader.readLine();
	    if (type.equals("null")) {
		return null;
	    }
	    subType = reader.readLine();
	} catch(IOException e) {
	    return null;
	}
	// and finally
	return new MIMEType(type, subType);
    }
} // FileClassifierProxy

On the server side will be running the service. This will run in its own thread (inheriting from Thread) and will listen for connections. When one is received, it will create a new Connection object also in its own thread to handle the message exchange. (This creation of another thread is probably overkill here where the entire message exchange is very short, but is better practice for more complex situations.)

/**
 * FileServerImpl.java
 */

package socket;

import java.net.*;
import java.io.*;

public class FileServerImpl extends Thread {
    
    protected ServerSocket listenSocket;

    public FileServerImpl() {
	try {
	    listenSocket = new ServerSocket(FileClassifierProxy.PORT);
	} catch(IOException e) {
	    e.printStackTrace();
	}
    }

    public void run() {
	try {
	    while(true) {
		Socket clientSocket = listenSocket.accept();
		new Connection(clientSocket).start();
	    }
	} catch(Exception e) {
	    e.printStackTrace();
	}
    }
} // FileServerImpl

class Connection extends Thread {

    protected Socket client;

    public Connection(Socket clientSocket) {
	client = clientSocket;
    }

    public void run() {
	String contentType = null;
	String subType = null;

	try {
	    InputStreamReader inputReader = 
		new InputStreamReader(client.getInputStream());
	    BufferedReader reader = new BufferedReader(inputReader);
	    OutputStreamWriter outputWriter = 
		new OutputStreamWriter(client.getOutputStream());
	    BufferedWriter writer = new BufferedWriter(outputWriter);

	    String fileExtension = reader.readLine();

	    if (fileExtension.equals("gif")) {
		contentType = "image";
		subType = "gif";
	    } else if (fileExtension.equals("txt")) {
		contentType = "text";
		subType = "plain";
	    } // etc

	    if (contentType == null) {
		writer.write("null");
	    } else {
		writer.write(contentType);
		writer.newLine();
		writer.write(subType);
	    }
	    writer.newLine();
	    writer.close();
	} catch(IOException e) {
	    e.printStackTrace();
	}
    }
}

The Jini server must start a FileServerImpl to listen for later connections. Then it can register a proxy object FileClassifierProxy with each lookup service which will send it on to interested clients. The proxy object must know where the service is listening in order to attempt a connection to it, and this is given by first making a query for the local host and then passing the hostname to the proxy in its constructor.

package socket;

import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import net.jini.core.lease.Lease;
// import com.sun.jini.lease.LeaseRenewalManager;  // Jini 1.0
// import com.sun.jini.lease.LeaseListener;        // Jini 1.0
// import com.sun.jini.lease.LeaseRenewalEvent;    // Jini 1.0
import net.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseListener;
import net.jini.lease.LeaseRenewalEvent;
import java.rmi.RMISecurityManager;
import java.net.*;

/**
 * FileClassifierServer.java
 */

public class FileClassifierServer implements DiscoveryListener, LeaseListener {

    protected FileClassifierProxy proxy;
    protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();
    
    public static void main(String argv[]) {
	new FileClassifierServer();
	try {
	    Thread.sleep(1000000L);
	} catch(Exception e) {
	}
    }

    public FileClassifierServer() {
	try {
	    new FileServerImpl().start();
	} catch(Exception e) {
            System.err.println("New impl: " + e.toString());
            System.exit(1);
	}

	// set RMI scurity manager
	System.setSecurityManager(new RMISecurityManager());

	// proxy primed with address
	String host = null;
	try {
	    host = InetAddress.getLocalHost().getHostName();
	} catch(UnknownHostException e) {
	    e.printStackTrace();
	    System.exit(1);
	}

	proxy = new FileClassifierProxy(host);
	// now continue as before
	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);
    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();

        for (int n = 0; n < registrars.length; n++) {
System.out.println("found registrars");
            ServiceRegistrar registrar = registrars[n];

