Java Remote Method Invocation RMI

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Java Remote Method Invocation (RMI)

• Yuan Chen
• (yuanchen_at_cc.gatech.edu)

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Related Technologies

• Message Passing (sockets)
• Flexible and sufficient for general communication
• Exchange protocol complicated and error-prone
• RPC (Remote Procedure Call)
• Platform-specific
• Procedure call
• Static stub
• CORBA
• System heterogeneity, language heterogeneity
• Language neutral object model
• Access to non-Java objects (as well as Java
  objects)
• DCOM
• Access to Win32 objects

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Goals

• Language integration simplicity and naturalness
• Enable distributed computing and communication
  but maintain language-specific (Java) object
  semantics as much as possible
• Similar interfaces as if object local, preserve
  some behavior
• Keep it simple expose exceptions related to
  distributed interaction
• System Integration
• Use safety, runtime class loading, security,
  garbage collection already provided by JVM
• Support different invocation semantics
• unicast, multicast (like with Subcontracts in
  Spring)
• Support different reference semantics
• persistent, non-persistent references to remote
  objects
• Multiple transports for pair of endpoints
• UDP, TCP (like in Network Objects)

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Distributed Object Model

• Distributed Object Model
• Remote object
• Live on server
• Object whose methods can be accessed from another
  address space
• Remote interface
• Declare exposed methods
• Like a proxy for the remote object
• Remote References
• Refer to remote objects
• Invoked on client exactly like local object
  references
• RMI
• Non-remote object passed by value
• Remote object passed by reference
• RemoteException

• Java Object Model
• Object
• Interface
• declare a set of methods for Java object without
  implementation
• Method Invocation
• primitive type passed by value
• object passed by reference

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Object
classes
Interfaces
Remote
RemoteObject
IOException
RemoteServer
RemoteException
UnicastRemoteObject
Remote Interface
extension
implementation
Stub
Server Impl
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• Stub
• Live on client
• Pretends to be remote object
• Skeleton
• Live on server
• Receive requests from stub
• Talk to true remote object
• Deliver response to stub

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Java RMI Details

• Remote reference
• Client stubs have same remote interface as the
  remote object, therefore Java treats them as
  objects with same type
• Remote method invocation
• Same syntax, but exceptions have to be caught
• Exception tells only that something happened
• Some semantics have to change
• Client has a reference to remote object
• Methods of class Object (equal, hashCode) dont
  involve a remote call, so can operate on
  reference
• Registry
• A registry is a running process on a host machine
• Name and look up remote objects
• Servers can register their objects
• Clients can find server objects and obtain a
  remote reference

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Remote Interface vs. Remote Class

• Remember that the reference is to an interface
• You must make references, arrays, etc. out of the
  interface type, not the implementation type
• You cant cast the remote reference to a normal
  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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Java RMI Layers
Java Virtual Machine
Java Virtual Machine
Client Object
Remote Object
Stub
Skeleton
TCP
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System Architecture
Client
Server
Application
Stubs
Skeleton
Remote Reference layer
RMI System
Transport

• Stub/Skeleton layer client proxy and server
  dispatcher
• Remote reference layer
• Invocation semantics unicast, multicast,
  replication strategy
• Reference semantics live or persistent,
  reconnection strategies
• Transport layer actual connection setup and
  management
• Deals with communications
• Connection management
• Dispatching messages between stub and skeleton
• Distributed Garbage Collection

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Pickling

• The process of creating a serialized
  representation of objects
• Pickling defines the serialized form
• Meta information type of each object,
  relationships between objects
• Values
• Used for object transmission across address space
• Serialized representation of objects
• object type
• object persistent state
• relationships between objects within a stream
• object implementation info
• Pickling remote object
• Stubs/skeletons marshal/unmarshal pickled
  streams
• When marshalling remote object, marshal stream
  embeds URL info of the stub code
• On unmarshalling, marshal stream loads stub code
  if its not available locally

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Dynamic Stub Loading
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Remote Method Call
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Enhancements in J2SE 5.0

• Dynamic Generation of Stub Classes
• Standard SSL/TLS Socket Factory Classes
• Launching rmid or a Java RMI Server from
  inetd/xinetd
• Serialization Enhancements

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Summary

• RMI is a very clean API
• Access to Remote Object
• Java-to-Java only
• Client-Server Protocol
• High-level API
• Transparent
• Lightweight
• Easy way to write distributed programs
• Wire protocol may need improvement for
  large-scale problems

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JEDI Example
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Summary

• Simple and flexible scheme
• Dynamic method invocation
• Scalable
• interacting with many remote objects at the same
  time
• Lack some RMI functionality
• Static type-check
• Large data (JEDI cannot support more than 64K of
  data)
• Advanced security capability
• Persistent objects
• Natural remote call semantics