Remote Method Invocation
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Java's Remote Method Invocation (commonly referred to as RMI) is used for client and server models. RMI is the object oriented equivalent to RPC (Remote procedure call).
The Java Remote Method Invocation (RMI) system allows an object running in one Java Virtual Machine (VM) to invoke methods on an object running in another Java VM. RMI provides for remote communication between programs written in the Java programming language.
RMI is defined to use only with the Java platform. If you need to call methods between different language environments, use CORBA. With CORBA a Java client can call C++ server and/or a C++ client can call a Java server. With RMI that can not be done.
STUB and SKELETON
The remote method invocation goes through a STUB on the client side and a so called SKELETON on the server side.
CLIENT --> STUB --> ... Network ... --> SKELETON --> REMOTE OBJECT
Prior to Java 1.2 the skeleton had to be explicitly generated with the rmic tool. Since 1.2 a dynamic skeleton is used, which employs the features of Java Reflection to do its work.
Remote objects can be listed in the RMI Registry. Clients can get a reference to the remote object by querying the Registry. After that, the client can call methods on the remote objects. (Remote object references can also be acquired by calling other remote methods. The Registry is really a 'bootstrap' that solves the problem of where to get the initial remote reference from.)
The RMI Registry can either be started within the server JVM, via the LocateRegistry.createRegistry() API, or a separate process called rmiregistry that has to be started before remote objects can be added to it, e.g. by the command line in Unix:
|rmiregistry on Unix
rmiregistry <port> &
or under Windows:
|rmiregistry on Windows
start rmiregistry <port>
If port is not specified the default 1099 is used. The client will need to connect to this port to access the Registry.
The Registry can also be started from a program by calling the following code:
|Code section 1: rmiregistry starting
import java.rmi.registry.LocateRegistry; ... Registry reg = LocateRegistry.createRegistry(iPort);
Objects passed in as parameters to the remote objects's methods will be passed by value. If the remote object changes the passed-in object values, it won't be reflected on the client side, this is opposite what happens when a local object is called. Objects that used as parameters for remote methods invocation must implement the
java.io.Serializable interface, as they are going to be serialized when passed through the network, and a new object will be created on the other side.
However, exported remote objects passed as parameters are passed by remote reference.
RMI Remote object
The remote object has to either extend the
java.rmi.server.UnicastRemoteObject object, or be explicitly exported by calling the
Here is an example of RMI client:
|Code listing 7.10: HelloClient.java