RMI Concept

Hi Everybody,
I have a general question.
First try to visualize a scenariio, 3-tier application. Where there is a server and unknown number of clients. Suppose all the clients need to call a method from a object that connects to data source (any Database) In such case how do i design the RMI object, as the dillema is-
same object might be called by different clients and same method might be called as well in that case will there be any problem. If so a working advise will be very helpful.
I gave it a lot of thought and came up with a idea that, for each client the RMI object may intiate a Thread; but the implementation is not worked with yet. It would be great if anybody helped.
Thanks everybody.
Imram

Imram,
I am not sure I completely understand your question, but as I understand it here goes:
If you have a middle-ware object between the clients and the database, which has a method that can be called by multiple clients; RMI will take care of the thread creation automatically for you. Simultaneous client calls will execute concurrently. Your concern needs to be to synchronize any critical sections of the object's functionality.
I hope I understood your question correctly, and I hope that helps.
John
PS I would also recommend the following site as a resource: https://cajo.dev.java.net

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Java.rmi.ServerException:

I am new to RMI concept
i stuck with the below problem
when i execute
E:\MyProjects\StockRMI>java -Djava.security.policy=policy.all StockMarketServerStockMarketServer is as shown below
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
import SimpleStocks.*;
public class StockMarketServer {
public static void main(String[] args) throws Exception {
    if(System.getSecurityManager() == null) {
      System.setSecurityManager( new RMISecurityManager() );
    StockMarketImpl myObject = new StockMarketImpl( "NASDAQ" );
    System.out.println( "RMI StockMarketServer ready..." );
} i got the following error
java.rmi.ServerException: RemoteException occurred in server thread; nested exce
ption is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested excep
tion is:
        java.lang.ClassNotFoundException: SimpleStocks.StockMarketImpl_Stub
RMI StockMarketServer ready...Interface class, Interface implementation class and stub all reside in MyProjects\StockRMI\SimpleStocks directory
where as Server class(StockMarkerServer) is in MyProjects\StockRMI
even i place the stub in MyProjects\StockRMI directory i got the same error.
Can any one please save me out of this problem

hurry!
Thank u First of all
I got it.. finally after 2 days
The error is in CLASSPATH only , i set it
Thanks once again

Is RMI (provided in the java package ) stable??

I'm starting to write an application that use the RMI concept. Because the guy at SUN's training center said that RMI is not very stable, he suggested to use CORBA.
About 1000 java application could use the RMI at the same time.
thanks

Most of the EJB application servers are using an RMI protocol to do the bean lookups and communication. I would have to think RMI is quite stable.

How to RMI Classes and sigar interface

HI all,
Actually i want check CPU usage and i am using sigar api for this..
i made a class using sigar api and now i want to use that class in RMI but i don't know to use that class in RMI concept.
i just want to use that class which shows CPU usage in RMI classes which i will make..
thanks

HI all,
Actually i want check CPU usage and i am using sigar api for this..
i made a class using sigar api and now i want to use that class in RMI but i don't know to use that class in RMI concept.
i just want to use that class which shows CPU usage in RMI classes which i will make..
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RMI beginner

Hi all,
I want to learn RMI.
Can u suggest me how to learn RMI concept.

This link seems a good beginning :
http://java.sun.com/j2se/1.4.2/docs/guide/rmi/getstart.doc.html

Remote system state

i want to know whether the remote system state on or off. any body help me to find remote system on or off state using java.net package. no need to use RMI concepts.

udhaya_Sri wrote:
i want to know whether the remote system state on or off. any body help me to find remote system on or off state using java.net package. no need to use RMI concepts.I doubt it (not a foolproof one anyway), and it would be likely to be system-dependent (and if I was the administrator, I'd probably turn it off).
ping usually works, but again, some systems may have it turned off. Some systems (like Unix) also provide an 'isalive' command, but it's probably based around ping.
Winston

Client & stub

hi...friends
i'm a new learner to rmi concepts. can anybody help me in knowing the client & stub concepts in rmi.

stub is a client side proxy for a remote object ( instance of a class that implements a remote interface ) which is responsible for communicating method invocations on remote objects to the server where the actual remote object implementation resides and are generated with the command -rmic class_name. Client gets reference to the remote object with the help of stub.
More information can be obtained from either Java Tutorial or the Java Documentation.