	    // export the proxy service
	    ServiceItem item = new ServiceItem(null,
					       proxy, 
					       null);
	    ServiceRegistration reg = null;
	    try {
		reg = registrar.register(item, Lease.FOREVER);
	    } catch(java.rmi.RemoteException e) {
		System.err.print("Register exception: ");
		e.printStackTrace();
		// System.exit(2);
		continue;
	    }
	    try {
		System.out.println("service registered at " +
				   registrar.getLocator().getHost());
	    } catch(Exception e) {
	    }
	    leaseManager.renewUntil(reg.getLease(), Lease.FOREVER, this);
	}
    }

    public void discarded(DiscoveryEvent evt) {

    }

    public void notify(LeaseRenewalEvent evt) {
	System.out.println("Lease expired " + evt.toString());
    }
        
} // FileClassifierServer

We have the classes

1. common.MIMEType
2. common.FileClassifier
3. socket.FileClassifierProxy
4. socket.FileServerImpl
5. socket.FileClassifierServer
6. client.TestFileClassifier

1. The server machine for FileClassifierServer
2. The HTTP server, which may be on a different machine
3. The machine for the lookup service
4. The machine running the client TestFileClassifier

The server running FileClassifierServer needs to know the following classes and interfaces

1. The common.FileClassifier interface
2. The class common.MIMEType
3. The class socket.FileClassifierServer
4. The class socket.FileClassifierProxy
5. The class socket.FileServerImpl

The lookup service does not need to know any of these classes. It just deals with them in the form of a java.rmi.MarshalledObject

The client needs to know

1. The common.FileClassifier interface
2. The class common.MIMEType

In addition, the HTTP server needs to be able to load and store classes. It needs to be able to access

1. The socket.FileClassifierProxy interface
2. The common.FileClassifier interface
3. The class common.MIMEType

Again we have a server and a client to run. Calling the client is unchanged as it does not care which server implementation is used

java -Djava.security.policy=policy.all client.TestFileClassifier

The value of java.rmi.server.codebase must specify the protocol used by the HTTP server to find the class files. This could be the file protocol or the http protocol. If the class files are stored on my Web server's pages under classes/socket/FileClassifierProxy.class. the codebase would be specified as

java.rmi.server.codebase=http://myWebHost/classes/

The server also sets a security manager. This is a restrictive one, so it needs to be told to allow access. This can be done by setting the property java.security.policy to point to a security policy file such as policy.all.

Combining all these points leads to startups such as

java -Djava.rmi.server.codebase=http://myWebHost/classes/ \
     -Djava.security.policy=policy.all \
     FileClassifierServer

An alternative that is often used for client-server systems instead of message passing is remote procedure calls (RPC). This involves a client that does some local processing and makes some RPC calls to the server. We can also bring this into the Jini world by using a proxy that does some processing on the client side, and makes use of an RMI proxy/stub when it needs to make calls back to the service.

Some file types are more common than others: gifs, doc files, html files, etc abound. But there are many more types, ranging from less common ones such as FrameMaker MIF files through to downright obscure ones such as PDP11 overlay files. An implementation of a file classifier might place the common types in a proxy object which makes them quickly available to clients, and the less common ones back on the server, accessible through a (slower) RMI call.

The proxy object will implement FileClassifier in order that clients can find it. The implementation will handle some file types locally, but others it will pass on to another object, that implements the ExtendedFileClassifier interface. The ExtendedFileClassifier has one method, getExtraMIMEType(). The proxy is told about this other object at constructor time. This class is

/**
 * FileClassifierProxy.java
 */

package extended;

import common.FileClassifier;
import common.ExtendedFileClassifier;
import common.MIMEType;
import java.rmi.RemoteException;

public class FileClassifierProxy implements FileClassifier, java.io.Serializable {
    
