A Note on Remote Method Invocation (RMI) When passing parameters to remote methods, things get a bit more complex. First, we're (usually) dealing with passing data between two independent virtual machines, which might be on separate physical machines as well. Passing the value of a pointer wouldn't do any good, as the target virtual machine doesn't have access to the caller's heap.
You'll often hear "pass by value" and "pass by reference" used with respect to RMI. These terms have more of a "logical" meaning, and really aren't correct for the intended use.
Here's what is usually meant by these phrases with regard to RMI. Note that this is not proper usage of "pass by value" and "pass by reference" semantics:
RMI Pass-by-value The actual parameter is serialized and passed using a network protocol to the target remote object. Serialization essentially "squeezes" the data out of an object/primitive. On the receiving end, that data is used to build a "clone" of the original object or primitive. Note that this process can be rather expensive if the actual parameters point to large objects (or large graphs of objects).
This isn't quite the right use of "pass-by-value"; I think it should really be called something like "pass-by-memento". (See "Design Patterns" by Gamma et al for a description of the Memento
pattern).
RMI Pass-by-reference The actual parameter, which is itself a remote object, is represented by a proxy. The proxy keeps track of where the actual parameter lives, and anytime the target method uses the formal parameter, another remote method invocation occurs to "call back" to the actual parameter. This can be useful if the actual parameter points to a large object (or graph of objects) and there are few call backs.
This isn't quite the right use of "pass-by-reference" (again, you cannot change the actual parameter itself). I think it should be called something like "pass-by-proxy". (Again, see "Design Patterns" for descriptions of the Proxy pattern).
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http://java.sys-con.com/read/35848.htm Some other non-CORBA compliant Object Request Brokers also support passing objects by value. For example, in Remote Method Invocation (RMI), objects are categorized as either local or remote at the time the object is implemented. This is done by implementing the Java interface java.rmi.Remote to indicate that an object is remote. All other objects are treated as local objects. When determining what argument passing policy to use, RMI depends on these categories. Remote objects are passed by reference, while local objects are passed by value.
[ July 25, 2006: Message edited by: My First My Last ]
