Remote methods

Remote interfaces

1. The remote interface must be public (it cannot have “package access,” that is, it cannot be “friendly”). Otherwise, a client will get an error when attempting to load a remote object that implements the remote interface.
2. The remote interface must extend the interface java.rmi.Remote.
3. Each method in the remote interface must declare java.rmi.RemoteException in its throws clause in addition to any application-specific exceptions.
4. A remote object passed as an argument or return value (either directly or embedded within a local object) must be declared as the remote interface, not the implementation class.

//: PerfectTimeI.java
// The PerfectTime remote interface
package c15.ptime;
import java.rmi.*;

interface PerfectTimeI extends Remote {
  long getPerfectTime() throws RemoteException;
} ///:~ 

Implementing the remote interface

//: PerfectTime.java
// The implementation of the PerfectTime 
// remote object
package c15.ptime;
import java.rmi.*;
import java.rmi.server.*;
import java.rmi.registry.*;
import java.net.*;

public class PerfectTime 
    extends UnicastRemoteObject
    implements PerfectTimeI {
  // Implementation of the interface:
  public long getPerfectTime() 
      throws RemoteException {
    return System.currentTimeMillis();
  }
  // Must implement constructor to throw
  // RemoteException:
  public PerfectTime() throws RemoteException {
    // super(); // Called automatically
  }
  // Registration for RMI serving:
  public static void main(String[] args) {
    System.setSecurityManager(
      new RMISecurityManager());
    try {
      PerfectTime pt = new PerfectTime();
      Naming.bind(
        "//colossus:2005/PerfectTime", pt);
      System.out.println("Ready to do time");
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~ 

1. Create and install a security manager that supports RMI. The only one available for RMI as part of the Java distribution is RMISecurityManager.
2. Create one or more instances of a remote object. Here, you can see the creation of the PerfectTime object.
3. Register at least one of the remote objects with the RMI remote object registry for bootstrapping purposes. One remote object can have methods that produce handles to other remote objects. This allows you to set it up so the client must go to the registry only once, to get the first remote object.

Setting up the registry

1. localhost does not work with RMI. Thus, to experiment with RMI on a single machine, you must provide the name of the machine. To find out the name of your machine under 32-bit Windows, go to the control panel and select “Network.” Select the “Identification” tab, and you’ll see your computer name. In my case, I called my computer “Colossus” (for all the hard disks I’ve had to put on to hold all the different development systems). It appears that capitalization is ignored.
2. RMI will not work unless your computer has an active TCP/IP connection, even if all your components are just talking to each other on the local machine. This means that you must connect to your Internet service provider before trying to run the program or you’ll get some obscure exception messages.

Creating stubs and skeletons

PerfectTime_Stub.class
PerfectTime_Skel.class

Using the remote object

//: DisplayPerfectTime.java
// Uses remote object PerfectTime
package c15.ptime;
import java.rmi.*;
import java.rmi.registry.*;

public class DisplayPerfectTime {
  public static void main(String[] args) {
    System.setSecurityManager(
      new RMISecurityManager());
    try {
      PerfectTimeI t = 
        (PerfectTimeI)Naming.lookup(
          "//colossus:2005/PerfectTime");
      for(int i = 0; i < 10; i++)
        System.out.println("Perfect time = " +
          t.getPerfectTime());
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
} ///:~ 

Alternatives to RMI