I'm having trouble with the rmiregistry, it won't seem to run!

Chances are that someone else is running it on the default port. You can run your own copy on whatever port you want (just specify a port number on the command line), but you need to include a port number in the rmi URLS in both your client and server (that you give to Naming.bind() and Naming.lookup()). In the server you can use a URL like "//:8765/Objname" to specify the port, (with no hostname it is assumes you want the local host).

There is a complete RMI example that supports having the registry tun on an alternate port here: http://monte.cs.rpi.edu/~hollingd/netprog/code/java/SimpleRMIport/

I get a runtime error when I call a remote method that creates an object I've defined to hold the components of a URL. The error message says that my class is not "serializable", how do I fix this?

If you want to send (as a parameter) or receive (as a return value) an object whose class you have defined, your class must implement the interface "Serializable". Note that unless you are doing something tricky, you can do this by simply declaring that the class "implements Serializable", no additional code is necessary. The interface Serializable is in java.io, so you need to import java.io.*.

You can also wait and try again later, or move to a different machine...

I want to use RPC, how do I get started?

Step 1: First write and debug a program that can parse and build URLs. Don't worry about RPC until you have this part done.

Step 2: Develop a protocol definition file (a .x file) that includes declarations for 2 remote procedures, one should return a string and accept a structure that includes fields for each component of a URL, the other should accept a string and return a structure. You need to define the structure in the protocol definition file as well.

Step 3: Run rpcgen -C on your protocol definition file. If you file is named hw7.x, you need to run: rpcgen -C hw7.x.

Step 4: Look at the .h file generated by rpcgen (the name will be the same name as your protocol definition file). Here you will see what your structure looks like as a C data structure and the prototypes for the functions your client will call.

Step 5: Make a copy of the program you developed in step 1 that will become your client (name it hw7client.c or something like that). You need to add some RPC initializtion stuff to the main(), something like this (which assumes the hostname of the server comes from argv[1]):

 CLIENT *clnt;
 clnt = clnt_create(argv[1], HW7_PROG, HW7_VERSION, "udp");
 

Note that HW7_PROG and HW7_VERSION must match whatever you specified in your .x file.

Step 6: Change the rest of the client program so that it calls the client stubs instead of your actual functions (which you should remove from the client).

Step 7: Run rpcgen -C -Ss hw7.x > hw7service.c, this will create a dummy server file. Edit this file, changing the code inside each of the remote procedures created for you so that they include the actual code (the code you removed from the client in step 6). You probably need to do a little work matching up data types, etc.

Step 8: Build the client, probably something like this:

  gcc -o client hw7client.c hw7_clnt.c hw7_xdr.c -lnsl
 

Build the server:

  gcc -DRPC_SVC_FG  -o server hw7service.c hw7_svc.c hw7_xdr.c -lrpcsvc -lnsl
  

Note: -DRPC_SVC_FG will force your server to run in the foreground, if you don't add this it will become a daemon (and it will be harder to debug and test).

Step 9: Run the server, run the client to test everything.

Step 10: Make sure you don't forget to write up detailed instructions on how we should build and test your program. Include a few sample command lines for the client. Submit your project, and start studying for the Netprog comprehensive final exam that will take place every day of finals week from 6AM-9AM in the Gym. Make sure your sneakers are in good shape, you have protective eye gear, and that have bring plenty of GatorAde to bring to the final.

I want to use RMI, how do I get started?

Step 1. Write and debug a Java program (a class with a main()) that supports the operations requested. Although you can use a Java URL object to do the work, you need to write your own class (you need to write some code!).

Step 2. Create an interface that extends java.rmi.Remote and includes the 2 methods you need to support (one that parses urls, the other constructs urls). Make sure you declare that each of the methods throws RemoteException.

Step 3. Create a class that implements your interface (that you created in step 2) and extends the class java.rmi.server.UnicastRemoteObject. The code for each method you define here can be taken from the class you developed in step 1. You should include a constructor that calls super() (this calls the constuctor for UnicastRemoteObject). You can also turn this into a server program by including a main() that creates an object and registers it with the naming service. Check the sample code for an example.

Step 4. Compile your interface (step 2) and class (step 3). Now run rmic and give it the name of your class (step 2). This will generate the client stubs and server skeleton classes.

Step 5. Write a class that will act as a client to test your remote object. Most of this can come from the code your wrote in step 1. To use a remote object you need to ask the naming service for a remote object reference, then cast that object to an object of type that supports your interface (defined in step 2). Check one of the samples for the details.

Step 6. Start up the rmi registry with the command rmiregistry. If the registry won't run, chances are someone else is running it on the same machine - see the top question in the FAQ for info on how to run yours on an alternate port.

Step 7. Start up your server (the class created in step 3), and test with your client (created in step 5).

NOTE: If any of your remote methods return an object, the object must be serializable (or it won't work with RMI). You need to have the class definition for the object type returned implement java.io.Serializable