Why stub and skeleton ?

Hi,
I am new RMI concepts...
i am not able to understand why we need stub and skeleton...
we can can put all the stub codes/operations in our client program itself...
similarly for skeleton...
then why we are having a seperate things like stub and skeletons??
thanks

heloo firend,
stub is gonna be in the client side and skeleton i s gonna be in the server side...
stub is a proxy 4 the remotemethod in the server....

Binding by the server

hi,
i have a question......while i read RMI i saw that server code binds the object . and client looks up the object......but in EJB there is no binding ! ...client looks up. why it is different ? after all EJB usues RMI...does the container takes care of binding the beans in the background ? or i am lacking something ?

It depends on what you mean by automatically. The
binding is defined in the deployment descriptor
written by the developer. The container reads the
descriptor and binds the EJB.
/Kaji guess you mean alternative of the hardcoded bind() method is here.......
<ejb-name>SignOnEJB</ejb-name>
      <home>day05.SignOnHome</home>
      <remote>day05.SignOn</remote>
      <ejb-class>day05.SignOnEJB</ejb-class>you mean these tags are responsible for the alternative of the bind() method. ?
actually i became dissatisfied bcoz of the absense of so called bind() method in RMI.....as EJB usues RMI concept so i thought bind() should have been there .......
well , OK
thanks for the time.

I need to run rmiregistry automatically.. any one help me...

hi
I am new to RMI, i created a simple RMI concept and it is working fine. I need to start rmiregistry by manually. Is there way to run rmiregistry as automatic when i execute the server program..
please clear me..

To start the rmiregistry programmatically:
LocateRegistry.createRegistry(1099);To stop it programmatically:
UnicastRemoteObject.unexportObject(registry, true);However, you can also do it "dirty" (I tried this successfully):
Runtime.getRuntime().exec(rmiRegistry);
// rmiRegistry is e.g. ".../rmireghistry.exe"The benefit of the last method is that "rmiregistry" is its own process which you can monitor (e.g. TaskManager in Windows) and kill seperatly if necessary during debugging.

Question about the concept of RMI reverse calls.

Hello guys.
I'd appreciate if someone could clarify something to me. As far as I understand , RMI reverse call in concept, is the mean thought witch a server can remotely call a method belonging to the client.
I've done that like this: the client called a method from to the server(classic rmi) , method that had the particularity to received as a parameter an instance of a serialized .class belonging to client. Through that received remote object it seems the server can call specific methods belonging to the client .class.
I'm not sure it's the correct way to do reverse calls, but what puzzles me is that shouldn't the server be able to call a client method without first needing to receive that instance of the client's .class and implicitly being dependent on that client initial call (maybe to get that .class in a different way)?

I'm not sure it's the correct way to do reverse callsIt's not. If the object passed to the server is serializable, the server can call methods on it, but they will execute at the server.
The object you pass to the server must be an exported remote object. Then the server callbacks will execute at the client.
but what puzzles me is that shouldn't the server be able to call a client method without first needing to receive that instance of the client's .classNo, why?
and implicitly being dependent on that client initial call (maybe to get that .class in a different way)?When you get the exported remote object part right, the remote method via which the client passes the callback to the server will declare the callback as a remote interface, not as the implementing class at the client.

