Can I invoke a client´s method from a server with RMI

Question

I would like to know if with RMI a server can remotely invoke a method from any of his clients, or just the clients can do this with their server.

Answer 1

Yes, firewalls permitting. The client has to export its callbacks as remote objects and supply them to the server, typically via some registerCallback() API. Then the server just calls the methods.

However firewalls to the Internet typically don't permit callbacks, and if they do there may be issues with port numbers: you will probably need the clients to export their remote objects on a specific port which is opened in the firewall, typically 1099.

Answer 2

Regardless of any implications of such a design decision, it is possible to use RMI between two JVMs that are accessible through the network. This means that if the server can access its clients through the network, and the clients have a JVM available and can act as RMI servers, then it is possible to make a client or each of the clients an RMI server and have the "server" communicate with each one of them.

Assuming the server is an application server i advise you to use the Java Messaging Service (JMS) to allow the server communicate with the client systems.

Answer 3

Besides the solution provided by melc, it is also possible to achieve a full duplex communication between the client and server by using WebSockets. They are now part of the Java EE 7 specification, you can read something more about them here: http://docs.oracle.com/javaee/7/tutorial/doc/websocket.htm. There are also other solutions, like Comet, or JMS (which is used like webosckets in RichFaces).