Transcript Remote Method Invocation

Advanced Programming
Rabie A. Ramadan
Lecture 4
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A Simple Use of
Java Remote Method Invocation (RMI)
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“The network is the computer”*
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Consider the following program organization:
method call
SomeClass
AnotherClass
returned object
computer 1
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computer 2
If the network is the computer, we ought to be able to put the two
classes on different computers
RMI is one technology that makes this possible
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RMI and Other Technologies
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CORBA (Common Object Request Broker Architecture) has long
been king
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CORBA supports object transmission between virtually any languages
Objects have to be described in IDL (Interface Definition Language), which
looks a lot like C++ data definitions
CORBA is complex and flaky
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Microsoft supported CORBA, then COM, now .NET
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RMI is purely Java-specific
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Java to Java communications only
As a result, RMI is much simpler than CORBA
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What is needed for RMI?
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Java makes RMI (Remote Method Invocation) fairly easy, but there are
some extra steps
To send a message to a remote “server object,”
• The “client object” has to find the object
• Do this by looking it up in a registry
• The client object then has to marshal the parameters (prepare them for
transmission)
• Java requires Serializable parameters
• The server object has to unmarshal its parameters, do its computation,
and marshal its response
• The client object has to unmarshal the response
Much of this is done for you by special software
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Terminology
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A remote object is an object on another computer
The client object is the object making the request (sending a
message to the other object)
The server object is the object receiving the request
As usual, “client” and “server” can easily trade roles (each can
make requests of the other)
The rmiregistry is a special server that looks up objects by
name
• Hopefully, the name is unique!
rmic is a special compiler for creating stub (client) and skeleton
(server) classes
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Processes
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For RMI, you need to be running three processes
• The Client
• The Server
• The Object Registry, rmiregistry, which is like a DNS
service for objects
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You also need TCP/IP active
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Interfaces
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Interfaces define behavior
Classes define implementation
Therefore,
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In order to use a remote object, the client must know its behavior
(interface), but does not need to know its implementation (class)
In order to provide an object, the server must know both its interface
(behavior) and its class (implementation)
In short,
• The interface must be available to both client and server
• The class should only be on the server
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Classes
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A Remote class is one whose instances can be accessed
remotely
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On the computer where it is defined, instances of this class can be
accessed just like any other object
On other computers, the remote object can be accessed via object
handles
A Serializable class is one whose instances can be marshaled
(turned into a linear sequence of bits)
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Serializable objects can be transmitted from one computer to
another
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Conditions for serializability
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If an object is to be serialized:
• The class must be declared as public
• The class must implement Serializable
• The class must have a no-argument constructor
• All fields of the class must be serializable: either
primitive types or serializable objects
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The Remote Interface
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In RMI, a common remote interface is the minimum amount of information
that must be shared in advance between “client” and “server” machines. It
defines a high-level “protocol” through which the machines will communicate.
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A remote interface is a normal Java interface, which must extent the marker
interface java.rmi.Remote.
• Corollaries: because the visible parts of a remote object are defined
through a Java interface, constructors, static methods and non-constant
fields are not remotely accessible (because Java interfaces can’t contain
such things).
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All methods in a remote interface must be declared to throw the
java.rmi.RemoteException exception.
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A Simple Example
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A file MessageWriter.java contains the interface
definition:
import java.rmi.* ;
public interface MessageWriter extends Remote {
void writeMessage(String s) throws RemoteException ;
}
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This interface defines a single remote method,
writeMessage().
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java.rmi.Remote
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The interface java.rmi.Remote is a marker
interface.
It declares no methods or fields; however,
extending it tells the RMI system to treat the
interface concerned as a remote interface.
• In particular we will see that the rmic compiler
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generates extra code for classes that implement remote
interfaces.
This code allows their methods to be called remotely.
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java.rmi.RemoteException
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Requiring all remote methods be declared to throw RemoteException was a
philosophical choice by the designers of RMI.
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RMI makes remote invocations look syntactically like local invocation.
In practice, though, it cannot be defend from problems unique to distributed
computing—unexpected failure of the network or remote machine.
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Forcing the programmer to handle remote exceptions helps to encourage
thinking about how these partial failures should be dealt with.
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See the influential essay: “A Note on Distributed Computing” by Waldo et al,
republished in The Jini Specification:
http://java.sun.com/docs/books/jini
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The Remote Object
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A remote object is an instance of a class that implements a remote interface.
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Most often this class also extends the library class
java.rmi.server.UnicastRemoteObject.
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This class includes a constructor that exports the object to the RMI system
when it is created, thus making the object visible to the outside world.
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Usually you will not have to deal with this class explicitly—your remote
object classes just have to extend it.
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One fairly common convention is to name the class of the remote object after
the name of the remote interface it implements, but append “Impl” to the end.
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A Remote Object
Implementation Class
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The file MessageWriterImpl.java contains the class declaration:
import java.rmi.* ;
import java.rmi.server.* ;
public class MessageWriterImpl extends UnicastRemoteObject
implements MessageWriter {
public MessageWriterImpl() throws RemoteException {
}
public void writeMessage(String s) throws RemoteException {
System.out.println(s) ;
}
}
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Compiling the Remote Object
Class
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To compile classes that implement Remote,
you must use the rmic compiler. The reasons
will be discussed later. For example:
rabie$ rmic MessageWriterImpl
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Client and Server Programs
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We have completed the Java files for the remote object class
itself, but we still need the actual client and server programs
that use this class.
In general there are some pieces of administrivia one has to
deal with—publishing class files and installing security
managers.
We initially make the simplifying assumption that both client
and server have copies of all class files for MessageWriter.
• Then “publishing class files” is not an issue, and we also
don’t need a security manager, because all code is “local”,
and therefore trusted.
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A Server Program
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We assume the file HelloServer.java contains the class
declaration:
import java.rmi.* ;
public class HelloServer {
public static void main(String [] args) throws Exception {
MessageWriter server = new MessageWriterImpl() ;
Naming.rebind(“messageservice”, server) ;
}
}
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Remarks
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This program does two things:
• It creates a remote object with local name server.
• It publishes a remote reference to that object with external
name “MessageWriter”.
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The call to Naming.rebind() places a reference to
server in an RMI registry running on the local host (i.e.,
the host where the HelloServer program is run).
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Client programs can obtain a reference to the remote
object by looking it up in this registry.
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A Client Program
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We assume the file HelloClient.java contains the class declaration:
import java.rmi.* ;
public class HelloClient {
public static void main(String [] args) throws Exception {
MessageWriter server =
(MessageWriter) Naming.lookup(
“rmi://rabieramadan.org/messageservice”) ;
server.writeMessage(“Hello, other world”) ;
}
}
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Remarks
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Again the program does two things:
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It looks up a reference to a remote object with external name
“MessageWriter”, and stores the returned reference with local name
server.
Finally (!), it invokes the remote method, writeMessage(), on server.
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The call to Naming.lookup() searches in a remote RMI registry. Its
argument is a URL, with protocol tag “rmi”.
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This example assumes the remote object lives on the host
“rabieramadan.org”, and has been registered in the default RMI
registry (which happens to listen on port 1099) on that machine.
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Compiling and Running the
Example
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Compile HelloServer and HelloClient on their
respective hosts, e.g.:
sirah$ javac HelloServer
merlot$ javac HelloClient
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Either ensure client and server share the current
directory, or copy all files with names of the form
MessageWriter * .class to the client’s current
directory.
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Running HelloClient/HelloServer
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