Remote Method Invocation

What is a RMI?

Before JDK1.1, the networked applications were sockets-based programming. With RMI, Java objects can easily invoke the methods of remote objects as if they were locally available.

The compute engine server accepts tasks from clients, runs the tasks, and returns any results. The server is comprised of an interface and a class. The interface provides the definition for the methods that can be called from the client. Essentially the interface defines the client's view of the remote object. The class provides the implementation.

How does it work?

Define the remote interfaces (include all functions that can be called remotely):

  package examples.hello; 
  import java.rmi.Remote; 
  import java.rmi.RemoteException; 
  public interface Hello extends Remote { 
    String sayHello() throws RemoteException; 
  }

Provide the implementation for the remote interface.
Create and install a security manager
Create one or more instances of a remote object
Register at least one of the remote objects with the RMI remote object registry (or another naming service such as one that uses JNDI), for bootstrapping purposes

  package examples.hello; 
  import java.rmi.Naming; 
  import java.rmi.RemoteException; 
  import java.rmi.RMISecurityManager; 
  import java.rmi.server.UnicastRemoteObject; 
  public class HelloImpl extends UnicastRemoteObject implements Hello{ 
    public HelloImpl() throws RemoteException { 
      super(); //default and can be omitted
    } 
    public String sayHello () { 
      return "Hello World!"; 
    } 

    public static void main (String args []) { 
      // Create and install a security manager, 
      // if no, only classpath will be found for the request
      //it protects access to system resources from untrusted downloade
      //code running within the virtual machine. 
      //it determines whether downloaded code has access to 
      //the local filesystem or can perform any other privileged ops. 
      if (System.getSecurityManager() == null) { 
        System.setSecurityManager(new RMISecurityManager()); 
      } 

      try { 
        HelloImpl obj = new HelloImpl() ; 
        // Bind this object instance to the name "HelloServer" 
        //callers on any host can look up the remote object by name
        //obtain its reference, invoke remote methods on the object
        Naming.rebind("//myhost/HelloServer", obj);
        //myhost defaults to the local host 
        System.out.println("HelloServer bound in registry"); 
      } catch (Exception e) { 
        System.out.println("HelloImpl err: " + e.getMessage()); 
        e.printStackTrace(); 
      } 
    } 
  } 

Client program (Clients program to remote interfaces, not to the implementation classes of those interfaces.)

package examples.hello; 
import java.applet.Applet; 
import java.awt.Graphics; 
import java.rmi.Naming; 
import java.rmi.RemoteException; 

public class HelloApplet extends Applet { 
  String message = "blank"; 
  // "obj" is the identifier that we"ll use to refer 
  // to the remote object that implements the "Hello" interface 
  Hello obj = null ; 

  public void init() { 
    try { 
      obj = (Hello)Naming.lookup("//" + 
        getCodeBase().getHost() + "/HelloServer"); 
      message = obj.sayHello(); 
    } catch (Exception e) { 
      System.out.println("HelloApplet exception:"+e.getMessage()); 
      e.printStackTrace(); 
    } 
  } 

  public void paint(Graphics g) { 
    g.drawString(message, 25, 50); 
  } 
} 

The client may begin by installing a security manager. This is necessary because RMI could be downloading code to the client. The client constructs a name used to look up a remote object.

To generate a stub for the remote object so that clients can contact the remote object. Before starting the compute engine, you need to start RMI's registry, using the rmiregistry command.

Web Sites

Sun's RMI