RMI: Programmer's Checklist

Source

The interface

Create or modify public interface XYZ extends java.rmi.Remote

Add the Remote method.

The return type and all the parameters need to either:

implement java.io.Serializable or be a subclass of a class that does, or
be a Remote object (i.e., implement an interface that extends Remote directly or indirectly).

The server (implementation of the interface)

implements XYZ.
has a no-argument constructor that throws RemoteException.
The main method that instantiates a server object:

sets a security manager ( needed, only if the server loads classes not found in its CLASSPATH) :

if ( System.getSecurityManager() == null )

{

System.setSecurityManager( new java.rmi.RMISecurityManager() );

}

constructs the remote object (can throw a RemoteException: declare this in the throws clause or place within a try-catch).
may need to bind the object to a service reference String in its machine's rmiregistry with the bind or rebind methods.

If an external rmiregistry is running on the machine:

java.rmi.Naming.rebind("HelloServer", obj);

The server can start an rmiregistry internally (i.e., within its own JVM) on some port. Then, we don't have rmiregistries running in the background on machines where people forgot to terminate the rmiregistries before they logged out. See janet.services.Hsp.java for an example of this (in its main method).

The client

sets a SecurityManager (see server instructions).
gets a reference to the remote object

Hello hello = (Hello) Naming.lookup("//" + machineName + "/HelloServer");

invokes a remote method on the reference:

The invocation must either

be within a try-catch block for RemoteException
include the RemoteException in the invoking method's throws clause

Compiling

compile all the .java files as usual.
rmic all remote objects.

Running

Establish the code base:

put stub class files in your codebase, as well as any other classes that the client does not need to have in its classpath.
these files need to be readable by all.
the path to them needs to be readable and executable by all.
Check that this has been done successfully by pointing a web browser to the directory that contains the downloadable files. You must get a directory list that includes the necessary files. If you get any other response, either the files are missing or the permissions are not set correctly on the files or the directories.

Specify a policy file that allows downloading of your classes. An example of a policy file that is sufficient (but dangerously permissive) is:

grant
{
// Allow everything for now
permission java.security.AllPermission;
};

Specify the codebase when you run the server.

For example, the command below (which should be all on 1 line) runs a Java application called Application. The system expects the policy file called policy to be in the directory from which the java command executes.

java -Djava.rmi.server.codebase=http://www.cs.ucsb.edu/~cappello/classes/ \

-Djava.security.policy=policy \

Application <command-line parameters go here>

I need class files in my public_html/classes directory to be readable by all. The classes directory needs to be readable and executable by all. The trailing slash on the codebase specification is crucial.

Runtime Exceptions

java.io.NotSerializableException: The object in question does not implement java.io.Serializable
java.security.AccessControlException: access denied: The JVM may not have access to a policy file, either because it is missing or unreadable, or the specification of where it is (on the command line) is incorrect.