    /**
     * The service object that knows lots more MIME types
     */
    protected ExtendedFileClassifier extension;

    public FileClassifierProxy(ExtendedFileClassifier ext) {
	this.extension = ext;
    }

    public MIMEType getMIMEType(String fileName) 
	throws RemoteException {
            if (fileName.endsWith(".gif")) {
            return new MIMEType("image", "gif");
        } else if (fileName.endsWith(".jpeg")) {
            return new MIMEType("image", "jpeg");
        } else if (fileName.endsWith(".mpg")) {
            return new MIMEType("video", "mpeg");
        } else if (fileName.endsWith(".txt")) {
            return new MIMEType("text", "plain");
        } else if (fileName.endsWith(".html")) {
            return new MIMEType("text", "html");
        } else {
	    // we don't know it, pass it on to the service
	    return extension.getExtraMIMEType(fileName);
	}
    }
} // FileClassifierProxy

The ExtendedFileClassifier interface will be the top level interface for the service and an RMI proxy for the service. It will be publically available for all clients to use. An immediate sub-interface, RemoteExtendedFileClassifier will add the Remote interface

/**
 * ExtendedFileClassifier.java
 */

package common;

import java.io.Serializable;
import java.rmi.RemoteException;

public interface ExtendedFileClassifier extends Serializable {
    
    public MIMEType getExtraMIMEType(String fileName)
	throws RemoteException;
    
} // ExtendedFileClassifier

/**
 * RemoteExtendedFileClassifier.java
 */

package extended;

import java.rmi.Remote;

interface RemoteExtendedFileClassifier extends common.ExtendedFileClassifier, Remote {

} // RemoteExtendedFileClassifier

The implementation of the ExtendedFileClassifier interface is done by an ExtendedFileClassifierImpl object. This will also need to extend UnicastRemoteObject in order that the RMI runtime can create an RMI proxy for it. Since this object may handle requests from many proxies, an alternative implementation of searching for MIME types that is more efficient for repeated searches is used

/**
 * ExtendedFileClassifierImpl.java
 */

package extended;

import java.rmi.server.UnicastRemoteObject;
import common.MIMEType;
import java.util.HashMap;
import java.util.Map;

public class ExtendedFileClassifierImpl extends UnicastRemoteObject
    implements RemoteExtendedFileClassifier {

    /**
     * Map of String extensions to MIME types
     */
    protected Map map = new HashMap();
    
    public ExtendedFileClassifierImpl() throws java.rmi.RemoteException {
	/* This object will handle all classification attempts
	 * that fail in client-side classifiers. It will be around
	 * a long time, and may be called frequently. So it is worth
	 * optimising the implementation by using a hash map
	 */
	map.put("rtf", new MIMEType("application", "rtf"));
	map.put("dvi", new MIMEType("application", "x-dvi"));
	map.put("png", new MIMEType("image", "png"));
	// etc
    }
    
    public MIMEType getExtraMIMEType(String fileName) 
	throws java.rmi.RemoteException {
	MIMEType type;
	String fileExtension;
	int dotIndex = fileName.lastIndexOf('.');

	if (dotIndex == -1 || dotIndex + 1 == fileName.length()) {
	    // can't find suitable suffix
	    return null;
	}

	fileExtension= fileName.substring(dotIndex + 1);
	type = (MIMEType) map.get(fileExtension);
	return type; 
    }
} // ExtendedFileClassifierImpl

The final piece in this jigsaw is the server that creates the service (and implicitly the RMI proxy for the service) and also the proxy primed with knowledge of the service.

package extended;

import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import net.jini.core.lease.Lease;
// import com.sun.jini.lease.LeaseRenewalManager;
// import com.sun.jini.lease.LeaseListener;
// import com.sun.jini.lease.LeaseRenewalEvent;
import net.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseListener;
import net.jini.lease.LeaseRenewalEvent;
import java.rmi.RMISecurityManager;

/**
 * FileClassifierServer.java
 */

public class FileClassifierServer implements DiscoveryListener, LeaseListener {

    protected FileClassifierProxy proxy;
    protected ExtendedFileClassifierImpl impl;
    protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();
    
    public static void main(String argv[]) {
	new FileClassifierServer();
	// RMI keeps this alive
    }

    public FileClassifierServer() {
	try {
	    impl = new ExtendedFileClassifierImpl();
	} catch(Exception e) {
            System.err.println("New impl: " + e.toString());
            System.exit(1);
	}

