Java Reference
In-Depth Information
turn any results. (Traditionally, the application invoking a remote meth-
od is called the
client
of the invocation, and the application executing the
method is called the
server,
although a client of one invocation may be
the server of another.)
Task
extends
Serializable
because all local types
passed to or returned from a remote method must be serializable.
Each method of a remote interface is required to throw
RemoteException
because any invocation of a remote method can have failures that must
be signaled. Recovery from these failures is unlike recovery in local com-
putation, where methods always arrive at their intended objects and
either results are always returned or you clearly know that they were
not returned.
When you invoke a remote method, network failures create new uncer-
taintiesif the network fails after the request is transmitted from the cli-
ent, the client doesn't know whether the request was received. The fail-
ure may have happened before the invocation reached the server, or it
may have happened after it reached the server but before results could
be returned. There is no way for the client to know which of these two
cases actually happened. If the request is to withdraw money from your
bank account, you would not want to simply retransmit the requestit
might get there twice. Remote interfaces must be designed to allow
clients to recover from such
partial failures.
Methods may be
idempo-
tent
meaning the method can safely be reinvokedor some other recovery
method may be provided for the interface, such as transactions that can
be aborted when non-idempotent methods fail. The users of a remote
object must recover from such failures, and the declared
RemoteException
helps them do so.
Here is a simple implementation of the
ComputeServer
interface:
import java.rmi.*;
import java.rmi.server.*;
public class ComputeServerImpl
extends UnicastRemoteObject
implements ComputeServer
{
public ComputeServerImpl() throws RemoteException { }
