Java Remote Method Invocation: 6 - Registry Interfaces

• The Registry Interface
• The LocateRegistry Class
• The RegistryHandler Interface

The RMI system uses the java.rmi.registry.Registry interface and the java.rmi.registry.LocateRegistry class to provide a well-known bootstrap service for retrieving and registering objects by simple names.

A registry is a remote object that maps names to remote objects. Any server process can support its own registry or a single registry can be used for a host.

The methods of LocateRegistry are used to get a registry operating on a particular host or host and port. The methods of the java.rmi.Naming class makes calls to a remote object that implements the Registry interface using the appropriate LocateRegistry.getRegistry method.

6.1 The Registry Interface

See the Registry API documentation.

6.2 The LocateRegistry Class

The class java.rmi.registry.LocateRegistry is used to obtain a reference (construct a stub) to a bootstrap remote object registry on a particular host (including the local host), or to create a remote object registry that accepts calls on a specific port.

The registry implements a simple flat naming syntax that associates the name of a remote object (a string) with a remote object reference. The name and remote object bindings are not remembered across server restarts.

Note that a getRegistry call does not actually make a connection to the remote host. It simply creates a local reference to the remote registry and will succeed even if no registry is running on the remote host. Therefore, a subsequent method invocation to a remote registry returned as a result of this method may fail.

package java.rmi.registry;

public final class LocateRegistry {

        public static Registry getRegistry()
                throws java.rmi.RemoteException {...}
        public static Registry getRegistry(int port)
                throws java.rmi.RemoteException {...}
        public static Registry getRegistry(String host)
                throws java.rmi.RemoteException {...}
        public static Registry getRegistry(String host, int port)
                throws java.rmi.RemoteException {...}
        public static Registry getRegistry(String host, int port,
                                           RMIClientSocketFactory csf)
                throws RemoteException {...}
        public static Registry createRegistry(int port)
                throws java.rmi.RemoteException {...}
        public static Registry createRegistry(int port,
                                              RMIClientSocketFactory csf,
                                              RMIServerSocketFactory ssf)
                throws RemoteException {...}
}

The first four getRegistry methods return a reference to a registry on the current host, current host at a specified port, a specified host, or at a particular port on a specified host. What is returned is the remote stub for the registry with the specified host and port information.

The fifth getRegistry method (that takes an RMIClientSocketFactory as one of its arguments), returns a locally created remote stub to the remote object Registry on the specified host and port. Communication with the remote registry whose stub is constructed with this method will use the supplied RMIClientSocketFactory, csf, to create Socket connections to the registry on the remote host and port.

Note: A registry returned from the getRegistry method is a specially constructed stub that contains a well-known object identifier. Passing a registry stub from one JVM to another is not supported (it may or may not work depending on the implementation). Use the LocateRegistry.getRegistry methods to obtain the appropriate registry for a host.

The createRegistry method creates and exports a registry on the local host on the specified port.

The second createRegistry method allows more flexibility in communicating with the registry. This call creates and exports a Registry on the local host that uses custom socket factories for communication with that registry. The registry that is created listens for incoming requests on the given port using a ServerSocket created from the supplied RMIServerSocketFactory. A client that receives a reference to this registry will use a Socket created from the supplied RMIClientSocketFactory.

Note: Starting a registry with the createRegistry method does not keep the server process alive.

6.3 The RegistryHandler Interface

See the RegistryHandler API documentation.