	// set RMI scurity manager
	System.setSecurityManager(new RMISecurityManager());

	// proxy primed with impl
	proxy = new FileClassifierProxy(impl);

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);
    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();

        for (int n = 0; n < registrars.length; n++) {
System.out.println("found registrars");
            ServiceRegistrar registrar = registrars[n];

	    // export the proxy service
	    ServiceItem item = new ServiceItem(null,
					       proxy, 
					       null);
	    ServiceRegistration reg = null;
	    try {
		reg = registrar.register(item, Lease.FOREVER);
	    } catch(java.rmi.RemoteException e) {
		System.err.print("Register exception: ");
		e.printStackTrace();
		continue;
	    }
	    try {
		System.out.println("service registered at " +
				   registrar.getLocator().getHost());
	    } catch(Exception e) {
	    }
	    leaseManager.renewUntil(reg.getLease(), Lease.FOREVER, this);
	}
    }

    public void discarded(DiscoveryEvent evt) {

    }

    public void notify(LeaseRenewalEvent evt) {
	System.out.println("Lease expired " + evt.toString());
    }
        
} // FileClassifierServer

We have the classes

1. common.MIMEType
2. common.FileClassifier
3. common.ExtendedFileClassifier
4. extended.FileClassifierProxy
5. extended.RemoteExtendedFileClassifier
6. extended.ExtendedFileServerImpl
7. extended.FileClassifierServer
8. client.TestFileClassifier

The server running FileClassifierServer needs to know the following classes and interfaces

1. The common.FileClassifier interface
2. The class common.MIMEType
3. common.ExtendedFileClassifier
4. The class extended.FileClassifierServer
5. The class extended.FileClassifierProxy
6. extended.RemoteExtendedFileClassifier
7. The class extended.ExtendedFileServerImpl

The lookup service does not need to know any of these classes. It just deals with them in the form of a java.rmi.MarshalledObject

The client needs to know

1. The common.FileClassifier interface
2. The class common.MIMEType

In addition, the HTTP server needs to be able to load and store classes. It needs to be able to access

1. The extended.FileClassifierProxy interface
2. extended.RemoteExtendedFileClassifier
3. extended.ExtendedFileServerImpl_Stub
4. The common.FileClassifier interface
5. The class common.MIMEType

This section is probably not worth keeping any more, and hasn't been brought up to date for this version. I shall either discard it or rework for a later version.

The option 3 service and the last section exported an RMI proxy object directly. They were able to do this because the Jini protocol allows objects to be moved around to their destination, where the ``lookup'' to find the object has been done by the Jini discovery protocol. In more ``traditional' uses of RMI, a client wishing to use an RMI service has to first locate it using RMI lookup mechanisms. This is possible with Jini also, but it is a little more complex and probably not so useful.

An RMI exportable object can advertise itself in various ways, and Jini registration is only one of these. RMI comes with a simple lookup service, the Naming service. This uses methods such as Naming.bind() and Naming.rebind() to register a name for the RMI object in an RMI registry, and Naming.lookup() to retrieve it from there.

An RMI registry is started by the command rmiregistry. This registry must be run on the same machine as a service trying to bind to it. So if there are many services running on many machines, then there will be many RMI registries running, one per machine. A server will create an RMI object and then register it with its local RMI naming service. It will also still need to export a service registration to the Jini lookup service. So this method will export two services: an RMI service to an RMI naming registry, and a Jini service to a Jini lookup service.

On the other side, a client wishing to locate an object using this method will still need to find the Jini service, and will follow the methods of option 3. When it gets its Jini proxy, it will then need to locate its RMI proxy to make calls on it. But this time, it has not carried the RMI proxy along, and this must be found from the RMI registry. In order to use this, it must know the internet address of the registry's machine. By the time it has reached the client it is too late to have this information, so the Jini proxy must be ``primed'' with the address of the registry and the name of the RMI service while it is still in the service and before it is exported to the Jini lookup service.

