Learning Java, 4th Edition

Remote Method Invocation

The most fundamental means of communication in Java is method invocation. Mechanisms such as the Java event model are built on simple method invocations between objects in the same virtual machine. Therefore, when we want to communicate between virtual machines on different hosts, it’s natural to want a mechanism with similar capabilities and semantics—to run a method “over there.” Java’s RMI mechanism does just that. It lets us get a reference to an object on a remote host and use it almost as if it were in our own virtual machine. RMI lets us invoke methods on remote objects, passing real Java objects as arguments and getting real Java objects as returned values.

Remote invocation is nothing new. For many years, C programmers have used remote procedure calls (RPC) to execute a C function on a remote host and return the results. The primary difference between RPC in other languages and RMI is that RPC is usually primarily concerned with data structures. It’s relatively easy to pack up data and ship it around, but RMI tries to do one better. In Java, we don’t just work with data structures; we work with objects that contain both data and methods for operating on the data. Not only do we have to be able to ship the state of an object (the data) over the wire, but the recipient has to be able to interact with the object (use its methods) after receiving it. With Java RMI, you can work with network services in an object-oriented fashion, using real, extensible ...