RMI How can a Client reconnect to a server after connection(any)-error

I have the following problem:
My RMI-Server runs for ever. In a batch-queue I have a procedure which looks periodly wether rmiregistry
and RMI-Server exists. On error both processes are killed and restarted.
My client as a simple example is displaying the server time. If any communication-problem with the
server exists, I need an automatic reconnect to the server. I accept that into the time distanz of the
error the display is frozen. Its unacceptable to restart the client !!.
The following example demonstates my test-example:
Server:
1. Start
2. waits for connecting (factory)
3. answer time-requests
Client:
1. Start
2. create a time class initially 1.jan.1970 00:00:00
3. Start a timer displaying the time class every second
4. start a timer connecting/reconnecting to the server and ask the servers actual time every second
PS. Is the server to stubil programmed, so that a hang can exists?
It would be nice, if anybody could answer me !!
The following sources work correctly without solving the problem of the reconnect:
////////////// Echo.java
package emi.server;
import java.rmi.*;
import java.util.*;
public interface Echo
extends Remote
public Date getTime() throws RemoteException;
////////////// EchoClient.java
package emi.server;
//import emi.utility.basics.*;
public class EchoClient
public static void main(String args[]) throws Exception
EchoClient echoclient1 = new EchoClient();
//Check the argument count
if (args.length != 1)
System.err.println("Usage: EchoClient <server> --> EXIT");
System.exit(0);
// all of time relevant things
Etim acttim = new Etim();
// displaying continous the time
EchoClientDisplay disp = new EchoClientDisplay(acttim);
disp.StartTimer();
// transfering continous the time from the server
EchoClientTransfer trans = new EchoClientTransfer(acttim, args[0]);
trans.StartTimer();
////////////// EchoClientDisplay.java
package emi.server;
import java.awt.event.*;
import javax.swing.*;
// displaying every 750 Milliseconds the value of the time
public class EchoClientDisplay implements ActionListener
private Timer tim;
private Etim tact;
public EchoClientDisplay(Etim tact)
tim = new Timer(750, this);
this.tact = tact;
public void StartTimer()
tim.setRepeats(true);
tim.setInitialDelay(5);
tim.start();
public void actionPerformed(ActionEvent e )
System.out.println(tact.toString());
////////////// EchoClientTransfer.java
package emi.server;
import java.rmi.Naming;
import java.rmi.RMISecurityManager;
import java.awt.event.*;
import javax.swing.Timer;
import java.util.Date;
// transferring the actual time from the server
public class EchoClientTransfer implements ActionListener
private Etim tact;
private String hostname;
private Timer tim;
private boolean init = false;
private Echo echoRef1 = null;
public EchoClientTransfer(Etim tact, String hostname)
this.tact = tact;
this.hostname = hostname;
this.tim = new Timer(500, this);
public void StartTimer()
tim.setRepeats(true);
tim.setInitialDelay(5);
tim.start();
public void actionPerformed(ActionEvent e )
//>>>>>>>>>>> this construction doesnt work correctly, its only good until the first
// network error
try
if( init == false )
// Create and install the security manager
System.setSecurityManager(new RMISecurityManager());
//get the remote factory object from the registry
String url = new String("rmi://"+ hostname +"/EchoFactory");
EchoFactory remoteFactory = (EchoFactory)Naming.lookup(url);
//get references to new EchoImpl instances
echoRef1 = remoteFactory.getEcho("User Meyer");
init = true;
if( init = true )
//make the remote calls
Date d = echoRef1.getTime();
tact.setDate(d);
catch(Exception ee)
System.out.println(ee.toString());
init = false;
//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
////////////// EchoFactory.java
package emi.server;
import java.rmi.RemoteException;
import java.rmi.Remote;
public interface EchoFactory extends Remote
Echo getEcho(String userName) throws RemoteException;
////////////// EchoFactoryImpl.java
package emi.server;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
public class EchoFactoryImpl extends UnicastRemoteObject implements EchoFactory
EchoFactoryImpl() throws RemoteException {};
public Echo getEcho(String userName) throws RemoteException
EchoImpl echoRef = new EchoImpl(userName);
return (Echo)echoRef;
////////////// EchoImpl.java
package emi.server;
import java.rmi.RemoteException;
import java.rmi.server.UnicastRemoteObject;
import java.util.*;
public class EchoImpl extends UnicastRemoteObject implements Echo
private String userName;
public EchoImpl() throws RemoteException
public EchoImpl(String userName) throws RemoteException
this.userName = userName;
public Date getTime()
Etim e = new Etim();
e.setTimeAct();
return e.get();
////////////// EchoServer.java
package emi.server;
import java.rmi.Naming;
import java.rmi.RMISecurityManager;
public class EchoServer
public static void main(String args[]) throws Exception
// Create and install the security manager
System.setSecurityManager(new RMISecurityManager());
// Create the servant instance for registration
EchoFactory factoryRef = new EchoFactoryImpl();
// Bind the object to the rmiregistry
Naming.rebind("EchoFactory", factoryRef);
System.out.println("Echo object ready and bound to the name 'EchoFactory'!");
////////////// Etim.java
package emi.server;
import java.util.*;
import java.text.*;
// this is my central class working up all time problems .. many hundred lines of code
// I think, you must not look at this code ist setting and reading time
// this is only a subset of methods for this example
public class Etim
private Date dat;
private Calendar cal;
public Etim()
cal = Calendar.getInstance(); // Gregorianischer Kalender
dat = new Date(0L); // January 1, 1970, 00:00:00
cal.clear();
* Zeit lesen.
public Date get()
return dat;
// setting the time
public void setDate( Date d )
dat.setTime( d.getTime() );
cal.setTime(dat);
// gets my time-class to the current system-clock
public void setTimeAct()
long millis;
millis = System.currentTimeMillis();
setMilli(millis);
* Zeit setzen.
public void setMilli(long millis)
dat.setTime(millis);
cal.setTime(dat);
// time in german format: day.month.year hour:minute:second:millisecond
public String toString()
return toStringTagMoJahr() + " " + toStringStdMiSek() +
":" + cal.get(Calendar.MILLISECOND);
* Ausgabeformat Tag.Monat.Jahr (z.B. 01.01.2001).
public String toStringTagMoJahr()
SimpleDateFormat s = new SimpleDateFormat("dd.MM.yyyy");
return s.format(dat);
* Ausgabeformat Stunde:Minute:Sekunde (00:00:00 - 23:59:59).
public String toStringStdMiSek()
SimpleDateFormat s = new SimpleDateFormat("HH:mm:ss");
return s.format(dat);