The implementation of the service remains unaltered as the rmi.FileClassifierImpl. The server must bind this implementation into the rmi naming registry using bind() or rebind(). It must also prime the proxy with the address of this rmi registry, and the name the implementation is bound to.

package rmi;

import java.rmi.Naming;
import java.net.InetAddress;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import net.jini.core.lease.Lease;
// import com.sun.jini.lease.LeaseRenewalManager;
// import com.sun.jini.lease.LeaseListener;
// import com.sun.jini.lease.LeaseRenewalEvent;
import net.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseListener;
import net.jini.lease.LeaseRenewalEvent;
import java.rmi.RMISecurityManager;

/**
 * FileClassifierServerNaming.java
 */

public class FileClassifierServerNaming implements DiscoveryListener, LeaseListener {

    // This is just a name for the service
    // It can be anything, just needs to shared by both
    // ends of the Naming service
    static final String serviceName = "FileClassifier";

    protected rmi.FileClassifierImpl impl;
    protected FileClassifierNamingProxy proxy;
    protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();
    
    public static void main(String argv[]) {
	new FileClassifierServerNaming();

        // no need to keep server alive, RMI will do that
    }

    public FileClassifierServerNaming() {
	try {
	    impl = new rmi.FileClassifierImpl();
	} catch(Exception e) {
            System.err.println("New impl: " + e.toString());
            System.exit(1);
	}

	// register this with RMI registry
	System.setSecurityManager(new RMISecurityManager());
	try {
	    Naming.rebind("rmi://localhost/" + serviceName, impl);
	} catch(java.net.MalformedURLException e) {
            System.err.println("Binding: " + e.toString());
            System.exit(1);
	} catch(java.rmi.RemoteException e) {
            System.err.println("Binding: " + e.toString());
            System.exit(1);
        }

	System.out.println("bound");
	// find where we are running
	String address = null;
	try {
	    address = InetAddress.getLocalHost().getHostName();
	} catch(java.net.UnknownHostException e) {
            System.err.println("Address: " + e.toString());
            System.exit(1);
        }

	String registeredName = "//" + address + "/" + serviceName;

	// make a proxy that knows the service address
	proxy = new FileClassifierNamingProxy(registeredName);

	// now continue as before
	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);
    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();

        for (int n = 0; n < registrars.length; n++) {
            ServiceRegistrar registrar = registrars[n];

	    // export the proxy service
	    ServiceItem item = new ServiceItem(null,
					       proxy, 
					       null);
	    ServiceRegistration reg = null;
	    try {
		reg = registrar.register(item, Lease.FOREVER);
	    } catch(java.rmi.RemoteException e) {
		System.err.print("Register exception: ");
		e.printStackTrace();
		// System.exit(2);
		continue;
	    }
	    try {
		System.out.println("service registered at " +
				   registrar.getLocator().getHost());
	    } catch(Exception e) {
	    }
	    leaseManager.renewUntil(reg.getLease(), Lease.FOREVER, this);
	}
    }

    public void discarded(DiscoveryEvent evt) {

    }

    public void notify(LeaseRenewalEvent evt) {
	System.out.println("Lease expired " + evt.toString());
    }
        
} // FileClassifierServerNaming

The proxy which uses the naming service can still continue to use the ``option 3'' implementation. The way it finds this is different, as it uses the naming service lookup() method.

package rmi;

import common.FileClassifier;
import common.MIMEType;

import rmi.RemoteFileClassifier;

import java.io.Serializable;
import java.io.IOException;
import java.rmi.Naming;

/**
 * FileClassifierNamingProxy
 */

public class FileClassifierNamingProxy implements FileClassifier, Serializable {
    
    protected String serviceLocation;
    transient RemoteFileClassifier server = null;

    public FileClassifierNamingProxy(String serviceLocation) {
	this.serviceLocation = serviceLocation;
    }

    private void readObject(java.io.ObjectInputStream stream) 
	throws java.io.IOException, ClassNotFoundException {
	stream.defaultReadObject();

	try {
	    Object obj = Naming.lookup(serviceLocation);
	    server = (RemoteFileClassifier) obj;
	} catch(Exception e) {
	    System.err.println(e.toString());
	    System.exit(1);
	}
    }

    public MIMEType getMIMEType(String fileName) 
	throws java.rmi.RemoteException {
	return server.getMIMEType(fileName);
    }
} // FileClassifierNamingProxy

In all the examples so far, a proxy has been created in a server and registered with a lookup service. Meanwhile, a service backend has usually been left behind in the server to handle calls from the proxy. However, there may be no need for the service to exist on the server, and the proxy could make use of other services elsewhere. This may be subject to security restrictions imposed by the client, which may disallow connections to some hosts.

