Java Reference
In-Depth Information
Next, we have the server
main()
method. It first sets a security manager.
The security manager controls what the VM is allowed to do. A number of
default security managers are provided, and here we use one that is designed
specifically to give safe and reasonable defaults for RMI applications. You can,
of course, write your own security manager. Security managers use “policy
specifications” to alter their capabilities. For now, we will explain enough to
run a simple example. See Section 5.8.4.2 for more information on policies for
our example.
Remember that
main()
is static, so there is no instance of
SessionImpl
yet, and thus also no instance of
Session
. We declare a variable of type
Session
, and set it to a new instance of
SessionImpl
. (There is no need
to typecast here because
SessionImpl
implements
Session
, therefore
SessionImpl
is, among other things, a
Session
.) We now have an instance
of the server class.
Next, the server must make itself available to the world. It does this by
registering itself with the RMI registry (see Section 5.8.3). This is done through
a static method of the
java.rmi.Naming
class,
rebind()
. Put simply, this
maps a remote object to a string name in the registry. When clients contact the
registry looking for a name then, if a remote object is mapped to that name,
the communication can take place (yes, we are simplifying at the moment).
The call to
rebind()
does not return. The server is up and running.
Finally, we have the implementation of our remote method,
add()
.
This looks like a lot of hassle to go through, and it is, but consider writing
an interface that offers, for example, methods like
getDirContents()
,
chDir()
,
downloadFile()
,
uploadFile()
. You've just written something
like an FTP server. No matter how many methods you add to your interface,
the complexity of the setup code does not increase. Maybe now it looks a little
more useful?
5.8.1.5
At this point, Example 5.12 should be fairly obvious. Our class has just a single
static method,
main()
. It, like our server side
main()
, sets up a security man-
ager. It then contacts a registry on the machine named
penfold
looking for an
instance of a remote interface named
Session
(again,
lookup()
is a static
method of the
java.rmi.Naming
class). We store that reference in a variable
of type
Session
called
sess
. We can then call the
add()
on
sess
. We'll show
the server and client running shortly.
Writing the Client Class
