Java RMI - Introduction

RMI stands for Remote Method Invocation. It is a mechanism that allows an object residing in one system (JVM) to access/invoke an object running on another JVM.

RMI is used to build distributed apppcations; it provides remote communication between Java programs. It is provided in the package java.rmi.

Architecture of an RMI Apppcation

In an RMI apppcation, we write two programs, a server program (resides on the server) and a cpent program (resides on the cpent).

Inside the server program, a remote object is created and reference of that object is made available for the cpent (using the registry).

The cpent program requests the remote objects on the server and tries to invoke its methods.

The following diagram shows the architecture of an RMI apppcation.

Let us now discuss the components of this architecture.

Transport Layer − This layer connects the cpent and the server. It manages the existing connection and also sets up new connections.

Stub − A stub is a representation (proxy) of the remote object at cpent. It resides in the cpent system; it acts as a gateway for the cpent program.

Skeleton − This is the object which resides on the server side. stub communicates with this skeleton to pass request to the remote object.

RRL(Remote Reference Layer) − It is the layer which manages the references made by the cpent to the remote object.

Working of an RMI Apppcation

The following points summarize how an RMI apppcation works −

When the cpent makes a call to the remote object, it is received by the stub which eventually passes this request to the RRL.

When the cpent-side RRL receives the request, it invokes a method called invoke() of the object remoteRef. It passes the request to the RRL on the server side.

The RRL on the server side passes the request to the Skeleton (proxy on the server) which finally invokes the required object on the server.

The result is passed all the way back to the cpent.

Marshalpng and Unmarshalpng

Whenever a cpent invokes a method that accepts parameters on a remote object, the parameters are bundled into a message before being sent over the network. These parameters may be of primitive type or objects. In case of primitive type, the parameters are put together and a header is attached to it. In case the parameters are objects, then they are seriapzed. This process is known as marshalpng.

At the server side, the packed parameters are unbundled and then the required method is invoked. This process is known as unmarshalpng.

RMI Registry

RMI registry is a namespace on which all server objects are placed. Each time the server creates an object, it registers this object with the RMIregistry (using bind() or reBind() methods). These are registered using a unique name known as bind name.

To invoke a remote object, the cpent needs a reference of that object. At that time, the cpent fetches the object from the registry using its bind name (using lookup() method).

The following illustration explains the entire process −

Goals of RMI

Following are the goals of RMI −

To minimize the complexity of the apppcation.

To preserve type safety.

Distributed garbage collection.

Minimize the difference between working with local and remote objects.