Java Remote Method Invocation (RMI)

Java Remote Method Invocation(RMI) Kupás Levente

Introduction • Distributed computing has become very popular in recent years for a few reasons.Java supports distributed computing in a variety of ways; one of the simplest is known as Remote Method Invocation (RMI). • RMI provides a way for Java programs to interact with classes and objects working on different virtual machines on other networked computers. From the perspective of the client, objects on the server may be referenced as if they were local.

Interfaces: The Heart of RMI Specifically, in RMI, the definition of a remote service is coded using a Java interface. The Java interface does not contain executable code. RMI supports two classes that implement the same interface. • The first class is the implementation of the behavior, and it runs on the server. • The second class acts as a proxy for the remote service and it runs on the client. This is shown in the following diagram:

The diagram A client program makes method calls on the proxy object, RMI sends the request to the remote JVM, and forwards it to the implementation. Any return values provided by the implementation are sent back to the proxy and then to the client's program.

RMI Architecture Layers Java RMI is comprised of three layers that support the interface.

Layers • The first layer is the Stub/Skeleton Layer. This layer is responsible for managing the remote object interface between the client and server. • The second layer is the Remote Reference Layer (RRL). This layer is responsible for managing the "liveliness" of the remote objects. It also manages the communication between the client/server and virtual machine s, for remote objects. • The third layer is the transport layer. This is the actual network/communication layer that is used to send the information between the client and server over the wire. It is currently TCP/IP based.

TCP/IP For any successfully communication the mutual language condition is necessary. The TCP is a higher-level protocol that manages state and error correction automatically. • TCP/IP provides a persistent, stream-based connection between two machines based on an IP address and port number at each end. • The IP is 32 bites number. This number is individual for any computer, but can be more IP number in same computer (depending for network card). • Usually a DNS name is used instead of an IP address. In the current release of RMI, TCP/IP connections are used as the foundation for all machine-to-machine connections.

Remote Classes and Interfaces A Remote class has two parts: • The interface • The class itself. • The Remote interface must have the following properties: • The remote interface must be public. • The remote interface must extend the interface java.rmi.Remote. • Each method in the remote interface must declare java.rmi.RemoteExceptionin its throwsclause in addition to any application-specific exceptions. • A remote object passed as an argument or return value must be declared as the remote interface, not the implementation class.

Remote Classes and Interfaces • The Remote class itself has the following properties: • It must implement a Remote interface. • It should extend the java.rmi.server.UnicastRemoteObject class. Objects of such a class exist in the address space of the server and can be invoked remotely.. • It can have methods that are not in its Remote interface. These can only be invoked locally.

Remote Classes and Interfaces • The Server requires the definition of both the Remote class and the Remote interface, but the Client only uses the Remote interface. If a remote object is being used remotely, its type must be declared to be the type of the Remote interface, not the type of the Remote class.

Using RMI In the next, we will build a simple RMI system in a step-by-step fashion. • Assuming that the RMI system is already designed, we take the following steps to build a system: • Write and compile Java code for interfaces • Write and compile Java code for implementation classes • Generate Stub and Skeleton class files from the implementation classes • Write Java code for a remote service host program • Develop Java code for RMI client program • Install and run RMI system

Running the RMI System • We need to start three consoles, one for the server, one for the client, and one for the RMIRegistry. • Start with the Registry. >rmiregistry The rmiregistry command creates and starts a remote object registry on the specified port (default=1099) on the current host. If all goes well, the registry will start running and we can switch to the next console.

Running the RMI System • In the second console start the server hosting the AddServer, and enter the following: >java AddServer It will start, load the implementation into memory and wait for a client connection. • In the last console, start the client program. >java AddClient • That's it; we have created a working RMI system. Even though we ran the three consoles on the same computer, RMI uses your network stack and TCP/IP to communicate between the three separate JVMs. This is a full-fledged RMI system

Java Remote Method Invocation (RMI)