gnu.cajo.utils.extra

Class ClientProxy

java.lang.Object

gnu.cajo.utils.extra.ClientProxy

All Implemented Interfaces:

Invoke, Serializable

public final class ClientProxy
extends Object
implements Invoke

This class is used to send server item callbacks to a firewalled client. A client whose firewall settings prohibit incoming socket connections is a common problem. This class is called ClientProxy, as it is a stand-in representation of a remote reference to a client, behind a firewall. This allows server objects to be designed without regard to client firewall issues. An intermediary process would give the server item a real remote reference to the client object when there is no client firewall, or a ClientProxy when there is. The client links the remote reference to this ClientProxy, to the locally firewalled client, using an ItemProxy object. The server item invokes methods on this client proxy which result in an immediate callback invocation on the client.

Note: this paradigm is not threadsafe! It is expected that callbacks to the remote client will not be invoked reentrantly. Correspondingly, a unique instance of this object must be given to each remote client object. A practical usage example is available online.

Version:

1.0, 28-Mar-04 Initial release

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
Remote	remoteThis This is the remoted reference to the server's ClientProxy.
int	timeout This is the longest value, in milliseconds, that the server will wait for a client invocation to execute before it aborts it.

Constructor Summary

Constructors

Constructor and Description
ClientProxy() A server creates this object, then provides the remote reference member remoteThis to the client.

Method Summary

Methods

Modifier and Type	Method and Description
void	cutOff() This method abruptly terminates the ClientProxy link to the server.
Object	invoke(String method, Object args) This method serves two fundamentally different, but symmetrical purposes.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

timeout

public int timeout

This is the longest value, in milliseconds, that the server will wait for a client invocation to execute before it aborts it. It is set by default to 5000 (5 seconds). Depending on the type of functionality being performed in the client methods, this time may require adjusting. When it is changed, the new value will apply for all subsequent calls.

remoteThis

public final Remote remoteThis

This is the remoted reference to the server's ClientProxy. It is passed back to the client, to be used int the ItemProxy constructor, to create a firewall traversing asynchronous callback link. In order to work, logically, it can only be passed to one remote client.

Constructor Detail

ClientProxy

public ClientProxy()
            throws RemoteException

A server creates this object, then provides the remote reference member remoteThis to the client. This creates the first half of the bridge, the ItemProxy class completes the second half.

Throws:

RemoteException - If the remote reference creation fails.

Method Detail

cutOff

public void cutOff()
            throws NoSuchObjectException

This method abruptly terminates the ClientProxy link to the server. The client can detect the detachment, if it wishes, by implementing a similar cutOff method. The client may also remotely invoke this method, if it wishes to sever its link to the server. Any subsequent invocations by the server will result in client timeout exceptions. This method also calls notify(), in case there was an invocation in process when this happened. Note: it would not make sense to call this method more than once, ever, for a given object instance.

Throws:

NoSuchObjectException - Should this method ever be called more than once, on the same object.

invoke

public Object invoke(String method,
            Object args)
              throws Exception

This method serves two fundamentally different, but symmetrical purposes. Initially a remote ItemProxy calls this method to have its calling thread blocked until the server item needs to make an asynchronous callback. Secondly, the server item will also invoke this method, and will have its thread blocked, until the resulting data, or exception, is returned from the firewalled client, via its ItemProxy.

Specified by:

invoke in interface Invoke

Parameters:

method - The name of the method on the firewalled remote client to be invoked asynchronously.

args - The data to be provided the method of the callback method, or data resulting from the client callback.

Returns:

The result of the client object callback.

Throws:

InterruptedException - If the client is not listening, or if the callback timeout has expired.

RemoteException - For any network related failures.

Exception - For any client specific reasons.