Java RMI (Remote Method Invocation)

RMI allows client Java objects to invoke remote Java objects as if they were local to the client. The RMI architecture separates the code that defines the behaviour and the code that implements the behaviour to remain separate and run on separate JVM's. Figure 2 illustrates this separation.

Figure 2. Separation of behaviour and implementation

It should be noted that a Java interface contains no executable code. Instead RMI supports two classes that implement the same interface. The first class, run on the server, is the implementation of the behaviour and the second class, run on the client, acts as a proxy for the remote service. Figure 3 illustrates this structure.

Figure 3. RMI implementation structure

When a method is 'called on the remote object' the call is actually made on the proxy object or stub object. RMI sends the request to the remote JVM, and forwards it to the implementation. Return values produced by the implementation are sent back to the proxy and then to the client [2].

Architecture
RMI is formed of three abstraction layers:
Stub and Skeleton - RMI uses a proxy pattern to forward method calls between the participating objects, Figure 4 illustrates the stub and skeleton role.

Figure 4. role of the stub and the skeleton

Remote Reference Layer - This layer understands how to interpret and manage references made from clients to the remote service objects.

Transport Layer - Based on TCP/IP connections between machines in a network, this layer provides basic connectivity, as well as some firewall penetration strategies.

Naming Remote Objects
When a server program creates an object (implementing a service) it registers the object in the RMI Registry under a public name.
The client locates the remote server via the static lookup() method of rmi.Naming class. The lookup() method accepts a URL that specifies the server host name and the name of the desired service. The method returns a remote reference to the service object. The URL takes the form:

rmi://<host_name>
[:<name_service_port>]
/<service_name>

where the host_name is a name recognized on the local area network (LAN) or a DNS name on the Internet. The name_service_port only needs to be specified only if the naming service is running on a different port to the default [2].

As with the implementation of traditional classes the server has to set a securityManager and bind itself to the local rmiregistry in order to be available to it's clients. Remote methods must throw RemoteException's to indicate problems in the remote connection.

All the required classes for RMI implementation are included in the JDK development kit (java.rmi.*)

 

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