CS 5523 Operating Systems
Simple Hello Server
Remote Invocation in Java

This handout describes the basics of remote invocation using Java. Three distinct hosts are involved:

The server host - machine on which the remote server and its registry are running
The client host - machine on which the client is running
The web server host - machine on which the web server that acts as a class repository resides.

The overall architecture is shown here:

In most actual implementations, the server handles many remote objects. In our example, the server will actually be the remote object itself. The remote object object has a single remote method called sayHello that takes no input arguments and returns a String "Hello World!".

The RMI in Java is very tricky to set up, because the paths are not what you might think they should be. We will not use packages in this example. We will also always be in the same directory as the source when we compile.

In this handout, we assume that your web page is in the public_html directory under your home directory. You should make a directory under public_html called moreclasses. This directory should be accessible via the web and will be the repository for the class files that you need. We assume that the web server repository resides on a machine called ringer.cs.utsa.edu.

Starting the Registry:

Change to your home directory.
Make sure that there are no class files in this directory.
Make sure that your $CLASSPATH is empty before you start (setenv CLASSPATH).
Start with the command rmiregistry port. For example to start the registry at port 14345, execute:
```
    rmiregistry 14345 &
```
If you make any changes to your server, you will need to kill and restart the registry.

Creating a Server:

Create a directory $HOME/mysrc/server and change to that directory.

Create the following files in server directory:

HelloImpl.java:

import java.rmi.Naming;
import java.rmi.RemoteException;
import java.rmi.RMISecurityManager;
import java.rmi.server.UnicastRemoteObject;
import java.net.*;

public class HelloImpl extends UnicastRemoteObject implements Hello {

   public HelloImpl() throws RemoteException {
     super();
   }

   public String sayHello() throws RemoteException { 
     return "Hello World!";
   }

   public static void main(String args[]) {
       // Figure out where server is running
       String port = "";
       String server = "localhost";
       try {
          InetAddress myAddr = InetAddress.getLocalHost();
          server = myAddr.getHostName();
        } catch (Exception e) {
          System.err.println("Couldn't determine hostname");
          System.exit(1);
       } 

       if (args.length == 1) {
           server = server + ":" + args[0];
       } else if (args.length != 0 &&  args.length != 1) {
           System.err.println ("Usage: java HelloImpl [rmiregistryport]");
           System.exit(1);
       }

      // Create and install a security manager
       if (System.getSecurityManager() == null) {
         System.setSecurityManager(new RMISecurityManager());
      }

      try {
         HelloImpl obj = new HelloImpl();
         System.out.println("Now binding to registry");
         Naming.rebind("rmi://" + server + "/HelloServer", obj);
         System.out.println("HelloServer bound in registry");
      } catch (Exception e) {
         System.out.println("HelloImpl err: " + e.getMessage());
         e.printStackTrace();
      }
   }   
}

Hello.java:

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface Hello extends Remote {
    String sayHello() throws RemoteException;
}

client.policy:

grant
{  permission java.net.SocketPermission
      "*:1024-65535", "connect,accept";
   permission java.net.SocketPermission
      "*:80", "connect";
};

Compile the server:
```
   javac Hello.java HelloImpl.java
```
Create the stubs using rmic:
```
    rmic HelloImpl
```
Copy all of the class files to your web repository:
```
  cp *.class $HOME/public_html/moreclasses/.
```
Be sure the files in your web repository are accessible. The class files must be world readable and the directories along the path must be world executable.
Start running the server (and hold your breath):
```
   java -Djava.rmi.server.codebase=http://ringer.cs.utsa.edu/~youracct/moreclasses/ -Djava.security.policy=client.policy HelloImpl 14345
```
Be sure to substitute your account name for ~youracct . Here ringer.cs.utsa.edu is the web server that hosts the repository but is not necessarily the machine that you will be running your server on. If you get an exception at this point, it means that you have a problem with paths or executable names.

Setting up a Client:

Create a directory $HOME/mysrc/client and change to that directory.

Create the following programs in that directory:

HelloClient.java

import java.rmi.*;
import java.rmi.Naming;
import java.rmi.RemoteException;

public class HelloClient {
    static String message = "blank";
    static Hello obj = null;

    public static void main(String args[]) {
        String server = "localhost";
        String port = ""; 
        if (args.length == 2) {
            server = args[0] + ":" + args[1]; 
        } else if (args.length != 0 &&  args.length != 2) {
           System.out.println ("Usage: java HelloClient [rmihost port]"); 
           System.exit(1); 
        }

        System.setSecurityManager(new RMISecurityManager());
        try {
           obj = (Hello)Naming.lookup("rmi://" + server + "/HelloServer");
           message = obj.sayHello();
           System.out.println(message);
        } catch (Exception e) {
           System.out.println("HelloClient exception: " + e.getMessage());
           e.printStackTrace();
        }
    }    
}

Hello.java:

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface Hello extends Remote {
    String sayHello() throws RemoteException;
}

client.policy:

grant
{  permission java.net.SocketPermission
      "*:1024-65535", "connect,accept";
   permission java.net.SocketPermission
      "*:80", "connect";
};

Compile the client code in the
```
javac Hello.java HelloClient.java
```
Run the client code:
```
java -Djava.rmi.server.codebase=http://ringer/~youracct/moreclasses/ -Djava.security.policy=client.policy HelloClient pandora 14345 
```
In this example, the server is running on pandora and the registry on pandora is listening on port 14345. The client is getting its code from your web page.

Last Revision: March 29, 5:30 pm

CS 5523 Operating Systems Simple Hello Server Remote Invocation in Java

CS 5523 Operating Systems
Simple Hello Server
Remote Invocation in Java