Download Java Remote Method Invocation (RMI)

Java Remote Method
Invocation (RMI)
Viraj Bhat ([email protected])
Cristina Schmidt ([email protected])
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Distributed Systems
„
a collection of independent computers that
appears to its users as a single coherent
system
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Models of Distribution
„
„
„
„
Message passing
Distributed objects
Event-based architectures
Space-based paradigms
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Distributed Object Model
„
„
„
Views a distributed system as a series of
interacting objects
Based on some underlying message
passing protocol invisible to the
programmer
Three main technologies: RMI, CORBA
and DCOM
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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 an
application to interact with these objects in the same way as
they 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.
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
What Is RMI?
„
„
„
„
„
„
A mechanism that allows the invocation of
a method that exists in another address
space
Java-to-Java only
Client-Server Protocol
High-level API
Transparent
Lightweight
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Examples of Use
„
„
„
„
Database access
Computations
Any custom protocol
Not for standard protocols (HTTP, FTP,
etc.)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Related Technologies
„
RPC
„
„
„
„
„
„
(“Distributed Common Object Model”)
Developed by Microsoft
Access to Win32 objects
LDAP
„
(“Common Object Request Broker Architecture”)
Developed by OMG
Access to non-Java objects (as well as Java)
DCOM
„
„
Developed by Sun
Platform-specific
CORBA
„
(“Remote Procedure Calls”)
(“Lightweight Directory Access Protocol”)
Finding resources on a network
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Part I: RMI Concepts
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
RMI
„
„
„
Client – the process that invokes a method on a
remote object
Server – the process that owns the remote
object
Object Registry – a name server that relates
objects with names
„
„
Objects are registered with the Object Registry, under
a unique name.
The Object Registry is used to obtain access to
remote objects, using their names.
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Objects (Diagram)
Java Virtual Machine
Java Virtual Machine
Client
Object
Remote
Object
TCP
Client
11/18/2003
Server
ECE 451:Introduction to Parallel
and Distributed Programming
RMI Layers
Java Virtual Machine
Java Virtual Machine
Client
Object
Remote
Object
Stub
Skeleton
Remote Reference Layer
Remote Reference Layer
Transport Layer
11/18/2003
TCP
ECE 451:Introduction to Parallel
and Distributed Programming
Transport Layer
RMI Architecture in the OSI model
Application Layer
Presentation Layer
User Application
Stub
Skeleton
Session Layer
Remote Reference Layer
Transport Layer
TCP
Network Layer
IP
Data-link layer
Physical Layer
Hardware Interface
Network
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Java Virtual Machine
Remote Objects
Java Virtual Machine
Client
Object
Remote
Object
Stub
Skeleton
Remote Reference Layer
Transport Layer
Client
„
Remote Objects
Live on server
„ Accessed as if they were local
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Reference Layer
TCP
Transport Layer
Server
Java Virtual Machine
Stubs and Skeletons
Java Virtual Machine
Client
Object
Remote
Object
Stub
Skeleton
Remote Reference Layer
Transport Layer
Remote Reference Layer
TCP
Transport Layer
Client
„
Stub
„
„
„
lives on client
pretends to be remote object - a proxy for the remote object
Skeleton
„
„
„
Server
lives on server
receives requests from stub, talks to the remote object and
delivers response to stub
Stubs and skeletons are not written by the programmer!