In this section we shall give an example of using a non-Jini service on another host. Recently an Australian, Pat Farmer, attempted to set a world record for jogging the longest distance. While he was running around, I became involved in a small project to broadcast his heart beart live to the Web: a heart monitor was attached to him, which talked on an RS232 link to a mobile phone he was carrying. This did a data transfer to a program running at www.micromed.com.au located at the Gold Coast, which forwarded the data to a machine at the Distributed Systems Technology Centre (DSTC) in Brisbane. This ran a Web server delivering an applet, and the applet talked back to a server on this DSTC machine which sent out the data to each applet as it was received from the heart monitor.

Now that the experiment is over, the broadcast data is sitting as a file at http://www.micromed.com.au/patfarmer/v2/patfhr.ecg , and it can be viewed on the applet from http://www.micromed.com.au/patfarmer/v2/heart.html. We can make it into a Jini service as follows

1. Have a service which we can locate using the service type (``display a heart monitor trace'') and information about it such as whose heart trace it is showing
2. The service can connect to an http address encoded into the service by its constructor (or other means), read from this and display the contents read assuming it is heart cardiograph data
3. The information about whose trace it is can be given by a Name entry

The heart monitor service can be regarded in a number of ways:

1. It is a full-blown service uploaded to the client, that just happens to use an external data source supplied from an HTTP server
2. It is a ``fat'' proxy to the HTTP service, which acts as a client to this service by displaying the data

The Heart interface only has one method, and that is to show() the heart trace in some manner.

/**
 * Heart.java
 */

package heart;

public interface Heart extends java.io.Serializable {
    
    public void show();
} // Heart

The HeartServer is similar to the ``option 2'' method, where it exports a complete service. This service, of type HeartImpl is primed with a URL of where the heart data is stored. This data will later be delivered by an HTTP server. This implementation is enough to locate the service. However, rather than just anyone's heart data, a client may wish to search for a particular persons data. This can be done by adding a Name entry as additional information to the service.

package heart;

import java.rmi.RMISecurityManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import net.jini.core.lease.Lease;
// import com.sun.jini.lease.LeaseRenewalManager;
// import com.sun.jini.lease.LeaseListener;
// import com.sun.jini.lease.LeaseRenewalEvent;
import net.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseListener;
import net.jini.lease.LeaseRenewalEvent;
import net.jini.core.entry.Entry;
import net.jini.lookup.entry.Name;

/**
 * HeartServer.java
 */

public class HeartServer implements DiscoveryListener, 
                                             LeaseListener {
    
    protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();

    public static void main(String argv[]) {
	new HeartServer();

        // keep server running forever to 
	// - allow time for locator discovery and
	// - keep re-registering the lease
	Object keepAlive = new Object();
	synchronized(keepAlive) {
	    try {
		keepAlive.wait();
	    } catch(InterruptedException e) {
		// do nothing
	    }
	}
    }

    public HeartServer() {

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);
    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();

        for (int n = 0; n < registrars.length; n++) {
            ServiceRegistrar registrar = registrars[n];

	    ServiceItem item = new ServiceItem(null,
					       // new HeartImpl("file:/home/jan/projects/jini/doc/heart/TECG3.ecg"),
					       new HeartImpl("http://www.micromed.com.au/patfarmer/v2/patfhr.ecg"), 
					       new Entry[] {new Name("Pat Farmer")});
	    ServiceRegistration reg = null;
	    try {
		reg = registrar.register(item, Lease.FOREVER);
	    } catch(java.rmi.RemoteException e) {
		System.err.println("Register exception: " + e.toString());
		continue;
	    }
	    System.out.println("service registered");

