Learning Java

Remote Method Invocation (RMI)

The most fundamental means of inter-object communication in Java is method invocation. Mechanisms like 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. Java’s Remote Method Invocation mechanism does just that. It lets us get a reference to an object on a remote host and use it 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 and RMI is that RPC, being an offshoot of the C language, is primarily concerned with data structures. It’s relatively easy to pack up data and ship it around, but for Java, that’s not enough. In Java we don’t just work with data structures; we work with objects, which 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 also the recipient has to be able to interact with the object (use its methods) after receiving it.

It should be no surprise that RMI uses object serialization, which allows us to send ...