CS4273: Distributed System Technologies and Programming I
Lecture 12: Java RMI (Remote Method Invocation)
Client-Server Programming without RMI (RPC)
Server
Client
Main ()
{ ……
read(file,buf,n);
…...
}
Main {
Loop on:
recv req
r = read(file, n);
pack rep
send rep to clnt
}
read(fd,buf,n)
{ make a socket to svr
prepare a req
socket
send req
recv rep
copy data to mybuf
}
stub
read(file,n)
{
……
get data from disk
……
}
write(file,data,n)
{
…….
}
stub
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Remote Method Invocation (RMI)
Stub objects are auto-generated by “rmic” from interface definition
Interface
implmentation
rmic
rmic
Client
RMI
Stub
Object
Server
Stub
Object
Server
Object
3
An Example of RMI
An interface
interface Product extends Remote {
String getDescription () throws
RemoteException;
String placeOrder () throws
RemoteException;
int getOrderNo () throws
RemoteException;
}
N.B. every rmi method must throw exceptions,
due to the un-reliability of RMI.
A service routine
public class ProductImpl extends
UnicastRemoteObject implements Product {
public ProductImpl (String pd) throws
RemoteException
{ descript = pd; }
public String getDescription () {
return “ProductDescipt:” + descript + “. ”; }
private String descript;
……….
}
A client RMI call
……
Product p = Naming.lookup(url-name);
String descript = p.getDescription ();
System.out.println(descript);
……
4
Inheritance Graph of Interface and Remote servers
Object
Remote
RemoteObject
RemoteStub
extends
RemoteServer
implements
UnicastRemoteObject
RMI interface
definition
interface implement
object
5
Naming Conventions for RMI classes and Files
Interface def file
Interface impl file
no suffix (e.g. Product)
Impl suffix (e.g. ProductImpl)
Server program file
Client program file
Stub program file
Server suffix (e.g. ProductServer)
Client suffix (e.g. ProductClient)
_Stub suffix (e.g. ProductImpl_Stub)
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Client-Server Binding in RMI
•
•
•
•
An RMI registration server (rmiregistry) on a site is responsible
for locating any appl servers on that site. This rmiregistry
should run on the server site before any appl servers start.
An appl server needs to register itself to rmiregistry on the
same site when it is created.
A client looks up an appl server from rmiregistry on that site by
specifying the server’s URL-addr
rmiregistry
lookup
(rmi://svr-site/svr-name).
A naming class
client
rebind
java.rmi.server.Naming
RMI
server
provides an interface for servers
to reg / de-reg and for clients to look up servers.
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An Example of RMI
A simple interface implementation program
import java.rmi.*; import java.rmi.server.*; // file: rmi/svr/ProductImpl.java
public class ProductImpl extends UnicastRemoteObject implements Product {
private String descript;
public ProductImpl (String pd) throws RemoteException {
descript = pd;
}
public String getDescription () {
return "ProductDescript" + descript + ".";
}
}
A server program // creates an impl obj and registers it
public class ProductSvr {// file: rmi/svr/ProductSvr.java
public static void main (String args[]) {
try { ProductImpl p1 = new ProductImpl (“Jia’s Warehouse");
Naming.rebind ("warehouse", p1); // register svr by name
} catch (Exception e) { System.out.println("Error:" +e);} }
}
8
Compile Server Program
We have the following files in server’s directory:
Product.java
// interface file
ProductImpl.java
// interface implementation file
ProductSvr.java
// server program file
1. Compile the interface file: javac Product.java
It can be skipped. When compiling ProductImpl.java, it is auto-compiled.
2. Compile the server implementation: javac ProductImpl.java
3. Generate stub class using RMIC: rmic ProductImpl
It generates file ProductImpl_Stub.class (for both client and server).
N.B. ProductImpl java must be compiled before using “rmic”.
4. Compile the server program: javac ProductSvr.java
N.B. the server links procedures in ProductImpl_Stub at run-time.