They are generated by a special compiler “rmic”
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Java Virtual Machine
Stubs and Skeletons
Client
Object
Remote
Object
Stub
Skeleton
Remote Reference Layer
Transport Layer
Client
„
Java Virtual Machine
Remote Reference Layer
TCP
Transport Layer
Server
Stub – responsibilities
Initiate remote calls
„ Marshals arguments to be sent
„ Informs the remote reference layer that a call
should be invoked on the server
„ Unmarshals a return value (or exception)
„ Informs the remote reference layer that the
call is complete
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Java Virtual Machine
Stubs and Skeletons
Client
Object
Remote
Object
Stub
Skeleton
Remote Reference Layer
Transport Layer
Client
„
Java Virtual Machine
Remote Reference Layer
TCP
Transport Layer
Server
Skeleton – responsibilities
Unmarshals incoming arguments
„ Calls the actual remote object implementation
„ Marshals return values for transport to the
client
„
„
Marshaling – definition
„
The process of converting native
programming language data types to a format
suitable for transmission across a network
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Interfaces and Stubs
Remote Interface
implements
Client
11/18/2003
Stub
implements
Skeleton
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Object
(Server)
Remote Interfaces
„
„
„
„
Declare exposed methods – the methods
that can be called from remote locations
Extend java.rmi.Remote
The remote object implements this
interface
Act like a proxy for the remote object
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Java Virtual Machine
Remote Reference Layer
Java Virtual Machine
Client
Object
Remote
Object
Stub
Skeleton
Remote Reference Layer
Transport Layer
Remote Reference Layer
TCP
Client
„
„
„
„
Sets up connections to remote address
spaces
Manages connections
Listens for incoming calls
Communicates via TCP/IP
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Transport Layer
Server
Object Registries
„
„
„
„
Name and look up remote objects
Remote objects register by name
Clients obtain a remote reference to the
remote object
A registry is a running process on the
same host as the RMI server
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
HTTP Tunneling
„
„
„
Cool: if it can’t make the connection
normally, it will tunnel through port 80
Allows clients behind firewall to make
remote calls to server
Note: does not work server -> client
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
RMI System Architecture
Client Virtual Machine
Server Virtual Machine
Client
Remote
Object
Skeleton
Stub
Server
Server
Client
“Fred”
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Registry Virtual Machine
Registry
RMI Flow
1. Server
Creates
Remote Object
Client Virtual
Machine
2. Server Registers Remote Object
Client
Server Virtual Machine
Remote
Object
1
Skeleton
Stub
Server
2
“Fred”
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Registry Virtual Machine
RMI Flow
Client Virtual Machine
Client
Server Virtual Machine
Remote
3. Client requests object from Registry
Object
4. Registry returns remote reference
Skeleton
Stub
3
Server
4
“Fred”
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Registry Virtual Machine
RMI Flow
Client Virtual Machine
Server Virtual Machine
Client
Remote
Object
5
7
6
Stub
Skeleton
Server
5. Client invokes stub method
6. Stub talks to skeleton
7. Skeleton invokes remote object
“Fred”
method
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Registry Virtual Machine
Part II: RMI Usage
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Interface Example
import java.rmi.*;
public interface Adder
extends Remote
{
public int add(int x, int y)
throws RemoteException;
}
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Class Example
import 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;
}
}
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Inheritance Diagram in Java
Object
Remote
RemoteObject
RemoteStub
RemoteServer
Unicast
RemoteObject
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Compiling Remote Classes
(Diagram)
Adder.java
(interface)
javac
AdderImpl.java
(remote class)
javac
Adder.class
(interface classfile)
AdderImpl.class
(classfile)
AdderImpl_Skel.class
(skeleton classfile)
rmic
AdderImpl_Stub.class
(stub classfile)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Registering Remote Classes
„
start the registry
„
„
running process
Unix:
rmiregistry &
„
Windows:
start /m rmiregistry
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Registry CLASSPATH
„
„
„
„
Registry VM needs to be able to find stub file(s)
You must set the CLASSPATH to include the
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)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Create the server
„
„
„
Creates a new instance of the remote
object
Registers it in the registry with a name
That’s it
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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();
}
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Launch the Server
% java AdderServer &
Adder bound
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Server Logging
invoke from command line
java
-Djava.rmi.server.logCalls=true
YourServerImpl
„
or enable inside program
RemoteServer.setLog(System.err);
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Creating an RMI Client
„
Install a Security Manager
„
„
Find a registry
„
„
„
„
to protect from malicious stubs
use java.rmi.Naming
Lookup the name, returns a reference
Cast the reference to the appropriate
Remote Interface
Just use it!
