CS696: Doc 7, RMI Parameters

	CS 696 Emerging Technologies: Distributed Objects

Spring Semester, 1998
RMI Parameters
To Lecture Notes Index
© 1998, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 21-Apr-98

Contents of Doc 7, RMI Parameters

References

Java Remote Method Invocation Specification, section 2.6

Doc 7, RMI Parameters Slide # 2

Parameters

Pass-by-Value (copy) verse Pass-by-Reference

public class Foo
   {
   public void method( Object parameter )
      {
      result = parameter.someMethod( echo );
      }
   }

Pass-by-Value (Copy)

A copy of the parameter is made

The method receives and acts on the copy

Pass-by-Reference

The method receives a reference to the parameter

The method uses the reference to access the parameter, which is held by the caller

Doc 7, RMI Parameters Slide # 3

Parameters and Remote Objects

public class Foo
      extends UnicastRemoteObject
      implements FooRemote
   {
   public void remoteMethod( Object parameter )
      {
      result = parameter.someMethod( echo );
      }
   }

If Foo is a server (remote) object on machine A and

If a client object on machine B calls Foo's remoteMethod

How to handle passing the parameter to the method?

Doc 7, RMI Parameters Slide # 4

Option 1 Pass-by-value
Send a copy of the parameter to machine A

This requires sending all of the parameters fields, and the fields of the fields, etc. to machine A

Doc 7, RMI Parameters Slide # 5

Option 2 Pass-by-reference
Send a reference of the parameter to machine A

This means that "parameter.someMethod( echo )" results in sending a request to machine B over the network to perform "someMethod" on the parameter

If someMethod( echo ) access "echo" then that results in a request over the network to perform a method call on echo

Doc 7, RMI Parameters Slide # 6

Parameters and RMI

In RMI objects that implement the Remote interface are passed by reference

All other parameters are passed by copy

A Simple Example

Client passes a vector to the server,
Server adds a string to the vector
Server returns the vector

package whitney.rmi.examples.basic;

import java.util.Vector;

public interface VectorParameter extends java.rmi.Remote 
   {
   public Vector modify( Vector input ) 
      throws java.rmi.RemoteException;
   }

Doc 7, RMI Parameters Slide # 7

The Server

package whitney.rmi.examples.basic;

import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
import java.rmi.registry.Registry;
import java.util.Vector;
import sdsu.rmi.registry.Registrar;
import sdsu.util.ProgramProperties;
import java.io.IOException;

public class VectorParameterServer 
      extends UnicastRemoteObject
      implements VectorParameter
   {
   public VectorParameterServer() throws RemoteException {}

   public Vector modify( Vector input ) throws RemoteException 
      {
      input.addElement( "Hi Mom");
      return  input;
      }


   public static String RMI_NAME = "cs696/VectorParameter";
   static String serverClass =
       "whitney.rmi.examples.basic.VectorParameterServer";
   
   public static void main(String args[]) throws 
      RemoteException, IOException
      {
      int port = getPort( args );

      String nameList = 
         Registrar.verboseRebind( port, 
                              RMI_NAME, serverClass);
      System.out.println( nameList );
      }
      
   private static int getPort( String args[] ) throws IOException
      {
      ProgramProperties flags = new ProgramProperties( args );
      int defaultPort = Registry.REGISTRY_PORT;
      int port = flags.getInt( "p", defaultPort );

      return port; 
      }
   }
Doc 7, RMI Parameters Slide # 8

The Client

package whitney.rmi.examples.basic;

import java.rmi.*;
import java.rmi.registry.Registry;
import java.net.MalformedURLException;
import java.io.IOException;
import java.util.Vector;
import sdsu.util.ProgramProperties;

public class VectorParameterClient 
   {
   public static void main(String args[]) 
      {
      try 
         {
         String serverLabel = 
            VectorParameterServer.RMI_NAME;
         String server = getServerURL( args, serverLabel );
         
         VectorParameter remote = 
            (VectorParameter) Naming.lookup( server );
            
         Vector data = new Vector();
         Vector remoteData = remote.modify( data );
         System.out.println( "remote " + remoteData);
         System.out.println( "local " + data);
         
         if ( data == remoteData )
            System.out.println( "equal" );
         else
            System.out.println( "not equal" );
         
         } 
      catch ( Exception error) 
         {  error.printStackTrace(); }
      }

   private static String getServerURL( String args[], 
                                    String serverLabel ) 
                                    throws IOException
      {
      String hostKey = "h";
      String portKey = "p";
      
      ProgramProperties flags = new ProgramProperties( args );
      
      String host = null;
      if ( flags.containsKey( hostKey ) )
         host = flags.getString( hostKey );
      else
         {
         System.out.println( "Missing flag " + hostKey );
         System.exit( 0 );
         }
      
      int defaultPort = Registry.REGISTRY_PORT;
      int port = flags.getInt( portKey, defaultPort );
      
      return "rmi://" + host + ":" + port + "/" + serverLabel;
      }
   }
Doc 7, RMI Parameters Slide # 9

