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Copyright 1997 Alex Chaffee

Java Remote Method Invocation

(RMI)

Alexander Day Chaffee

[email protected]

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Introduction

Java

Networking

Distributed Computing

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Who is in Attendance?

Find out any relevant background and

interest of the audience

Java Programmers?

C++ Programmers?

TCP/IP Gurus?

Smalltalk?

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Agenda

Background

APIs and Usage

Q&A

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Overview

TCP/IP

java.net

RMI JDBC CORBA

Network OS

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What Is RMI?

Access to Remote Objects

Java-to-Java only

Client-Server Protocol

High-level API

TransparentLightweight

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7/63Copyright 1997 Alex Chaffee

Examples of Use

Database access

Computations

Any custom protocol

Not for standard protocols (HTTP, FTP,

etc.)

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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

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Part I: RMI Concepts

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TCP

Remote Objects (Diagram)

Java Virtual Machine

ClientObject


RemoteObject

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RMI Layers

TCPRemote Reference Layer

Transport Layer


Client

Object

Remote Reference Layer

Transport Layer


Stub

Remote

Object

Skeleton

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Remote Objects

Remote Objects

Live on server

Accessed as if they were local

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Registries

Name and look up remote objects

Servers can register their objects

Clients can find server objects and obtain a

remote reference

A registry is a running process on a host

machine

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Remote References and

InterfacesRemote References

Refer to remote objects

Invoked on client exactly like local objectreferences

Remote Interfaces

Declare exposed methods

Implemented on client

Like a proxy for the remote object

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Stubs and Skeletons

Stub

lives on client

pretends to be remote object

Skeleton

lives on server

receives requests from stub

talks to true remote object

delivers response to stub

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Remote Interfaces and Stubs

Remote Interface

Stub Remote Object(Server)

Client Skeleton

implements implements

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Remote Reference Layer

Local pointers not good enough

Figures out which remote object is being

referenced

Could span multiple virtual machines

Communicates via TCP/IP

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Transport Layer

Deals with communications

Connection management

Dispatching messages between stub and

skeleton

Distributed Garbage Collection

Sits on top of java.net

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HTTP Tunneling

Cool: if it cant make the connection

normally, it will tunnel through port 80

Allows clients behind firewall to makeremote calls to server

Note: does not work server -> client

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RMI System Architecture

Client Virtual Machine

Client

Server Virtual Machine

Stub

Remote

Object

Skeleton

Registry Virtual Machine

Fred

Server

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RMI Flow


Client


Stub

Remote

Object

Skeleton


Fred

Server

1

2

1. Server Creates Remote Object2. Server Registers Remote Object

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RMI Flow


Client


Stub

Remote

Object

Skeleton


Fred

Server

4

3. Client requests object from Registry

4. Registry returns remote reference

(and stub gets created)

3

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RMI Flow


Client


Stub

Remote

Object

Skeleton


Fred

Server

6

5. Client invokes stub method6. Stub talks to skeleton

7. Skeleton invokes remote object

method

5 7

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Part II: RMI Usage

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Creating Remote Objects

Define a Remote Interface

extends java.rmi.Remote

Define a class that implements the Remote

Interface

extends java.rmi.RemoteObject

or java.rmi.UnicastRemoteObject

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Remote Interface Example

import java.rmi.*;

public interface Adder

extends Remote{

public int add(int x, int y)

throws RemoteException;}

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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;

}

}

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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

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Compiling Remote Classes

(Diagram)

Adder.java

(interface)

Adder.class

(interface classfile)

javac

AdderImpl.java

(remote class)

AdderImpl.class

(classfile)

javacrmic

AdderImpl_Skel.class

(skeleton classfile)

AdderImpl_Stub.class

(stub classfile)

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Registering Remote Classes

start the registry

running process

Unix:rmiregistry &

Windows:

start /m rmiregistry

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Registry CLASSPATH

Registry VM needs to be able to find stub

file(s)

You must set the CLASSPATH to includethe directory containing the stub file

An easy way to check CLASSPATH is to use the javap command,

supplying a 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 the

java.rmi.server.codebase System property

(more later)

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Create the server

Creates a new instance of the remote object

Registers it in the registry with a unique

name

Thats it

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RMI Server Example

try {

AdderImpl adder = new AdderImpl();

Naming.rebind("adder", adder);

System.out.println("Adder bound");}

catch (RemoteException re) {

re.printStackTrace();

}

catch (MalformedURLException me) {

me.printStackTrace();

}

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Launch the Server

% java AdderServer &

Adder bound

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Server Logging

invoke from command line

java

-Djava.rmi.server.logCalls=true

YourServerImpl

or enable inside program

RemoteServer.setLog(System.err);

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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 appropriate

Remote Interface

Just use it!

