Java Remote Method Invocation (RMI)

Java Remote MethodInvocation (RMI)

)

Distributed Systems

� a collection of independent computers thatappears to its users as a single coherentsystem

Models of Distribution

� Message passing� Distributed objects

� Event-based architectures� Space-based paradigms

Distributed Object Model� Views a distributed system as a series ofinteracting objects

� Based on some underlying messagepassing protocol invisible to theprogrammer

� Three main technologies: RMI, CORBAand DCOM

Distributed Object Computing� Enable any object in the local system to directly interact

with an object on a remote host� Goals:

� Let any object reside anywhere in the network, and allow anapplication to interact with these objects in the same way asthey do with a local object.� Provide the ability to construct an object on one host and

transmit it to another host.� Enable an agent on one host to create an object on another

host.

What Is RMI?� A mechanism that allows the invocation ofa method that exists in another addressspace

� Java-to-Java only� Client-Server Protocol� High-level API

� Transparent� Lightweight

Related Technologies� RPC (“Remote Procedure Calls”)

� Developed by Sun� Platform-specific

� CORBA (“Common Object Request Broker Architecture”)� Developed by OMG

� Access to non-Java objects (as well as Java)� DCOM (“Distributed Common Object Model”)

� Developed by Microsoft� Access to Win32 objects

� LDAP (“Lightweight Directory Access Protocol”)� Finding resources on a network

RMI� Client – the process that invokes a method on aremote object

� Server – the process that owns the remoteobject

� Object Registry – a name server that relatesobjects with names� Objects are registered with the Object Registry, undera unique name.

� The Object Registry is used to obtain access toremote objects, using their names.

Remote Objects (Diagram)Java Virtual Machine

ClientObject

Client

TCP

Java Virtual Machine

RemoteObject

Server

RMI LayersJava Virtual Machine

ClientObject

Stub

Remote Reference Layer

Transport Layer

11/18/2003

TCP

Java Virtual Machine

RemoteObject

Skeleton


Transport Layer

RMI Architecture in the OSI modelApplication Layer

Presentation Layer

Session Layer

Transport Layer

Network LayerData-link

layerPhysical Layer

User Application

Stub Skeleton


TCP

IP

Hardware Interface

Network

Remote Objects

� Remote Objects� Live on server

Java Virtual MachineClientObject

Stub


Transport Layer

Client

Java Virtual MachineRemoteObject

Skeleton

Remote Reference LayerTCPTransport Layer

Server

� Accessed as if they were local

Stubs and Skeletons� Stub

� lives on client


Stub


Transport Layer

Client


Skeleton


Server

� pretends to be remote object - a proxy for the remote object� Skeleton� lives on server

� receives requests from stub, talks to the remote object anddelivers response to stub

� Stubs and skeletons are not written by the programmer!They are generated by a special compiler “rmic”

Stubs and Skeletons� Stub – responsibilities

� Initiate remote calls


Stub


Transport Layer

Client


Skeleton


Server

� Marshals arguments to be sent� Informs the remote reference layer that a callshould be invoked on the server

� Unmarshals a return value (or exception)� Informs the remote reference layer that thecall is complete

Stubs and Skeletons� Skeleton – responsibilities


Stub


Transport Layer

Client


Skeleton


Server

� Unmarshals incoming arguments� Calls the actual remote object implementation� Marshals return values for transport to theclient

� Marshaling – definition� The process of converting native

programming language data types to a formatsuitable for transmission across a network

Remote Interfaces and StubsRemote Interface

implements implements

Remote ObjectClient Stub Skeleton(Server)

11/18/2003

Remote Interfaces� Declare exposed methods – the methodsthat can be called from remote locations

� Extend java.rmi.Remote� The remote object implements thisinterface

� Act like a proxy for the remote object



Stub


Transport Layer

Client


Skeleton


Server

� Sets up connections to remote addressspaces

� Manages connections� Listens for incoming calls� Communicates via TCP/IP

Object Registries� Name and look up remote objects� Remote objects register by name

� Clients obtain a remote reference to theremote object� A registry is a running process on thesame host as the RMI server

HTTP Tunneling� Cool: if it can’t make the connectionnormally, it will tunnel through port 80� Allows clients behind firewall to makeremote calls to server� Note: does not work server -> client

RMI System ArchitectureClient Virtual Machine

Client

Stub

Client

“Fred”

Server Virtual Machine

RemoteObject

SkeletonServer

Server

RegistryRegistry Virtual Machine

RMI Flow1. Server Creates Remote Object

2. Server Registers Remote ObjectClient

Stub

“Fred”


RemoteObject

1

SkeletonServer

2

Registry Virtual Machine

RMI FlowClient Virtual Machine

Client

Stub

3 4


3. Client requests object from Registry4. Registry returns remote reference

SkeletonServer

“Fred”


