In this project, you will implement a remote method invocation (RMI) library. RMI forwards method
calls over a network connection, permitting objects located on one Java virtual machine to call methods
on another. Users of the RMI library write code in the usual Java style: remote methods appear,
syntactially, as regular Java methods. The only difference is in how the objects on which the methods
are invoked are obtained. The usage is roughly as follows:
// Create a (contrived) object which can read
// files on the local filesystem.
Storage storage = new Storage();
// Invoke a method on the local object.
byte data = storage.retrieve(“/dir/file”);
Remote method invocation
// Create a “stub” object which “knows” how to
// communicate with the server. This object is
// generated for you automatically by the RMI
// library (“Stub” is a class in the RMI
// library) – you only need to define the
// interface “Server”.
InetSocketAddress address =
new InetSocketAddress(“127.0.0.1”, 80);
Server server =
// Invoke a method on the remote object. This
// method is executed remotely. The data and
// return value are automatically marshalled
// and unmarshalled for you by the RMI library.
// Return values and exceptions are passed
// back to the local VM and can be handled in
// the usual Java fashion.
byte data = server.retrieve(“/dir/file”);
A similar simplification takes place on the server side — the details are given later in the handout.
As you can probably see, RMI greatly reduces the difficulty of writing networked Java code. After you
complete the RMI library, you will use it to write a distributed filesystem in the next project.
Java already has a standard RMI library, but we will not be using it in this course. You may be
interested in browsing its documentation to get more ideas about RMI. A link is given in the references
You should work with a partner on this project. The starter code can be downloaded from http://idon’tknowwhere.
When you are finished, submit your code in a single zip archive to /afs/somewhere. Be sure to include
your modified Skeleton.java and Stub.java, and files containing any helper classes you have created.
Stub, Skeleton, and all helper classes must be in the package rmi. We will extract these classes and use
them to test your implementation. Your RMI library should work on Java 7 virtual machines.
RMI works, firstly, by exposing an object on one Java virtual machine as remotely-accessible, and
secondly, by providing other virtual machines with a way to access this object. The remotely-accessible
object can be thought of as a server in the abstract sense, since it provides some services through
its remotely-accessible methods. Each Java virtual machine that accesses this object is then a client.
Therefore, the RMI library has two major components: one that simplifies the task of making servers
remotely-accessible, and another that simplifies the writing of the corresponding clients.
Note that it is important not to think of the remotely-accessible object as a “server” in the low-level
(socket programming-level) sense. As you will soon see, a low-level TCP server is implemented in, and
hidden by, the RMI library.
On the Server: Skeleton
The server is a regular Java object of a regular Java class, with some public methods. Some of these
public methods are meant to be accessible remotely. To permit remote access, a skeleton object is
created for the server. The skeleton object is implemented in the RMI library. It is a multithreaded
TCP server which handles all the low-level networking tasks: it listens for incoming connections, accepts
them, parses method call requests, and calls the correct methods on the server object. When a method
returns, the return value (or exception) is sent over the network to the client, and the skeleton closes
Note that the server object itself need not perform any network I/O — this is all done entirely by
the skeleton, within the RMI library. The server object does not even have to be aware of the existence
of any skeletons that might invoke methods on it.
What determines which public methods of the server object are accessible remotely? The server
object implements a certain kind of interface called a remote interface, which will be detailed later.
The remote interface lists the remotely-accessible methods of the server object. The skeleton object is
created for this interface, and only forwards calls to the methods which are listed in it.
On the Client: Stub
Clients use stub objects to access the server. Stub objects are created by the RMI library. Each one
appears to implement a given remote interface. However, instead of implementing the interface in a
direct manner, each stub object forwards all method calls to a server by contacting a remote skeleton.
When the client invokes a method on a stub object, the stub opens a connection to the skeleton, and
sends the method name and arguments. As described in the section on skeletons, this causes the remote
skeleton to call the same method on the server object. When the method finishes, the skeleton transmits
the result, and the stub returns this result to the caller in the client virtual machine.
As with the skeleton, the user of the stub need not explicitly perform any network I/O — again, this
is done entirely by the stub object, and implemented within the RMI library.
The goal of the RMI library is, simply, that when the client calls a method on the stub, the skeleton
calls exactly the same method on the server. If no network error occurs, it should appear as if the client
made the call directly on the server, as if the server is a local object.
A remote interface is simply a regular interface with the additional requirement that every method be
marked as throwing a special exception, RMIException. This is because, when using RMI, a method
may fail due to a network error, a protocol incompatibility, or for other reasons that are related only to
RMI and have nothing to do with the functionality of the server object. These failures are, of course,
signalled by throwing RMIException.
Example: File Server Based on RMI
1. Defining a remote interface
public interface Server
public long size(String path) throws FileNotFoundException, RMIException;
public byte retrieve(String path) throws FileNotFoundException, RMIException;
2. Defining a server class
public class ServerImplementation implements Server
// Fields and methods.
public long size(String path) throws FileNotFoundException, RMIException
// Some implementation of the size method, which probably
// accesses the server machine’s local storage and does no
// network I/O.
public byte retrieve(String path) throws FileNotFoundException, RMIException
// Some implementation of the retrieve method.
3. Creating the server object and making it remotely-accessible
// Create the server object.
ServerImplementation server = new ServerImplementation(…);
// At this point, the server object is a regular local object, and is not accessible
// Create the skeleton object. Note that the type Skeleton is parametrized by the
// remote interface for which the skeleton is being created. Unfortunately, due to
// a limitation of the Java language, the class object for the same interface must
// also be passed as the first parameter to the constructor, as shown below.