	    // set lease renewal in place
	    leaseManager.renewUntil(reg.getLease(), Lease.FOREVER, this);
	}
    }

    public void discarded(DiscoveryEvent evt) {

    }

    public void notify(LeaseRenewalEvent evt) {
	System.out.println("Lease expired " + evt.toString());
    }   
    
} // HeartServer

The client searches for a service implementing the Heart interface, with the additional requirement that it be for a particular person. Once it has this, it just calls the method show() to display this in some manner.

package heart;

import heart.Heart;

import java.rmi.RMISecurityManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceTemplate;

import net.jini.core.entry.Entry;
import net.jini.lookup.entry.Name;

/**
 * HeartClient.java
 */

public class HeartClient implements DiscoveryListener {

    public static void main(String argv[]) {
	new HeartClient();

        // stay around long enough to receive replies
        try {
            Thread.currentThread().sleep(1000000L);
        } catch(java.lang.InterruptedException e) {
            // do nothing
        }
    }

    public HeartClient() {
	System.setSecurityManager(new RMISecurityManager());

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);

    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();
	Class [] classes = new Class[] {Heart.class};
	Entry [] entries = new Entry[] {new Name("Pat Farmer")};
	Heart heart = null;
	ServiceTemplate template = new ServiceTemplate(null, classes, 
						       entries);
 
        for (int n = 0; n < registrars.length; n++) {
	    System.out.println("Service found");
            ServiceRegistrar registrar = registrars[n];
	    try {
		heart = (Heart) registrar.lookup(template);
	    } catch(java.rmi.RemoteException e) {
		e.printStackTrace();
		continue;
	    }
	    if (heart == null) {
		System.out.println("Heart null");
		continue;
	    }
	    heart.show();
	    System.exit(0);
	}
    }

    public void discarded(DiscoveryEvent evt) {
	// empty
    }
} // HeartClient

The HeartImpl class opens a connection to an HTTP server and requests delivery of a file. For heart data it needs to display this at a reasonable rate, so it reads, draws, sleeps, in a loop. It acts as a fat client to the HTTP server, displaying the data in a suitable format (in this case, it uses HTTP as a transport mechanism for data delivery). As a ``client-aware'' service it customises this delivery to the characteristics of the client platform, just occupying a ``reasonable'' amount of screen space and using local colors and fonts.

/**
 * HeartImpl.java
 */

package heart;

import java.io.*;
import java.net.*;
import java.awt.*;

public class HeartImpl implements Heart {
    
    protected String url;

    /*
     * If we want to run it standalone we can use this
     */
    public static void main(String argv[]) {

	HeartImpl impl =
	    new HeartImpl("file:/home/jan/projects/jini/doc/heart/TECG3.ecg");
	impl.show();
    }

    public HeartImpl(String u) {
	url = u;
    }

    double[] points = null;
    Painter painter = null;

    String heartRate = "--";


    public void setHeartRate(int rate) {
	if (rate > 20 && rate <= 250) {
	    heartRate = "Heart Rate: " + rate;
	} else {
	    heartRate = "Heart Rate: --";
	}
	// ? ask for repaint? 
    }

    public void quit(Exception e, String s) {
	System.err.println(s);
	e.printStackTrace();
	System.exit(1);
    }

    public void show() {
	int SAMPLE_SIZE = 300 / Toolkit.getDefaultToolkit().
	                                            getScreenResolution();
	Dimension size = Toolkit.getDefaultToolkit().
	                         getScreenSize();
	int width = (int) size.getWidth();
	// capture points in an array, for redrawing in app if needed
	points = new double[width * SAMPLE_SIZE];
	for (int n = 0; n < width; n++) {
	    points[n] = -1;
	}


	URL dataUrl = null;
	InputStream in = null;

	try {
	    dataUrl = new URL(url);
	    in = dataUrl.openStream();
	} catch (Exception ex) {
	    quit(ex, "connecting to ECG server");
	    return;
	}