The Output of the Client
remote [Hi Mom]
local []
not equal

Doc 7, RMI Parameters Slide # 10

Trouble in RMI Land - Passing Foo

Foo

package whitney.rmi.examples.basic;

public class Foo
   {
   String name = "Roger";
   }

Remote Interface

package whitney.rmi.examples.basic;

public interface ParameterTrouble extends java.rmi.Remote 
   {
   public void tryMe( Foo input ) throws 
      java.rmi.RemoteException;
   }

Server

package whitney.rmi.examples.basic;

import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
import java.rmi.registry.Registry;
import sdsu.rmi.registry.Registrar;
import sdsu.util.ProgramProperties;
import java.io.IOException;

public class ParameterTroubleServer 
      extends UnicastRemoteObject
      implements ParameterTrouble
   {
   public ParameterTroubleServer() throws RemoteException {}

   public void tryMe( Foo input ) throws RemoteException 
      {}

   public static String RMI_NAME = "cs696/ParameterTrouble";
   static String serverClass =
       "whitney.rmi.examples.basic.ParameterTroubleServer";
   
   public static void main(String args[]) throws RemoteException, IOException
      {
      int port = getPort( args );

      String nameList = Registrar.verboseRebind( port, RMI_NAME, serverClass);
      System.out.println( nameList );
      }
      
   private static int getPort( String args[] ) throws IOException
      {
      ProgramProperties flags = new ProgramProperties( args );
      int defaultPort = Registry.REGISTRY_PORT;
      int port = flags.getInt( "p", defaultPort );

      return port; 
      }
   }
Doc 7, RMI Parameters Slide # 11

Passing Foo Client

package whitney.rmi.examples.basic;

import java.rmi.*;
import java.rmi.registry.Registry;
import java.net.MalformedURLException;
import java.io.IOException;
import sdsu.util.ProgramProperties;

public class ParameterTroubleClient 
   {
   public static void main(String args[]) 
      {
      try 
         {
         String serverLabel = 
               ParameterTroubleServer.RMI_NAME;
         String server = getServerURL( args, serverLabel );
         
         ParameterTrouble remote = 
            (ParameterTrouble) Naming.lookup( server );
            
         remote.tryMe( new Foo() );
            } 
      catch ( Exception error) 
         {
         error.printStackTrace();
         }
      }
   
   private static String getServerURL( String args[], String serverLabel ) throws IOException
      {
      String hostKey = "h";
      String portKey = "p";
      
      ProgramProperties flags = new ProgramProperties( args );
      
      String host = null;
      if ( flags.containsKey( hostKey ) )
         host = flags.getString( hostKey );
      else
         {
         System.out.println( "Missing flag " + hostKey );
         System.exit( 0 );
         }
      
      int defaultPort = Registry.REGISTRY_PORT;
      int port = flags.getInt( portKey, defaultPort );
      
      return "rmi://" + host + ":" + port + "/" + serverLabel;
      }
   }

Output

java.rmi.MarshalException: Error marshaling arguments; nested exception is:
        java.io.NotSerializableException: whitney.rmi.examples.basic.Foo
        at java.lang.Throwable.<init>(Compiled Code)
        at java.lang.Exception.<init>(Compiled Code)
        at java.io.IOException.<init>(Compiled Code)
        at java.rmi.RemoteException.<init>(Compiled Code)
        at java.rmi.MarshalException.<init>(Compiled Code)
        at whitney.rmi.examples.basic.ParameterTroubleServer_Stub.tryMe(Compiled
 Code)
        at whitney.rmi.examples.basic.ParameterTroubleClient.main(Compiled Code)

Doc 7, RMI Parameters Slide # 12

Parameter Passing in Remote Method Invocation

An argument to, or a return value from, a remote object can be any Java type that is serializable

An argument to, or a return value, a remote object can also be a remote object, but the declared type must be a Remote interface

Note, the remote object may return an implementation of a remote object, but only stub is received

Doc 7, RMI Parameters Slide # 13

Remote Example The Interface

public interface RemoteParameter extends java.rmi.Remote 
   {
   public RemoteParameter itWorks( ) throws
       java.rmi.RemoteException;
   }

The Client (parts missing)

public class RemoteParameterClient 
   {
   public static void main(String args[]) 
      {
      try 
         {
         String serverLabel = 
            RemoteParameterServer.RMI_NAME;
         String server = getServerURL( args, serverLabel );
         
         RemoteParameter remote = 
            (RemoteParameter) Naming.lookup( server );
            
         RemoteParameter   aReference = remote.itWorks( );
         System.out.println( "It Works");
         } catch ( Exception error) {}
      }

The Server (parts missing)

public class RemoteParameterServer 
      extends UnicastRemoteObject
      implements RemoteParameter
   {
   public RemoteParameter itWorks( ) throws RemoteException 
      {
      return this;
      }

visitors since 05-Feb-98