RMI FlowClient Virtual Machine

Client

5

Stub6


RemoteObject7

SkeletonServer

5. Client invokes stub method6. Stub talks to skeleton

7. Skeleton invokes remote objectmethod


Part II: RMI Usage

11/18/2003 and Distributed Programming

Creating Remote Objects� Define a Remote Interface

� extends java.rmi.Remote� Define a class that implements theRemote Interface

� extends java.rmi.RemoteObject� or java.rmi.UnicastRemoteObject

Remote Interface Exampleimport java.rmi.*;

public interface Adderextends Remote

{public int add(int x, int y)throws RemoteException;

}

ECE 451:Introduction to Parallel11/18/2003

Remote Class Exampleimport java.rmi.*;

import java.rmi.server.*;public class AdderImpl extends UnicastRemoteObject

implements Adder{

public AdderImpl() throws RemoteException{

}public int add(int x, int y)

throws RemoteException{

return x + y;}

}

Inheritance Diagram in JavaObject Remote

RemoteObject

RemoteStub RemoteServer

UnicastRemoteObject

Compiling Remote Classes� Compile the Java class

� javac� reads .java file

� produces .class file� Compile the Stub and Skeleton

� rmic� reads .class file

� produces _Skel.class and _Stub.class

Compiling Remote Classes(Diagram)

Adder.java(interface)

javacAdder.class(interface classfile)

javac

AdderImpl_Skel.class(skeleton classfile)

AdderImpl.java(remote class)

AdderImpl.class(classfile) rmic

AdderImpl_Stub.class(stub classfile)

ECE 451:Introduction to Parallel11/18/2003 and Distributed Programming

Registering Remote Classes� start the registry

� running process� Unix:rmiregistry &

� Windows:start /m rmiregistry

Registry CLASSPATH� Registry VM needs to be able to find stub file(s)� You must set the CLASSPATH to include thedirectory containing the stub file

� An easy way to check CLASSPATH is to use the javap command, supplyinga fully package qualified class name. It uses the current CLASSPATH to find

and print the interface to a class.

� Or, your server needs to specify thejava.rmi.server.codebase System property (morelater)

Create the server

� Creates a new instance of the remoteobject� Registers it in the registry with a name� That’s it


RMI Server Exampletry {

AdderImpl adder = new AdderImpl();Naming. rebind ("adder", adder);System.out.println("Adder bound");

}catch (RemoteException re) {

re.printStackTrace();}

catch (MalformedURLException me) {me.printStackTrace();

}

Launch the Server% java AdderServer &Adder bound


Server Logging� invoke from command linejava

-Djava.rmi.server.logCalls=trueYourServerImpl

� or enable inside programRemoteServer.setLog(System.err);

Creating an RMI Client� Install a Security Manager

� to protect from malicious stubs� Find a registry� use java.rmi.Naming

� Lookup the name, returns a reference� Cast the reference to the appropriateRemote Interface

� Just use it!

RMI URLsrmi://host[:port]/name� default port is 1099� Specifies hostname of registry

� can also use relative URLs� name only

� assumes registry is on local host

RMI Client ExampleSystem.setSecurityManager(new RMISecurityManager());Adder a = (Adder)

Naming.lookup("adder");

int sum = a.add(2,2);System.out.println("2+2=" + sum);

Remote Interfaces vs. RemoteClasses

� Remember that the reference is to an interface� You must make references, arrays, etc. out ofthe interface type, not the implementation type� You can’t cast the remote reference to a normalreference� So name your Remote Objects with “Impl” (soyou don’t get confused)

Parameter Passing� All parameters are passed by value

� Primitive types� passed by

value� Objects� passed by value

� use Java Object Serialization

Object Serialization� saves the state (data) of a particularinstance of an object

� serialize - to save� unserialize - to load

Java Serialization� writes object as a sequence of bytes� writes it to a Stream

� recreates it on the other end� creates a brand new object with the olddata

java.io.Serializable� Objects that implement the

java.io.Serializable interface are marked asserializable

� Also subclasses� empty interface - just a marker – no needto implement any special methods

RMI Security� Server is untrusted

� Stubs could be malicious� rmic is OK, but someone could custom-code an evil stub: it’s just a .class file

RMI Security Managers� AppletSecurityManager

� stub can only do what an applet can do� RMISecurityManager� disables all functions except class definition and

access� A downloaded class is allowed to make a connectionif the connection was initiated via the RMItransport.

� None� Stub loading disabled

� Stubs still work if they are in local classpath

Limitations of RMI� Java-only

� but you can use JNI on the server� Uses TCP, not UDP� At least two sockets per connection

� Untested for huge loads

Sun vs. Microsoft� RMI is not shipped as part of Microsoft’sproducts

� RMI will still work in applications� include java.rmi.* class files in your classpath� download rmi.zip from ftp.microsoft.com

� RMI will work in applets� include java.rmi.* class files (or rmi.zip) in yourcodebase

� extra download time

RMI Chat Server ObjectsMessage

interfaceMessageReceiver

- receiveMessage(Message)

ChatClient

implements

interfaceChatServer

- login(MessageReceiver)- sendMessage(Message)

ChatServerImpl

Dispatcherremote reference MessageQueuelocal reference

Summary� RMI is a very clean API

� Easy way to write distributed programs� Wire protocol may need improvement forlarge-scale problems

Documents

Java Remote Method Invocation (RMI)