Hello willy51,
Thank you for answering.
I think, your comment of the design is true - its a problem when starting up in a new enviroment and
you have nobody who shows you the right direction at the beginning. Talking personally together only
10 minutes is better than writing a noval.
I thing the following model of a client works better:
concept:
- visualize a personal time class continously evgery second
- if there is a connection to a server set the time-class with the server time
- if you loss connection, try to reconnect
question:
in which situation hangs connectToServer() ?
(whithout the simple errors : no rmiregistry, no rmi-server)
public class EchoClient
private String hostname;
public static void main(String args[]) throws Exception
// my internal TIME-Class
Etim acttim = new Etim();
// Create and install the security manager
System.setSecurityManager(new RMISecurityManager());
// remote call
Echo echoRef1 = null;
String url = new String("rmi://"+ servername:port +"/EchoFactory");
// displaying continous the time, technic = swing.timer
EchoClientDisplay disp = new EchoClientDisplay(acttim);
disp.StartTimer();
// transfering continous the time from the server, technic = swing.timer
// The state of transfer from server = offline
EchoClientTransfer trans = new EchoClientTransfer(acttim);
trans.StartTimer();
// Connect to server
connectToServer(url, echoRef1, trans);
// wait for ever, if connection failed, try every 5 seconds a reconnect to server
while(true)
// test, if connection failed. The connections fails if the Object EchoClientTransfer
// get a error, when it asks the server for the time ( remote call )
if(trans.getStatus() == false ) // test, if connection failed
connectToServer(url, echoRef1, trans);
// try it again after 5 seconds
Thread.sleep(5000);
private static void connectToServer(String url, Echo echoRef1, EchoClientTransfer trans)
System.out.println("Retry connection");
// Connect to server
while( true )
try
//get the remote factory object from the registry
EchoFactory remoteFactory = (EchoFactory)Naming.lookup(url);
//get references to new EchoImpl instances
echoRef1 = remoteFactory.getEcho("User Meyer");
// reactivate Datatransfer because I have now a new connection to the server
trans.reactivateAfterConnectionError(echoRef1);
// end of initialisation
break;
catch( Exception e )
//>>>>>>> Error initialising connection to server. try again after 5 seconds"
Thread.sleep(5000); // retry after 5 seconds

RMI Rebind problem

Hi
I'm a novice on RMI and have been trying to learn the concept by copying the HelloWorld RMI example from the Java Tutorials to run on my Windows 98 PC. However, when I issued a DOS command: Java HelloImpl, the following error came up:
" Access denied (java.net.SocketPermission .....1099 connect resolve)"
Any suggestions what possible cause of the problem ?
Thanks.

check the permissions in your policy file.