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
RMI URLs
rmi://host[:port]/name
„
„
„
default port is 1099
Specifies hostname of registry
can also use relative URLs
name only
„ assumes registry is on local host
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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);
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Remote Interfaces vs. Remote
Classes
„
„
„
„
Remember that the reference is to an interface
You must make references, arrays, etc. out of
the interface type, not the implementation type
You can’t cast the remote reference to a normal
reference
So name your Remote Objects with “Impl” (so
you don’t get confused)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Parameter Passing
„
All parameters are passed by value
„
Primitive types
„
„
passed by value
Objects
passed by value
„ use Java Object Serialization
„
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Object Serialization
„
„
„
saves the state (data) of a particular
instance of an object
serialize - to save
unserialize - to load
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
java.io.Serializable
„
„
„
Objects that implement the
java.io.Serializable interface are marked as
serializable
Also subclasses
empty interface - just a marker – no need
to implement any special methods
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Not All Objects Are Serializable
„
„
Any object that doesn’t implement Serializable
Any object that would pose a security risk
„
„
Any object whose value depends on VMspecific information
„
„
e.g. FileInputStream
e.g. Thread
Any object that contains a (non-static, non-transient)
unserializable object (recursively)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
NotSerializableException
„
„
thrown if you try to serialize or unserialize
an unserializable object
maybe you subclassed a Serializable
object and added some unserializable
members
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Incompatible Changes
„
„
If class has members added or removed, it
becomes incompatible
java.io.InvalidClassException thrown if you
try to deserialize an incompatible object
stream
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Serial Version
„
„
If the changes were actually compatible
find out the Serial Version UID of the original
class
„
„
use the serialver utility
add a member variable to the changed class
protected static final long serialVersionUID = 2215190743590612933L;
„
now it’s marked as compatible with the old class
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Using readObject
„
„
if you need to force an object to be
compatible
implement readObject() method to make
compatible changes
private void readObject(ObjectInputStream
stream) throws java.io.IOException
{
defaultReadObject(stream);
// do compatible stuff
}
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Callbacks
„
„
„
„
The server calls a remote method on the
client => the server becomes the client,
and the client is now the server
The client registers a notification method
with the server
Server invokes the method (“calls back”)
to notify the client
Registry is out of the loop
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
RMI Security
„
„
„
Server is untrusted
Stubs could be malicious
rmic is OK, but someone could customcode an evil stub: it’s just a .class file
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
RMI Security Managers
„
AppletSecurityManager
„
„
RMISecurityManager
„
„
„
stub can only do what an applet can do
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
„
„
11/18/2003
Stub loading disabled
Stubs still work if they are in local classpath
ECE 451:Introduction to Parallel
and Distributed Programming
Codebase Property
„
Stub classpaths can be confusing
„
„
„
„
The RMI class loader always loads stubs from the
CLASSPATH first
Next, it tries downloading classes from a web
server
„
„
3 VMs, each with its own classpath
Server vs. Registry vs. Client
(but only if a security manager is in force)
java.rmi.server.codebase specifies which web
server
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Codebase and Thread Safety
„
„
„
„
There’s a thread problem with codebase
With a single remote server, and multiple
remote objects on that server, each with its
own stub codebase
One server can step on the codebase property
and foul up the registration for the other
server
In practice, won’t happen very often
„
More likely to happen in servlets
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
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
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
RMI vs. COM
„
„
„
Very similar
remote interfaces ~ type libraries
COM is Win32-only (for now)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Sun vs. Microsoft
„
„
RMI is not shipped as part of Microsoft’s
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
„
„
11/18/2003
include java.rmi.* class files (or rmi.zip) in your
codebase
extra download time
ECE 451:Introduction to Parallel
and Distributed Programming
RMI Chat Server Objects
Message
interface
interface
MessageReceiver
ChatServer
- receiveMessage(Message)
- login(MessageReceiver)
- sendMessage(Message)
ChatClient
implements
remote reference
local reference
11/18/2003
ChatServerImpl
Dispatcher
ECE 451:Introduction to Parallel
and Distributed Programming
MessageQueue
Summary
„
„
„
RMI is a very clean API
Easy way to write distributed programs
Wire protocol may need improvement for
large-scale problems
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
Where to get more information
„
„
„
„
„
„
Core Java Volume II by Cay S Horstmann and
Gray Cornell
Harold, Java Network Programming (O’Reilly)
rmi-users mailing list ([email protected])
http://www.developer.com/ (Gamelan)
http://www.javaworld.com/ (magazine)
http://www.stinky.com/java/ (original Author’s
site)
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming
My sincere thanks
„
Alex Chaffee’s Purple Technology and his
RMI slides which I have used in my
Presentation
11/18/2003
ECE 451:Introduction to Parallel
and Distributed Programming