Java Network Programming (3rd ed) [Electronic resources] نسخه متنی

18.5 The java.rmi.registry Package

How does a client that needs a remote
object locate that object on a distant server? More precisely, how
does it get a remote reference to the object? Clients find out what
remote objects are available by querying the
server's registry. A registry
advertises the availability of the server's remote
objects. Clients query the registry to find out what remote objects
are available and to get remote references to those objects.
You've already seen one: the
java.rmi.Naming class for interfacing with
registries.

The Registry interface and the
LocateRegistry class allow clients to retrieve
remote objects on a server by name. A RegistryImpl
is a subclass of RemoteObject, which links names
to particular RemoteObject objects. Clients use
the methods of the LocateRegistry class to
retrieve the RegistryImpl for a specific host and
port.

18.5.1 The Registry Interface

The
java.rmi.registry.Registry interface has five
public methods: bind( ), to bind a name to a specific
remote object; list( ), to list all the names
bound in the registry; lookup( ), to find a
specific remote object given its URL; rebind( ),
to bind a name to a different remote object; and unbind(
), to remove a name from the registry. All of these behave
exactly as previously described in the
java.rmi.Naming class, which implements this
interface. Other classes that implement this interface may use a
different scheme for mapping names to particular objects, but the
methods still have the same meaning and signatures.

Besides these five methods, the Registry interface
also has one field,
Registry.REGISTRY_PORT, the default port on
which the registry listens. Its value is 1099.

18.5.2 The LocateRegistry Class

The
java.rmi.registry.LocateRegistry class lets the client find the registry
in the first place. This is achieved with five overloaded versions of
the static LocateRegistry.getRegistry() method:

public static Registry getRegistry( ) throws RemoteException
public static Registry getRegistry(int port) throws RemoteException
public static Registry getRegistry(String host) throws RemoteException
public static Registry getRegistry(String host, int port)
throws RemoteException
public static Registry getRegistry(String host, int port,  // Java 1.2 
RMIClientSocketFactory factory) throws RemoteException

Each of these methods returns a Registry object
that can be used to get remote objects by name.
LocateRegistry.getRegistry( ) returns a stub for
the Registry running on the local host on the
default port, 1,099.
LocateRegistry.getRegistry(int
port) returns a stub for the
Registry running on the local host on the
specified port. LocateRegistry.getRegistry(String
host) returns a stub for the
Registry for the specified host on the default
port, 1,099. LocateRegistry.getRegistry(String
host, int
port) returns a stub for the
Registry on the specified host on the specified
port. Finally, LocateRegistry.getRegistry(String
host, int
port, RMIClientSocketFactory
factory) returns a stub to the registry running on
the specified host and port, which will be contacted using sockets
created by the provided
java.rmi.server.RMIClientSocketFactory object. If
the host String is
null, getRegistry( ) uses the
local host; if the port argument is negative, it
uses the default port. Each of these methods can throw an arbitrary
RemoteException.

For example, a remote object that wanted to make itself available to
clients might do this:

Registry r = LocateRegistry.getRegistry( ); 
r.bind("My Name", this);

A remote client that wished to invoke this remote object might then
say:

Registry r = LocateRegistry.getRegistry("thehost.site.com"); 
RemoteObjectInterface tro = (RemoteObjectInterface) r.lookup("MyName"); 
tro.invokeRemoteMethod( );

The final two methods in the LocateRegistry class
are the overloaded LocateRegistry.createRegistry(
) methods. These create a
registry and start it listening on the specified port. As usual, each
can throw a RemoteException. Their signatures are:

public static Registry createRegistry(int port) throws RemoteException
public static Registry createRegistry(int port, 
RMIClientSocketFactory csf, RMIServerSocketFactory ssf) // Java 1.2
throws RemoteException