RMI or plain JDBC???

I'm in the planning stages for an application that will be run on client machines via Java Web Start. These applications are going to need to have access to a MS SQL Server, located on a separate machine, and I'm wondering if I should use RMI for all my database connections, queries, updates, etc., or should I just use JWS to launch my application, and include the location of the JDBC drivers in the .jnlp file, and handle all database calls and such on the client end. There are of course benefits and drawbacks to both. I would tend to lean towards RMI as it offers more security, but I'm new to the concept of RMI, and am clueless as to how to implement it, being the app is on a client machine and it needs access to the server. Don't know how to do that, but I'm searching the forums and Google for other cases of people doing the same thing I am looking at, and seeing how they implemented it. I'm not trolling for free code, just looking for examples that will help me create my own way of doing it. So, should I use RMI, or does my situation not warrant it and I can go with straight JDBC calls to the database. Thanks for any advice...
James

Unless there is a firewall between your client and server machines, or you anticipate being able to make significant use of server-side cacheing, then go with JDBC.
If security is a really big deal (specifically you want to encrypt communications between clients & the server) then RMI offers some options that aren't available with JDBC, but an external solution such as IPSec is probably going to be easier to implement.
D.

New @ RMI need help with java.rmi.UnmarshalException: error unmarshalling

Hi @ all out there,
I'm new with Java RMI and have to write a EventSystem for an college project where clients can subscribe to a topic and get notified when someone publishes a message to the subscribed topic.
At server-side I have a class called EventSystem that provides methods for subscribing and unsubscribing from topics, and also for posting messages (for publishers).
To subscribe i thought that the client must specify the topic and also itself ( means that a client calls in this way: obj.subscribe("mytopic", this).
The EventSystem handles a list of all clients, and whenever a new message is posted it goes trough all clients and invokes the handleMessage(String msg) method that all Clients have to provide.
On my local machine without RMi this concept works just great.
I now tried to get it working using RMI , but I get the following Exception when starting the client (the server starts fine) :
Looking up for rmiregistry at 138.232.248.22:1099
Subscriber exception:
java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.io.InvalidClassException: SubscriberImpl; SubscriberImpl; class invalid for deserialization
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:336)
        at sun.rmi.transport.Transport$1.run(Transport.java:159)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
        at java.lang.Thread.run(Thread.java:619)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(StreamRemoteCall.java:255)
        at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:233)
        at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:142)
        at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:178)
        at java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:132)
        at $Proxy0.subscribe(Unknown Source)
        at SubscriberImpl.main(SubscriberImpl.java:48)
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.io.InvalidClassException: SubscriberImpl; SubscriberImpl; class invalid for deserialization
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:293)
        at sun.rmi.transport.Transport$1.run(Transport.java:159)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
        at java.lang.Thread.run(Thread.java:619)
Caused by: java.io.InvalidClassException: SubscriberImpl; SubscriberImpl; class invalid for deserialization
        at java.io.ObjectStreamClass.checkDeserialize(ObjectStreamClass.java:713)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1733)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
        at sun.rmi.server.UnicastRef.unmarshalValue(UnicastRef.java:306)
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:290)
        ... 9 more
Caused by: java.io.InvalidClassException: SubscriberImpl; class invalid for deserialization
        at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:587)
        at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1583)
        at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1496)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1732)
        ... 13 moreI googled now for 2 hours but can't resolve the problem alone. As far as I can understand I have to serialize Objects that I want to send to the server, right?
So how can i do this? I've never used serialization till now.
any ideas how to solve this problem?
greets from italy and sorry for my very weak english
bd_italy

A class has been modified after deployment. Stop the Registry, clean, recompile, and redeploy.

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