RMI Concepts and Functions Quiz

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Questions and Answers

What is the primary function of the Stub/Skeleton Layer in RMI?

It intercepts method calls made by the client and redirects them to the remote object.

Explain how location transparency is achieved in RMI.

Location transparency is achieved as the client interacts with the remote object as if it were local, while the RMI infrastructure manages the underlying communication.

What is required for a client to invoke methods on a remote object in RMI?

The client must cast the reference to the remote interface of the object.

Identify the three abstraction layers of the RMI implementation.

The three layers are the Stub/Skeleton Layer, the Remote Reference Layer, and the Transport Layer. Signup and view all the answers

How does RMI handle garbage collection for remote objects?

RMI includes garbage collection management, even for remote objects, ensuring resources are freed appropriately. Signup and view all the answers

What is the primary purpose of RMI in Java?

The primary purpose of RMI is to enable remote object-to-object communication between different Java Virtual Machines, allowing methods to be invoked on objects located remotely. Signup and view all the answers

How does RMI achieve transparency in method invocation?

RMI achieves transparency by allowing method invocations on remote objects to use the same syntax as local method invocations, making it indistinguishable for the application whether the object is local or remote. Signup and view all the answers

What is a key difference between RMI and basic socket programming?

A key difference is that RMI provides an abstraction for remote method calls while sockets require custom application-level protocols for communication, which can be complex and error-prone. Signup and view all the answers

Define the roles of 'client' and 'server' in the context of RMI.

'Client' refers to the object that invokes a remote method, while 'server' refers to the remote object containing the methods that can be invoked. Signup and view all the answers

What makes RMI a powerful feature for distributed applications in Java?

RMI's ability to dynamically load new classes and pass objects that a foreign virtual machine has never encountered before enhances its flexibility and power in distributed computing. Signup and view all the answers

What role does the skeleton play in the RMI infrastructure when invoking a remote method?

The skeleton unmarshals the parameters, invokes the remote method on the implementation, and marshals the result back to the caller. Signup and view all the answers

Describe the function of the Remote Reference Layer in RMI.

The Remote Reference Layer defines the invocation semantics and maintains the session during the method call. Signup and view all the answers

How has the JRMP protocol evolved since JDK 1.2 in relation to the use of skeletons?

The JRMP protocol was modified to eliminate the need for skeletons, utilizing reflection to connect to remote service objects. Signup and view all the answers

Explain the significance of the transport layer in RMI.

The transport layer establishes TCP/IP connections between JVMs and manages those connections, even over networking obstacles. Signup and view all the answers

How do clients locate remote services in RMI, and what role does the naming service play?

Clients locate remote services using a naming service, which acts as a directory maintaining a mapping of names to remote objects. Signup and view all the answers

What is the role of the RMI Registry in the RMI architecture?

The RMI Registry acts as a directory for remote objects, allowing clients to look up remote objects by name and obtain references to them. Signup and view all the answers

How does a client obtain a reference to a remote object in RMI?

A client obtains a reference to a remote object by using the <code>java.rmi.Naming</code> class to perform a lookup with the object's name. Signup and view all the answers

Explain the purpose of the `java.rmi.Naming` class in the RMI system.

The <code>java.rmi.Naming</code> class serves as a client for the RMI Registry, used for binding remote objects on the server side and looking them up on the client side. Signup and view all the answers

What format must the name used in `Naming.bind()` follow?

The name in <code>Naming.bind()</code> must be in the RMI URL format, which specifies how remote objects are identified in the registry. Signup and view all the answers

What happens when a client performs a `lookup` using the `java.rmi.Naming` class?

When a client performs a <code>lookup</code>, it queries the RMI Registry and retrieves a stub reference to the specified remote object. Signup and view all the answers

Study Notes

RMI (Remote Method Invocation)

Java allows development of distributed applications.
Distributed computing spreads programs, data processing, and calculations across a network.
This can be for leveraging processing power or due to application design.
RMI enables object-to-object communication between Java Virtual Machines (JVMs).
JVMs can be on the same or different computers.
One JVM can invoke methods on an object in another JVM.
This allows dynamic loading of classes.

Other Alternatives to RMI

Sockets: Basic network programming; flexible but complex protocols increasing error proneness.
EJB: Enterprise JavaBeans.
CORBA: Common Object Request Broker Architecture.
DCOM: Distributed Component Object Model.

RMI Architecture

RMI aims to make objects in separate JVMs appear and act as local objects.
The JVM making the call is the client.
The JVM containing the object is the server.
RMI design prioritizes transparency.
Clients don't differentiate between local and remote objects.
Server refers to a single remote object with invokable methods.
Client refers to the object invoking a remote method.
Objects can act as both client and server.

RMI Layers

Stub/Skeleton Layer: This layer intercepts client method calls and redirects them to remote objects.
- Stubs are on the client side.
- Skeletons are on the server side.
- This utilizes proxy design pattern.
- Stub acts as a proxy for remote service.
- Skeletons handle method calls from the stub to the actual object.
Remote Reference Layer : Defines and manages the RMI connection semantics.
- Manages the session during method calls.
Transport Layer: Establishes stream-based connections (TCP/IP) between JVMs.
- Handles communication between client and server.
- TCP/IP is the communication protocol.
- Uses JRMP protocol (Java Remote Method Protocol).

Locating Remote Objects

Clients use a naming/directory service.
This service is typically a naming service that is run on a host with a publicly known port.
RMI Registry (java.rmi.registry.Registry): a registry object that serves as a directory service.
- Keeps a hash table mapping of names to remote objects. -Each machine can have its own registry.
- RMI Registry listens on default port 1099.
Remote objects have names in the registry.
- Clients look up these names to get object references.

The java.rmi.Naming class

This class acts as a client to the RMI Registry.
It can be used on server side or client side.
Allows binding and looking up remote objects.
- Static methods to bind, rebind, and lookup.
- URLs have the form rmi://<host_name>[:<name_service_port>]/<service_name>

Developing RMI Applications

Steps for creating RMI applications:
1. Define the remote interface.
2. Implement the remote interface.
3. Code for registering the object.
4. Write the client using the remote object.
5. Generate stubs and skeletons using the rmic tool, defining the classpath.
6. Run the RMI registry, server, and client.

Remote Interface Definition

The interface defines methods that can be invoked remotely.
It must extend java.rmi.Remote.
Method signatures must include java.rmi.RemoteException.

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Description

Test your knowledge on the Remote Method Invocation (RMI) in Java. This quiz covers key concepts such as the roles of the stub and skeleton layers, location transparency, and the RMI architecture. Dive into the intricacies of how RMI enables powerful distributed applications.