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Java RMI …related technologies

RPC  (“Remote Procedure Calls”) Developed by Sun

Platform-specific

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CORBA  (“Common Object Request Broker Architecture”)

Developed by OMG

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Access to non-Java objects (as well as Java)

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DCOM  (“Distributed Common Object Model”)

Developed by Microsoft

Access to Win32 objects

LDAP  (“Lightweight Directory Access Protocol”)

Finding resources on a network

Earlier days

Only local objects existed

My Machine

My Object

Today….

Network and Distributed Objects

My Machine

Remote Machine

Local Objects

Remote Objects

Java RMI allows...

• provide user with a “thin client “
• allows good performance on low end workstations
• run server on high end hardware
• maximize $ investment over many clients
• server remote from client
• Distributed network object

Architecture

Client

Server

Stubs

Skeletons

Remote Reference

Remote Reference

Transport

Architecture

The General Idea

• Instantiate an object on another machine
• Invoke methods on the remote object

The parts...

• Client - user interface
• Server - data source
• Stubs
• marshals argument data (serialization)
• unmarshals results data (deserialization)
• Skeletons (not reqd w/Java 2)
• unmarshals argument data
• marshals results data

The parts... (cont.)

• Remote Reference Layer
• provides a RemoteRef object that represents the link to the remote service implementation object.
• encodes and decodes the on-wire protocol
• implements the remote object protocols
• Transport layer
• The Transport Layer makes the connection between JVMs. All connections are stream-based network connections that use TCP/IP.
• handles the underlying socket handling required for communications
• sets up and maintains connections
• communications related error handling

The steps...

• Create the Interface to the server
• Create the Server
• Create the Client
• Compile the Interface (javac)
• Compile the Server (javac)
• Compile the Client (javac)
• Generate Stubs and Skeletons (rmic)

To run

• Start the RMI registry
• rmiregistry is in the JSDK bib directory
• Start the RMI Server
• Start the RMI Client

RMI Registry

• The RMI Registry is a naming service provided with the JDK as a teaching tool or for a small number of Remote Objects
• Uses port 1099 as its default port
• Can be considered to be a reference implementation
• runs out of steam above a 100 objects
• runs on same machine as the remote object

RMI Registry (more)

JNDI (Java Naming and Directory Interface)

• J2EE (Jakarta EE) uses JNDI as a more powerful alternative to the RMI Registry.
• JNDI integrates with directory services like LDAP, CORBA Naming Service, DNS, and NIS.
• Unlike the RMI Registry, JNDI provides:
• Persistent storage of registered objects.
• Distributed lookup of objects across different machines.
• Scalability to support large enterprise applications.
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Activatable Objects

• Added in Java 2 SDK
• Standard RMI objects exported as UnicastRemoteObject must run continuously
• instead by implementing java.rmi.activation.Activatable the object can be deactivated and reactivated remotely when a method call is made
• Must use the rmid server process to take advantage of this capability

Java Remote Method Protocol (JRMP)

• Proprietary, stream-based protocol that is only partially specified is now in two versions
• JDK 1.1 version of RMI and required the use of Skeleton classes on the server
• Java 2 SDK. It has been optimized for performance and does not require skeleton classes
• some implementations, such as BEA Weblogic and NinjaRMI don’t use JRMP instead use their own wire level protocol

Other JDK 1.1 and Java 2 Differences

• With the Java 2 SDK
• Service interfaces are not required to extend from java.rmi.Remote
• Service methods do not necessarily throw RemoteException.

Java 2 JSDK 1.3

• RMI-IIOP
• instead of using JRMP RMI will use Internet Inter-Orb Protocol (IIOP)
• IIOP is the wire protocol used for communication between Common Object Request Broker Architecture (CORBA) clients and servers
• CORBA is a distributed object technology from the Object Management Group (OMG)
• 800 member industry group
• vendor neutral architecture
• sets the direction for RMI/CORBA Integration
• more on CORBA later