Java RMI Interview Questions

Why does the RMI implementation create so many sockets when my application uses custom socket factories; or why are stubs (using a custom socket factory) that refer to the same remote object not equal; or why does the RMI implementation not reuse server-side ports when I use a custom server socket factory?

The RMI implementation attempts to reuse open sockets where possible for remote invocations. When a remote method is invoked on a stub that uses a custom socket factory, the RMI implementation will reuse an open connection (if any) as long as that socket was created by an equivalent socket factory. Since client socket factories are serialized to clients, a single client may have several distinct copies of the same logical socket factory. To ensure that the RMI implementation will reuse sockets created by custom socket factories, make sure your custom client socket factory classes implement the hashCode and equals methods appropriately. If the client socket factory does not implement these methods correctly, another ramification is that stubs (using the client socket factory) that refer to the same remote object will not be equal.
The RMI implementation attempts to reuse server-side ports as well. It will only do so if there is an existing server socket for the port created by an equivalent socket factory. Make sure the server socket factory class implements the hashCode and equals methods too.

If your socket factory has no instance state, a trivial implementation of the hashCode and equals methods are the following:

  publicinthashCode(){

Return57;

 }   
publicbooleanequals(Objecto){

 returnthis.getClass()==o.getClass()

}   

What is difference RMI registry and OSAgent ?

RMI Registry
The rmiregistry command creates and starts a remote object registry on the specified port on the current host. If port is omitted, the registry is started on port 1099. The rmiregistry command produces no output and is typically run in the background. For example:
start rmiregistry
A remote object registry is a bootstrap naming service that is used by RMI servers on the same host to bind remote objects to names. Clients on local and remote hosts can then look up remote objects and make remote method invocations.
The registry is typically used to locate the first remote object on which an application needs to invoke methods. That object in turn will provide application-specific support for finding other objects.
The methods of the java.rmi.registry.LocateRegistry class are used to get a registry operating on the local host or local host and port.
The URL-based methods of the java.rmi.Naming class operate on a registry and can be used to look up a remote object on any host, and on the local host: bind a simple (string) name to a remote object, rebind a new name to a remote object (overriding the old binding), unbind a remote object, and list the URLs bound in the registry.

OSAgent
The osagent is a process that allows CORBA servers to register their objects and assists client applications in the location of objects. To avoid a single point of failure, the osagent is designed to be operated on more than one node. Osagent processes (and the clients and servers that access them) follow a set of rules to discover each other and cooperate in the location of registered objects.

Difference in between Unicast and Multicast object ?

A unicast packet is the complete opposite: one machine is talking to only one other machine. All TCP connections are unicast, since they can only have one destination host for each source host. UDP packets are almost always unicast too, though they can be sent to the broadcast address so that they reach every single machine in some cases.

A multicast packet is from one machine to one or more. The difference between a multicast packet and a broadcast packet is that hosts receiving multicast packets can be on different lans, and that each multicast data-stream is only transmitted between networks once, not once per machine on the remote network. Rather than each machine connecting to a video server, the multicast data is streamed per-network, and multiple machines just listen-in on the multicast data once it's on the network.

What is the main functionality of the Remote Reference Layer ?

RRL exists in both the RMI client and server. It is used by the stub or skeleton protocol layer and uses the transport layer. RRL is reponsible for transport-independent functioning of RMI, such as connection management or unicast/multicast object invocation.

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