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A Simple Client Program
// file: ~java/rmi/clnt/ProductClnt.java
//usage: java ProductClnt server-hostname
import java.rmi.*; import java.rmi.server.*;
public class ProductClnt {
public static void main (String args[]) {
String url = "rmi://"+args[0];
try {
Product p1 = (Product) Naming.lookup (url + "/warehouse");
System.out.println(p1.getDescription());
System.out.println(p1.placeOrder());
System.out.println("OrderNo: " + p1.getOrderNo());
} catch (Exception e) { System.out.println("Error:" +e);} }
}
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Build Client Program
1. Copy interface file Product.java to local directory.
2. Copy the client stub file ProductImpl_Stub.class (generated by
rmic) to the local directory.
3. Compile the client source code: javac ProductClnt.java
•
•
when compiling ProductClnt.java, it automatically compiles the
interface Product.java if it has not been compiled yet. The compiler
locates the interface Product.java by naming conventions.
client calls stub procedures in ProductImpl_Stub.class at run-time. It is
in a separate class file, without linking to the client at compilation time.
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Run the Example of RMI
•
Start RMI registry program on the server site:
rmiregistry &
•
Start the server program on the server site:
java ProductSvr &
•
Start the client program on the client site:
java ProductClnt args
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Naming class for client-server Binding
RMI uses class “java.rmi.server.Naming” for client-server binding
functions, which includes:
• static Remote lookup (String url)
returns the remote object for the URL
• static void bind(String name, Remote obj)
binds name to the remote object obj
• static void rebind(String name, Remote obj)
binds name to remote obj & replaces any existing one
• static void unbind(String name)
unbinds the name
• static String[] list(String url)
returns an array of strings of urls registered at the registry server.
13
View Database of RMI Registration Server
// file: ~java/rmi/showBinding.java
import java.rmi.*; import java.rmi.server.*;
public class showBindings {
public static void main ( String[] args) {
try {
String [] bindings = Naming.list("");
for (int i = 0; i < bindings.length; i++)
System.out.println(bindings[i]);
} catch(Exception e)
{System.out.println("Error"+e);} }
}
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Summary of Programming with RMI
On Server Site
1. write an interface file.
2. write the interface implementation program, which
implements all methods in the interface.
3. write a server program, which initializes a server-impl
object and registers the server object with the registry.
4. compile all the source files on server side.
5. generate stub class by using rmic.
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Summary of Programming with RMI (Cont.)
On Client Site
6. copy the interface file and the generated stub class file to
the local directory (from the server site).
7. write a client program that uses rmi.
8. compile the client program.
Run the Client & Server
9. run the registry server on server site.
10. run server and client on server and client sites respectively.
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Applets Using RMI
server
client
applet
RMI
HTTP
CORBA
IIOP
CGI
server
server
17
Programming Applets using RMI
The server (and rmiregistry server) must run on the Web server, bcs applets can
only communicate with their home server!!!
1. Copy the interface file (Product.java) and the stub class file
(ProductImpl_Stub.class) to the local directory.
2. Write applet "ProductApplet.java" and compile it.
3. Run "rmiregistry" and server "ProductSvr" on the Web Server site.
4. Run applet in Web Browser.
N.B. No need to change the server program
rmiregistry
when moving a client to an applet.
lookup
applet
rebind
RMI
server
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An Example of RMI Applet Program
// file: java/rmi/applet/ProductApplet.java
import java.rmi.*; import java.rmi.server.*; import java.applet.Applet;
public class ProductApplet extends JApplet {
private String message = "Initializing…";
public void init() {
String url = "rmi://" + getCodeBase().getHost(); // web-site !!!
try { Product p1 = (Product) Naming.lookup (url + "warehouse");
message = p1.getDescription(); // rmi
repaint();
} catch (Exception e) { System.out.println("Error:" +e);}
}
public synchronized void paint(Graphics g) {
g.drawString(message, 5, 50); }
}
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