	Frame frame = new Frame("Heart monitor");
	frame.setSize((int) size.getWidth()/2, (int) size.getHeight()/2);
	try {
	    painter = new Painter(this, frame, in);
	    painter.start();
	} catch (Exception ex) {
	    quit(ex, "fetching data from ECG server");
	    return;
	}
	frame.setVisible(true);
    }
} // HeartImpl

class Painter extends Thread {

    static final int DEFAULT_SLEEP_TIME = 25; // milliseconds
    static final int CLEAR_AHEAD = 15;
    static final int MAX = 255;
    static final int MIN = 0;
    final int READ_SIZE = 10;

    protected HeartImpl app;
    protected Frame frame;

    protected InputStream in;
    protected final int RESOLUTION = Toolkit.getDefaultToolkit().
	                                            getScreenResolution();
    protected final int UNITS_PER_INCH = 125;
    protected final int SAMPLE_SIZE = 300 / RESOLUTION;
    protected int sleepTime = DEFAULT_SLEEP_TIME;
    
    public Painter(HeartImpl app, Frame frame, InputStream in) throws Exception {
	this.app = app;
	this.frame = frame;
	this.in = in;
    }

    public void run() {

	while (!frame.isVisible()) {
	   try {
	       Thread.sleep(1000);
	   } catch(Exception e) {
	       // ignore
	   }
	}

	int height = frame.getSize().height;
	int width = frame.getSize().width;
	int x = 1; // start at 1 rather than 0 to avoid drawing initial line
	           // from -128 .. 127
	int magnitude;
	int nread;
	int max = MIN; // top bound of magnitude
	int min = MAX;  // bottom bound of magnitude
	int oldMax = MAX + 1;
	byte[] data = new byte[READ_SIZE];
	Graphics g = frame.getGraphics();
	g.setColor(Color.red);
	try {
	    Font f = new Font("Serif", Font.BOLD, 20);
	    g.setFont(f);
	} catch (Exception ex) {
	    // ....
	}



	try {
            boolean expectHR = false;   // true ==> next byte is heartrate

	    while ((nread = in.read(data)) != -1) {
		for (int n = 0; n < nread; n++) {
		    int thisByte = data[n] & 0xFF;
		    if (expectHR) {
			expectHR = false;
			app.setHeartRate(thisByte);
			continue;
		    } else if (thisByte == 255) {
			expectHR = true;
			continue;
		    }
		    
		    // we are reading bytes, from -127..128
		    // conver to unsigned
		    magnitude = thisByte;

		    // then convert to correct scale
		    magnitude -= 128;
		    // scale and convert to window coord from the top downwards
		    int y = ((128 - magnitude) * RESOLUTION) / UNITS_PER_INCH;
		    app.points[x] = y;

		    // draw only on multiples of sample size
		    if (x % SAMPLE_SIZE == 0) {
			// delay to draw at a reasonable rate
			Thread.sleep(sleepTime);

			int x0 = x / SAMPLE_SIZE;
			g.clearRect(x0, 0, CLEAR_AHEAD, height);
			if (oldMax != MAX + 1) {
			    g.drawLine(x0-1, oldMax, x0, min); 
			}
			g.drawLine(x0, min, x0, max);
			oldMax = max;
			min = 1000;
			max = -1000;
			if (app.heartRate != null) {
			    g.setColor(Color.black);
			    g.clearRect(0, 180, 200, 100);
			    g.drawString(app.heartRate, 0, 220);
			    g.setColor(Color.red);
			}
		    } else {
			if (y > max) max = y;
			if (y < min) min = y;
		    }
		    if (++x >= width * SAMPLE_SIZE) {
			x = 0;
		    }
		}
	    }
	} catch(Exception ex) {
	    if (! (ex instanceof SocketException)) {
		System.out.println("Applet quit -- got " + ex);
	    }
	} finally {
	    try {
		if (in != null) {
		    in.close();
		    in = null;
		}
	    } catch (Exception ex) {
		// hide it
	    }
	}
    }
}

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