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RMI URLs

rmi://host[:port]/name

default port is 1099

Specifies hostname ofregistry

can also use relative URLs

name only

assumes registry is on local host

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RMI Client Example

System.setSecurityManager(

new RMISecurityManager());

Adder a = (Adder)Naming.lookup("adder");

int sum = a.add(2,2);

System.out.println("2+2=" + sum);

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Remote Interfaces vs. Remote

ClassesRemember that the reference is to an

interface

You must make references, arrays, etc. outof the interface type, not the implementation

type

You cant cast the remote reference to anormal reference

So name your Remote Objects with Impl

(so you dont get confused)

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Parameter Passing

Primitive types

passed by value

Remote objects

passed by reference

Non-remote objects

passed by value

uses Java Object Serialization

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Object Serialization

aka Persistence

saves the state (data) of a particular instance

of an object

serialize - to save

unserialize - to load

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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 old data

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java.io.Serializable

Objects that implement the

java.io.Serializable interface are marked as

serializableAlso subclasses

Magically, all non-static and non-transient

data members will be serializedActually, its not magic, itsReflection(its

done with mirrors)

empty interface - just a marker

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Not All Objects Are

SerializableAny object that doesnt implementSerializable

Any object that would pose a security riske.g. FileInputStream

Any object whose value depends on VM-

specific informatione.g. Thread

Any object that contains a (non-static, non-

transient)

unserializable object(recursively)

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NotSerializableException

thrown if you try to serialize or unserialize

an unserializable object

maybe you subclassed a Serializable objectand added some unserializable members

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Incompatible Changes

If class has members added or removed, it

becomes incompatible

java.io.InvalidClassException thrown if youtry to deserialize an incompatible object

stream

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Serial Version

If the changes were actually compatible

find out the Serial Version UID of the

original classuse the serialver utility

add a member variable to the changed class

protected static final long serialVersionUID =-2215190743590612933L;

now its marked as compatible with the old

class

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Using readObject

if you need to force an object to be

compatible

implement readObject() method to makecompatible changes

private void readObject(ObjectInputStream

stream) throws java.io.IOException

{

defaultReadObject(stream);

// do compatible stuff

}

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Callbacks

They just work

Pass in a remote reference to a client object

Server object can call its methods

transparently

Registry is out of the loop

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RMI Security

Server is untrusted

Stubs could be malicious

rmic is OK, but someone could custom-

code an evil stub: its just a .class file

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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 connection if

the connection was initiated via the RMI transport.

None

Stub loading disabled

Stubs still work if they are in local classpath

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Codebase Property

Stub classpaths can be confusing

3 VMs, each with its own classpath

Server vs. Registry vs. Client The RMI class loader always loads stubs from the

CLASSPATH first

Next, it tries downloading classes from a web

server

(but only if a security manager is in force)

java.rmi.server.codebase specifies which web

server

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Stub File Configuration

The best way to configure it is as follows.

NEVER have stub class files in ANY classpath

make the stub files accessible via a web server

set the java.rmi.server.codebase property, in

the application creating the server object, to the

web server's URL

The stubs will be downloaded from your

HTTP server on demand (Thanks to [email protected] for advice)

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Codebase and Thread Safety

Theres a thread problem with codebase

With a single remote server, and multiple

remote objects on that server, each with itsown stub codebase

One server can step on the codebase

property and foul up the registration for theother server

In practice, wont happen very often

More likely to happen in servlets

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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

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RMI vs. COM

Very similar

remote interfaces ~ type libraries

COM is Win32-only (for now)

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Sun vs. Microsoft

RMI is not shipped as part of Microsofts products

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 your codebase

IE4: only if theyre signed

extra download time

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Part III: RMI Chat Server

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RMI Chat Server Objects

Message

interface

MessageReceiver- receiveMessage(Message)

ChatClient

interface

ChatServer- login(MessageReceiver)

- sendMessage(Message)

ChatServerImpl

Dispatcher MessageQueueimplementsremote reference

local reference

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Summary

RMI is a very clean API

Easy way to write distributed programs

Wire protocol may need improvement for

large-scale problems

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Where to get more

informationOther training sessions

Harold,Java Network Programming

(OReilly)

rmi-users mailing list ([email protected])

http://www.developer.com/ (Gamelan)

http://www.javaworld.com/ (magazine)

http://www.stinky.com/java/ (Authors site)

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Q&A

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